Photovoltaic system

From Wikipedia, the free encyclopedia
  (Redirected from Solar photovoltaic)
Jump to: navigation, search
A photovoltaic system mounted in front of a bank of windows.

A photovoltaic system, also photovolatic power system, solar PV system, PV system or casually solar array, is a power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and directly convert sunlight into electricity, a solar inverter to change the electrical current from DC to AC, as well as mounting, cabling and other electrical accessories to set-up a working system. It may also use a solar tracking system to improve the system's overall performance or include an integrated battery solution, as prices for storage devices are expected to decline.

Strictly speaking, a solar array only encompasses the ensemble of solar panels, the visible part of the PV system, and does not include all the other hardware, often summarized as balance of system (BOS).

PV systems range from small, roof-top mounted or building-integrated systems with capacities from a few to several tens of kilowatts, to large utility-scale power stations of hundreds of megawatts. Nowadays, most PV systems are connected to the electrical grid, while stand-alone or off-grid systems only account for a small portion of the market.

Prices for PV systems have rapidly declined in recent years and strongly depend on the market, the size of the system, and used technology. In the United States, prices for utility-scale systems were around $2.50–$4.00 per watt in 2012,[1] while prices for smaller roof-top systems in the highly penetrated German market fell below €1.40 per watt in 2014.[2] In that market, solar panels make up for 40 to 50 percent of the overall cost, leaving the rest to installation labor and to the PV system's remaining components.[3]

Solar power based on thermal energy, such as concentrated solar power or panels for water heating, are not components of a photovoltaic system.


A photovoltaic system converts the sun's radiation into usable electricity. It comprises the solar array and the balance of system components. PV systems may be built in various configurations:

  • Grid-connected optionally using a battery storage
  • Off-grid without battery (array-direct)
  • Off-grid with battery storage, optionally converting to AC

Besides these basic configurations, PV systems can be categorized by various aspects, such as, building-integrated vs. rack-mounted systems, residential vs. utility systems, rooftop vs. ground-mounted systems, tracking vs. fixed-tilt systems, new construction vs. retrofit, systems with microinverters vs. central inverter, systems using polysilicon vs. thin-film technology, and systems with Chinese vs. European and US-manufactured modules.

Overall situation[edit]

Schematics of a typical residential PV system

The current situation for PV systems presents itself as follows: more than 95 percent of all PV systems are grid-connected. Most PV system do not use a battery storage. However, this may change soon, as governmental incentives are being implemented. The solar array of a typical residential PV system is rack-mounted on the rooftop, rather than integrated into the roof or facade of the building, as this is significantly more expensive. Utility-scale solar power stations are ground-mounted, with fixed tilted solar panels, rather than using a solar tracking device. Crystalline silicon is the predominant material used in 90 percent of worldwide produced solar modules, while rival thin-film has lost market-share in recent years. About 70 percent of all solar cells and modules are produced in China and Taiwan, leaving 5 percent to European and US-manufacturers. Both, small rooftop systems and large solar power plants are growing rapidly, and both countribute equally to the generated solar electricity, that accounts for about 1 percent of the overall, worldwide electricity generation.

Driven by advances in technology and increases in manufacturing scale and sophistication, the cost of photovoltaics has declined continuosly. There are several million PV systems distributed all over the world, mostly in Europe, with 1.4 million systems in Germany alone–as well as in China, North America and Japan, regions with a faster growing market than in Europe. The energy conversion efficiency of a conventional solar cell increased from 15 to 20 percent over the last 10 years and a PV system recoups the energy needed for its manufacture in about 2 years. In exceptionally irradiated locations, or when thin-film technology is used, the so-called energy payback time decreases to one year or less. Net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have also greatly supported installations of PV systems in many countries. The levelised cost of electricity (LCOE) from PV systems has become competitive with conventional electricity sources in an expanding list of geographic regions, as grid parity has been achived many differnt markets.

Diagram of the possible components of a photovoltaic system.

From a solar cell to a PV system[edit]

Conventional solar cells, normally wired in series, are encapsulated in a solar module to protect them from the weather. The module consists of a tempered glass as cover, a soft and flexible encapsulant, a rear backsheet made of a weathering and fire-resistant material and an alumnium frame around the outer edge. Electrically connected and mounted on a supporting structure, solar modules build a string of modules, often called solar panel. A solar array consists of one or many such panels.

Complete PV system

A photovoltaic system for residential, commercial, or industrial energy supply consists of the solar array, one or more DC to AC power converters, also known as inverters, a racking system that supports the solar modules, electrical wiring and interconnections, and mounting for other components.

Optionally, a photovoltaic system may include any or all of the following: renewable energy credit revenue-grade meter, maximum power point tracker (MPPT), battery system and charger, GPS solar tracker, energy management software, solar concentrators, solar irradiance sensors, anemometer, or task-specific accessories designed to meet specialized requirements for a system owner. The number of modules in the system determines the total DC watts capable of being generated by the solar array; however, the inverter ultimately governs the amount of AC watts that can be distributed for consumption. For example: A PV system comprising 11 kilowatts DC (kWDC) worth of PV modules, paired with one 10-kilowatt AC (kWAC) inverter, will be limited by the maximum output of the inverter: 10 kW AC.

Used terms

The terms "solar array" and "PV system" are often used interchangeable, despite the fact that the solar array does not encompass the entire system. Moreover, "solar panel" is often used as a synonym for "solar module", alhough former consists of a string of several items of the latter.

Small systems[edit]

A small, roof-top mounted PV system of approx. 6 kilowatts.

A small PV system is capable of providing enough AC electricity to power a single home, or even an isolated device in the form of AC or DC electric. For example, military and civilian Earth observation satellites, street lights, construction and traffic signs, electric cars, solar-powered tents,[4] and electric aircraft may contain integrated photovoltaic systems to provide a primary or auxiliary power source in the form of AC or DC power, depending on the design and power demands.

Large systems[edit]

The community-owned Westmill Solar Park in South East England

Large grid-connected photovoltaic power systems are capable of providing an energy supply for multiple consumers. The electricity generated can be stored, used directly (island/standalone plant), fed into a large electricity grid powered by central generation plants (grid-connected or grid-tied plant), or combined with one, or many, domestic electricity generators to feed into a small electrical grid (hybrid plant).[5][6] PV systems are generally designed in order to ensure the highest energy yield for a given investment.

US installation process[edit]

In the United States, the Authority Having Jurisdiction (AHJ) will review designs and issue permits, before construction can lawfully begin. Electrical installation practices must comply with standards set forth within the National Electrical Code (NEC) and be inspected by the AHJ to ensure compliance with building code, electrical code, and fire safety code. Jurisdictions may require that equipment has been tested, certified, listed, and labeled by at least one of the Nationally Recognized Testing Laboratories (NRTL). Despite the complicated installation process, a recent list of solar contractors shows a majority of installation companies were founded since 2000.[7]


Solar array[edit]

A photovoltaic array in the Negev Desert, Israel
Further information: photovoltaics, solar module and solar cell

A photovoltaic array (or solar array) is a linked collection of solar panels.[8] The power that one module can produce is seldom enough to meet requirements of a home or a business, so the modules are linked together to form an array. Most PV arrays use an inverter to convert the DC power produced by the modules into alternating current that can power lights, motors, and other loads. The modules in a PV array are usually first connected in series to obtain the desired voltage; the individual strings are then connected in parallel to allow the system to produce more current. Solar panels are typically measured under STC (standard test conditions) or PTC (PVUSA test conditions), in watts.[9] Typical panel ratings range from less than 100 watts to over 400 watts.[10] The array rating consists of a summation of the panel ratings, in watts, kilowatts, or megawatts.

Mounting systems[edit]

A 23-year old, ground mounted PV system on a North Frisian Island, Germany

Modules are assembled into arrays on some kind of mounting system, which may be classified as ground mount, roof mount or pole mount. For solar parks a large rack is mounted on the ground, and the modules mounted on the rack. For buildings, many different racks have been devised for pitched roofs. For flat roofs, racks, bins and building integrated solutions are used.[citation needed] Solar panel racks mounted on top of poles can be stationary or moving, see Trackers below. Side-of-pole mounts are suitable for situations where a pole has something else mounted at its top, such as a light fixture or an antenna. Pole mounting raises what would otherwise be a ground mounted array above weed shadows and livestock, and may satisfy electrical code requirements regarding inaccessibility of exposed wiring. Pole mounted panels are open to more cooling air on their underside, which increases performance. A multiplicity of pole top racks can be formed into a parking carport or other shade structure. A rack which does not follow the sun from left to right may allow seasonal adjustment up or down.

Solar trackers[edit]

A 1998 model of a passive solar tracker, viewed from underneath.
Main article: Solar tracker

A solar tracking system tilts a solar panel throughout the day. Depending on the type of tracking system, the panel is either aimed directly at the sun or the brightest area of a partly clouded sky. Trackers greatly enhance early morning and late afternoon performance, increasing the total amount of power produced by a system by about 20–25% for a single axis tracker and about 30% or more for a dual axis tracker, depending on latitude.[11][12] Trackers are effective in regions that receive a large portion of sunlight directly. In diffuse light (i.e. under cloud or fog), tracking has little or no value. Because most concentrated photovoltaics systems are very sensitive to the sunlight's angle, tracking systems allow them to produce useful power for more than a brief period each day.[13] Tracking systems improve performance for two main reasons. First, when a solar panel is perpendicular to the sunlight, it receives more light on its surface than if it were angled. Second, direct light is used more efficiently than angled light[citation needed]. Special Anti-reflective coatings can improve solar panel efficiency for direct and angled light, somewhat reducing the benefit of tracking.[14]

Solar inverters[edit]

Inverter for grid connected PV

Systems designed to deliver alternating current (AC), such as grid-connected applications need an inverter to convert the direct current (DC) from the solar modules to AC. Grid connected inverters must supply AC electricity in sinusoidal form, synchronized to the grid frequency, limit feed in voltage to no higher than the grid voltage and disconnect from the grid if the grid voltage is turned off.[15] Islanding inverters need only produce regulated voltages and frequencies in a sinusoidal waveshape as no synchronisation or co-ordination with grid supplies is required.

Inverter (left), generation meter, and AC disconnect (right). A modern 2013 installation in Vermont, United States.

A solar inverter may connect to a string of solar panels. In some installations a solar micro-inverter is connected at each solar panel.[16] For safety reasons a circuit breaker is provided both on the AC and DC side to enable maintenance. AC output may be connected through an electricity meter into the public grid.[17]

Maximum power point tracking

Maximum power point tracking (MPPT) is a technique that grid connected inverters use to get the maximum possible power from the photovoltaic array. In order to do so, the Inverter's MPPT system digitally samples the solar array's ever changing power output and applies the proper resistance to find the optimal maximum power point.[18]

Charge controller[edit]

Main article: Charge controller

PV systems with integrated battery solutions also need a charge controller, as the varying voltage and current from the solar array requires constant adjustment to prevent damage from overcharging.[19] Basic charge controllers may simply turn the PV panels on and off, or may meter out pulses of energy as needed, a strategy called PWM or pulse-width modulation. More advanced charge controllers will incorporate MPPT logic into their battery charging algorithms. Charge controllers may also divert energy to some purpose other than battery charging. Rather than simply shut off the free PV energy when not needed, a user may choose to heat air or water once the battery is full.


Although still expensive, PV systems increasingly use rechargeable batterie to store a surplus to be later used at night. Batteries also stabilize the electrical grid by leveling out peak loads, as they can charge during periods of low demand and feed their stored energy into the grid when demand is high. Batteries used for grid-storage also help to eliminate the need for expensive peaking power plants and to amortize the cost of generators over more hours of operation.

Monitoring and metering[edit]

The metering must be able to accumulate energy units in both directions or two meters must be used. Many meters accumulate bidirectionally, some systems use two meters, but a unidirectional meter (with detent) will not accumulate energy from any resultant feed into the grid.[20]

In some countries, for installations over 30 kWp a frequency and a voltage monitor with disconnection of all phases is required. This is done where more solar power is being generated than can be accommodated by the utility, and the excess can not either be exported or stored. Grid operators historically have needed to provide transmission lines and generation capacity. Now they need to also provide storage. This is normally hydro-storage, but other means of storage are used. Initially storage was used so that baseload generators could operate at full output. With variable renewable energy, storage is needed to allow power generation whenever it is available, and consumption whenever it is needed. The two variables a grid operator have are storing electricity for when it is needed, or transmitting it to where it is needed. If both of those fail, installations over 30kWp can automatically shut down, although in practice all inverters maintain voltage regulation and stop supplying power if the load is inadequate. Grid operators have the option of curtailing excess generation from large systems, although this is more commonly done with wind power than solar power, and results in a substantial loss of revenue.[21] Three-phase inverters have the unique option of supplying reactive power which can be advantageous in matching load requirements.[22]

Standalone systems[edit]

Solar parking meter in Edinburgh, Scotland.
A mobile charging station for electric vehicles in France.

A stand-alone, or off-grid system is not connected to the electrical grid. Standalone systems vary widely in size and application from wristwatches or calculators to remote buildings or spacecraft. If the load is to be supplied independently of solar insolation, the generated power is stored and buffered with a battery. In non-portable applications where weight is not an issue, such as in buildings, lead acid batteries are most commonly used for their low cost and tolerance for abuse.

A charge controller may be incorporated in the system to: a) avoid battery damage by excessive charging or discharging and, b) optimizing the production of the cells or modules by maximum power point tracking (MPPT). However, in simple PV systems where the PV module voltage is matched to the battery voltage, the use of MPPT electronics is generally considered unnecessary, since the battery voltage is stable enough to provide near-maximum power collection from the PV module. In small devices (e.g. calculators, parking meters) only direct current (DC) is consumed. In larger systems (e.g. buildings, remote water pumps) AC is usually required. To convert the DC from the modules or batteries into AC, an inverter is used.

Solar vehicles[edit]

The Japanese winner of 2009 World Solar Challenge in Australia.
Main article: Solar vehicle

Ground, water, air or space vehicles may obtain some or all of the energy required for their operation from the sun. Surface vehicles generally require higher power levels than can be sustained by a practically sized solar array, so a battery is used to meet peak power demand, and the solar array recharges it. Space vehicles have successfully used solar photovoltaic systems for years of operation, eliminating the weight of fuel or primary batteries.

Micro systems[edit]

Profile picture of a mobile solar powered generator
The solar panels on this small yacht at sea can charge the 12 volt batteries at up to 9 amperes in full, direct sunlight.
Main article: Microgeneration

With a growing DIY-community and an increasing interest in environmentally friendly "green energy", some hobbyists have endeavored to build their own PV solar systems from kits[23] or partly diy.[24] Usually, the DIY-community uses inexpensive[25] or high efficiency systems[26] (such as those with solar tracking) to generate their own power. As a result, the DIY-systems often end up cheaper than their commercial counterparts.[27] Often, the system is also hooked up into the regular power grid, using net metering instead of a battery for backup. These systems usually generate power amount of ~2 kW or less. Through the internet, the community is now able to obtain plans to construct the system (at least partly DIY) and there is a growing trend toward building them for domestic requirements. Small scale solar systems are now also being used both in developed countries and in developing countries, for residences and small businesses.[28][29]


One of the most cost effective solar applications is a solar powered pump, as it is far cheaper to purchase a solar panel than it is to run power lines.[30][31][32] They often meet a need for water beyond the reach of power lines, taking the place of a windmill or windpump. One common application is the filling of livestock watering tanks, so that grazing cattle may drink. Another is the refilling of drinking water storage tanks on remote or self-sufficient homes.


Connecting a photovoltaic panel directly to a DC mechanical fan motor can provide air movement when it is most needed during the day. Common applications include both attic and greenhouse ventilation. Increased efficiency can be obtained by interposing a linear current booster (LCB) between the solar panel and the fan motor, to more closely coordinate varying panel output with motor energy requirements. Other controls sometimes used are timers and thermostats, so that the fan does not run when not wanted, even if the sun is shining.

Grid-connected systems[edit]

Diagram of a residential grid-connected PV system

A grid connected system is connected to a larger independent grid (typically the public electricity grid) and feeds energy directly into the grid. This energy may be shared by a residential or commercial building before or after the revenue measurement point. The difference being whether the credited energy production is calculated independently of the customer's energy consumption (feed-in tariff) or only on the difference of energy (net metering). Grid connected systems vary in size from residential (2-10kWp) to solar power stations (up to 10s of MWp). This is a form of decentralized electricity generation. The feeding of electricity into the grid requires the transformation of DC into AC by a special, synchronising grid-tie inverter.[33] In kW sized installations the DC side system voltage is as high as permitted (typically 1000V except US residential 600V) to limit ohmic losses. Most modules (72 crystalline silicon cells) generate 160W to 300W at 36 volts. It is sometimes necessary or desirable to connect the modules partially in parallel rather than all in series. One set of modules connected in series is known as a 'string'.[34]

Connection to DC grids[edit]

DC grids are found in electric powered transport: railways trams and trolleybuses. A few pilot plants for such applications have been built, such as the tram depots in Hannover Leinhausen, using photovoltaic contributors[35] and Geneva (Bachet de Pesay).[36] The 150 kWp Geneva site feeds 600V DC directly into the tram/trolleybus electricity network whereas before it provided about 15% of the electricity at its opening in 1999.

Building-mounted and building-integrated systems[edit]

In urban and suburban areas, photovoltaic arrays are commonly used on rooftops to supplement power use; often the building will have a connection to the power grid, in which case the energy produced by the PV array can be sold back to the utility in some sort of net metering agreement. Some utilities, such as Solvay Electric in Solvay, NY, use the rooftops of commercial customers and telephone poles to support their use of PV panels.[37] Solar trees are arrays that, as the name implies, mimic the look of trees, provide shade, and at night can function as street lights.

In agricultural settings, the array may be used to directly power DC pumps, without the need for an inverter. In remote settings such as mountainous areas, islands, or other places where a power grid is unavailable, solar arrays can be used as the sole source of electricity, usually by charging a storage battery.[citation needed] There is financial support available for people wishing to install PV arrays. Incentives range from federal tax credits to state tax credits and rebates to utility loans and rebates. A listing of current incentives can be found at the Database of State Incentives for Renewables and Efficiency.

In the UK, households are paid a 'Feedback Fee' to buy excess electricity at a flat rate per kWh. This is up to 44.3p/kWh which can allow a home to earn double their usual annual domestic electricity bill.[38] The current UK feed-in tariff system is due for review on 31 March 2012, after which the current scheme may no longer be available.[39][need quotation to verify]

Power plants[edit]

A photovoltaic power station (solar park or solar farm) is a power station using photovoltaic modules and inverters for utility scale electricity generation, connected to an electricity transmission grid. Some large photovoltaic power stations like Waldpolenz Solar Park and Topaz Solar Farm cover tens or hundreds of hectares and have power outputs up to hundreds of megawatts.

Planning and permit[edit]

While article 690 of the National Electric Code provides general guidelines for the installation of photovoltaic systems, these guidelines may be superseded by local laws and regulations. Often a permit is required necessitating plan submissions and structural calculations before work may begin. Additionally, many locales require the work to be performed under the guidance of a licensed electrician. Check with the local City/County AHJ (Authority Having Jurisdiction) to ensure compliance with any applicable laws or regulations.


Insolation and energy[edit]

Global solar potential

Solar insolation is made up of direct radiation, diffuse radiation and reflected radiation (or albedo).The absorption factor of a PV cell is defined as the fraction of incident solar irradiance that is absorbed by the cell.[40] At high noon on a cloudless day at the equator, the power of the sun is about 1 kW/m²,[41] on the Earth's surface, to a plane that is perpendicular to the sun's rays. As such, PV arrays can track the sun through each day to greatly enhance energy collection. However, tracking devices add cost, and require maintenance, so it is more common for PV arrays to have fixed mounts that tilt the array and face solar noon (approximately due south in the Northern Hemisphere or due north in the Southern Hemisphere). The tilt angle, from horizontal, can be varied for season,[42] but if fixed, should be set to give optimal array output during the peak electrical demand portion of a typical year for a stand alone system. This optimal module tilt angle is not necessarily identical to the tilt angle for maximum annual array energy output.[43] The optimization of the a photovoltaic system for a specific environment can be complicated as issues of solar flux, soiling, and snow losses should be taken into effect. In addition, recent work has shown that spectral effects can play a role in optimal photovoltaic material selection. For example, the spectral albedo can play a significant role in output depending on the surface around the photovoltaic system [44] and the type of solar cell material.[45]

For the weather and latitudes of the United States and Europe, typical insolation ranges from 4 kWh/m²/day in northern climes to 6.5 kWh/m²/day in the sunniest regions. Typical solar panels have an average efficiency of 15%, with the best commercially available panels at 21%. Thus, a photovoltaic installation in the southern latitudes of Europe or the United States may expect to produce 1 kWh/m²/day. A typical "150 watt" solar panel is about a square meter in size. Such a panel may be expected to produce 0.75 kWh every day, on average, after taking into account the weather and the latitude, for an insolation of 5 sun hours/day. A typical 1 kW photovoltaic installation in Australia or the southern latitudes of Europe or United States, may produce 3.5-5 kWh per day, dependent on location, orientation, tilt, insolation and other factors.[46] In the Sahara desert, with less cloud cover and a better solar angle, one could ideally obtain closer to 8.3 kWh/m²/day provided the nearly ever present wind would not blow sand onto the units. The area of the Sahara desert is over 9 million km². 90,600 km², or about 1%, could generate as much electricity as all of the world's power plants combined.[47]

Tracking the sun[edit]

Trackers and sensors to optimise the performance are often seen as optional, but tracking systems can increase viable output by up to 45%.[8][48] PV arrays that approach or exceed one megawatt often use solar trackers. Accounting for clouds, and the fact that most of the world is not on the equator, and that the sun sets in the evening, the correct measure of solar power is insolation – the average number of kilowatt-hours per square meter per day. For the weather and latitudes of the United States and Europe, typical insolation ranges from 2.26 kWh/m²/day in northern climes to 5.61 kWh/m²/day in the sunniest regions.[49][50]

For large systems, the energy gained by using tracking systems can outweigh the added complexity (trackers can increase efficiency by 30% or more). For very large systems, the added maintenance of tracking is a substantial detriment.[51] Tracking is not required for flat panel and low-concentration photovoltaic systems. For high-concentration photovoltaic systems, dual axis tracking is a necessity.[52]

Pricing trends affect the balance between adding more stationary solar panels versus having fewer panels that track. When solar panel prices drop, trackers become a less attractive option.

Shading and dirt[edit]

Photovoltaic cell electrical output is extremely sensitive to shading. The effects of this shading are well known.[53][54][55] When even a small portion of a cell, module, or array is shaded, while the remainder is in sunlight, the output falls dramatically due to internal 'short-circuiting' (the electrons reversing course through the shaded portion of the p-n junction). If the current drawn from the series string of cells is no greater than the current that can be produced by the shaded cell, the current (and so power) developed by the string is limited. If enough voltage is available from the rest of the cells in a string, current will be forced through the cell by breaking down the junction in the shaded portion. This breakdown voltage in common cells is between 10 and 30 volts. Instead of adding to the power produced by the panel, the shaded cell absorbs power, turning it into heat. Since the reverse voltage of a shaded cell is much greater than the forward voltage of an illuminated cell, one shaded cell can absorb the power of many other cells in the string, disproportionately affecting panel output. For example, a shaded cell may drop 8 volts, instead of adding 0.5 volts, at a particular current level, thereby absorbing the power produced by 16 other cells.[56] It is, thus important that a PV installation is not shaded by trees or other obstructions. Several methods have been developed to determine shading losses from trees to PV systems over both large regions using LiDAR,[57] but also at an individual system level using sketchup.[58] Most modules have bypass diodes between each cell or string of cells that minimize the effects of shading and only lose the power of the shaded portion of the array. The main job of the bypass diode is to eliminate hot spots that form on cells that can cause further damage to the array, and cause fires. Sunlight can be absorbed by dust, snow, or other impurities at the surface of the module. This can reduce the light that strikes the cells. In general these losses aggregated over the year are small even for locations in Canada.[59] Maintaining a clean module surface will increase output performance over the life of the module. Google found that cleaning the flat mounted solar panels after 15 months increased their output by almost 100%, but that the 5% tilted arrays were adequately cleaned by rainwater.[60][61]


Module output and life are also degraded by increased temperature. Allowing ambient air to flow over, and if possible behind, PV modules reduces this problem.

Photovoltaic efficiency[edit]

Main article: Solar cell efficiency

In 2012, solar panels available for consumers can have an efficiency of up to about 17%,[62] while commercially available panels can go as far as 27%. It has been recorded that a group from the The Fraunhofer Institute for Solar Energy Systems have created a cell that can reach 44.7% efficiency, which makes scientists' hopes of reaching the 50% efficiency threshold a lot more feasible.[63][64][65][66]


Photovoltaic systems need to be monitored to detect breakdown and optimize their operation. Several photovoltaic monitoring strategies depending on the output of the installation and its nature. Monitoring can be performed on site or remotely. It can measure production only, retrieve all the data from the inverter or retrieve all of the data from the communicating equipment (probes, meters, etc.). Monitoring tools can be dedicated to supervision only or offer additional functions. Individual inverters and battery charge controllers may include monitoring using manufacturer specific protocols and software.[67] Energy metering of an inverter may be of limited accuracy and not suitable for revenue metering purposes. A third-party data acquisition system can monitor multiple inverters, using the inverter manufacturer's protocols, and also acquire weather-related information. Independent smart meters may measure the total energy production of a PV array system. Separate measures such as satellite image analysis or a solar radiation meter (a pyranometer) can be used to estimate total insolation for comparison.[68] Data collected from a monitoring system can be displayed remotely over the World Wide Web. For example, the Open Solar Outdoors Test Field (OSOTF)[69] is a grid-connected photovoltaic test system, which continuously monitors the output of a number of photovoltaic modules and correlates their performance to a long list of highly accurate meteorological readings. The OSOTF is organized under open source principles—All data and analysis is be made freely available to the entire photovoltaic community and the general public.[70] The Fraunhofer Center for Sustainable Energy Systems maintains two test systems, one in Massachusetts, and the Outdoor Solar Test Field OTF-1 in Albuquerque, New Mexico, which opened in June 2012. A third site, OTF-2, also in Albuquerque, is under construction.[71] Some companies offer analysis software to analyze system performance. Small residential systems may have minimal data analysis requirements other than perhaps total energy production; larger grid-connected power plants can benefit from more detailed investigations of performance.[72][73][74]

Performance factors[edit]

Uncertainties in revenue over time relate mostly to the evaluation of the solar resource and to the performance of the system itself. In the best of cases, uncertainties are typically 4% for year-to-year climate variability, 5% for solar resource estimation (in a horizontal plane), 3% for estimation of irradiation in the plane of the array, 3% for power rating of modules, 2% for losses due to dirt and soiling, 1.5% for losses due to snow, and 5% for other sources of error. Identifying and reacting to manageable losses is critical for revenue and O&M efficiency. Monitoring of array performance may be part of contractual agreements between the array owner, the builder, and the utility purchasing the energy produced.[citation needed] Recently, a method to create "synthetic days" using readily available weather data and verification using the Open Solar Outdoors Test Field make it possible to predict photovoltaic systems performance with high degrees of accuracy.[75] This method can be used to then determine loss mechanisms on a local scale - such as those from snow [59][61] or the effects of surface coatings (e.g. hydrophobic or hydrophilic) on soiling or snow losses.[76] Access to the Internet has allowed a further improvement in energy monitoring and communication. Dedicated systems are available from a number of vendors. For solar PV system that use microinverters (panel-level DC to AC conversion), module power data is automatically provided. Some systems allow setting performance alerts that trigger phone/email/text warnings when limits are reached. These solutions provide data for the system owner and the installer. Installers are able to remotely monitor multiple installations, and see at-a-glance the status of their entire installed base.[citation needed]

Module life[edit]

Effective module lives are typically 25 years or more.[77] The payback period for an investment in a PV solar installation varies greatly and is typically less useful than a calculation of return on investment.[78] While it is typically calculated to be between 10 and 20 years, the payback period can be far shorter with incentives.[79]

Hybrid systems[edit]

Hybrid Power System.gif

A hybrid system combines PV with other forms of generation, usually a diesel generator. Biogas is also used. The other form of generation may be a type able to modulate power output as a function of demand. However more than one renewable form of energy may be used e.g. wind. The photovoltaic power generation serves to reduce the consumption of non renewable fuel. Hybrid systems are most often found on islands. Pellworm island in Germany and Kythnos island in Greece are notable examples (both are combined with wind).[80][81] The Kythnos plant has reduced diesel consumption by 11.2%.[82]

There has also been recent work showing that the PV penetration limit can be increased by deploying a distributed network of PV+CHP hybrid systems in the U.S.[83] The temporal distribution of solar flux, electrical and heating requirements for representative U.S. single family residences were analyzed and the results clearly show that hybridizing CHP with PV can enable additional PV deployment above what is possible with a conventional centralized electric generation system. This theory was reconfirmed with numerical simulations using per second solar flux data to determine that the necessary battery backup to provide for such a hybrid system is possible with relatively small and inexpensive battery systems.[84] In addition, large PV+CHP systems are possible for institutional buildings, which again provide back up for intermittent PV and reduce CHP runtime.[85]


Increasing use of photovoltaic systems and integration of photovoltaic power into existing structures and techniques of supply and distribution increases the value of general standards and definitions for photovoltaic components and systems.[citation needed] The standards are compiled at the International Electrotechnical Commission (IEC) and apply to efficiency, durability and safety of cells, modules, simulation programs, plug connectors and cables, mounting systems, overall efficiency of inverters etc.[86]

Costs and economy[edit]

Pricing for PV systems in Germany
(euros per kilowatt)

History of PV roof-top prices in euros per kilowatt (€/kW) since 2006.[87]

Costs of production have been reduced dramatically in recent years for more widespread use through production and technological advances. For large-scale installations, prices below $1.00 per watt are now common.[88] A price decrease of 50% had been achieved in Europe from 2006 to 2011 and there is a potential to lower the generation cost by 50% by 2020.[89] Crystal silicon solar cells have largely been replaced by less expensive multicrystalline silicon solar cells, and thin film silicon solar cells have also been developed recently at lower costs of production. Although they are reduced in energy conversion efficiency from single crystalline "siwafers", they are also much easier to produce at comparably lower costs.[90]

The table below shows the total cost in US cents per kWh of electricity generated by a photovoltaic system.[91][92] The row headings on the left show the total cost, per peak kilowatt (kWp), of a photovoltaic installation. Photovoltaic system costs have been declining and in Germany, for example, were reported to have fallen to USD 2200/kWp by the second quarter of 2012.[93] The column headings across the top refer to the annual energy output in kWh expected from each installed kWp. This varies by geographic region because the average insolation depends on the average cloudiness and the thickness of atmosphere traversed by the sunlight. It also depends on the path of the sun relative to the panel and the horizon. Panels are usually mounted at an angle based on latitude, and often they are adjusted seasonally to meet the changing solar declination. Solar tracking can also be utilized to access even more perpendicular sunlight, thereby raising the total energy output.

The calculated values in the table reflect the total cost in cents per kWh produced. They assume a 10% total capital cost (for instance 4% interest rate, 1% operating and maintenance cost,[94] and depreciation of the capital outlay over 20 years). Normally, photovoltaic modules have a 25 year warranty.[95][96]

Cost per kilowatt hour (US cents/kWh)
20 years 2400
kWh/kWp y
kWh/kWp y
kWh/kWp y
kWh/kWp y
kWh/kWp y
kWh/kWp y
kWh/kWp y
kWh/kWp y
kWh/kWp y
$200 /kWp 0.8 0.9 1.0 1.1 1.3 1.4 1.7 2.0 2.5
$600 /kWp 2.5 2.7 3.0 3.3 3.8 4.3 5.0 6.0 7.5
$1000 /kWp 4.2 4.5 5.0 5.6 6.3 7.1 8.3 10.0 12.5
$1400 /kWp 5.8 6.4 7.0 7.8 8.8 10.0 11.7 14.0 17.5
$1800 /kWp 7.5 8.2 9.0 10.0 11.3 12.9 15.0 18.0 22.5
$2200 /kWp 9.2 10.0 11.0 12.2 13.8 15.7 18.3 22.0 27.5
$2600 /kWp 10.8 11.8 13.0 14.4 16.3 18.6 21.7 26.0 32.5
$3000 /kWp 12.5 13.6 15.0 16.7 18.8 21.4 25.0 30.0 37.5

During the cells' lifetime, repairs will be necessary to keep the system running well. Research has shown that some repairs have been based on irradiation in high efficiency metals which can be measured by admittance spectroscopy.[97] Although these repairs can be expensive, thermal events such as the one in Bakersfield, California can occur, as faulty arrays made the system overheat and catch flame. This lead to expose more faulty areas and a larger fire.[98]


United Kingdom[edit]

In the UK, PV installations are generally considered permitted development and don't require planning permission. If the property is listed or in a designated area (National Park, Area of Outstanding Natural Beauty, Site of Special Scientific Interest or Norfolk Broads) then planning permission is required.[99]

Typical KW peak for a PV pannel 1m2 is 0.2KW peak in the uk.

United States[edit]

In the US Many localities require a license to install a photovoltaic system. A grid-tied system normally requires a licensed electrician to make the connection between the system and the grid-connected wiring of the building.[100] Installers who meet these qualifications are located in almost every state.[7]

The State of California prohibits Homeowners' associations from restricting solar devices.[101]


Although Spain uses about 20% of its energy via photovoltaics as cities such as Huelva and Seville boast nearly 3,000 hours of sunshine per year, Spain has issued a solar tax to account for the debt created by the investment done by the Spanish government. Those who do not connect to the grid can face up to a fine of 30 million euros ($40 million USD).[102][103]

See also[edit]


  1. ^ Installed Price of Solar Photovoltaic Systems in the U.S. Continues to Decline at a Rapid Pace
  2. ^ [1] History of monthly turn-key prices for roof-top PV systems up to 100 kWp in euros per kilowatt
  3. ^ Fraunhofer ISE Levelized Cost of Electricity Study, Novermber 2013, p. 19
  4. ^ Army evaluating transportable solar-powered tents | Article | The United States Army. (2010-12-08). Retrieved on 2013-07-17.
  5. ^ Types of PV systems. Florida Solar Energy Center (FSEC), a research institute of the University of Central Florida.
  6. ^ Rahmani, R.; Fard, M.; Shojaei, A.A.; Othman, M.F.; Yusof, R., A complete model of stand-alone photovoltaic array in MATLAB-Simulink environment, 2011 IEEE Student Conference on Research and Development (SCOReD), pp 46–51, 2011.
  7. ^ a b Solar Power World
  8. ^ a b "Small Photovoltaic Arrays". Research Institute for Sustainable Energy (RISE), Murdoch University. Retrieved 5 February 2010. 
  9. ^ Key Factors in selecting solar components
  10. ^ List of Eligible SB1 Guidelines Compliant Photovoltaic Modules
  11. ^ A Performance Calculator. Retrieved on 2012-04-23.
  12. ^ Technological advantages. Retrieved on 2012-04-23.
  13. ^ o Al-Mohamad, Ali. "Efficiency improvements of photo-voltaic panels using a Sun-tracking system." Applied Energy 79, no. 3 (2004): 345-354.
  14. ^ Reflective Coating Silicon Solar Cells Boosts Absorption Over 96 Percent. (2008-11-03). Retrieved on 2012-04-23.
  15. ^ Grid-Tied Inverter Safety. Retrieved on 2012-04-23.
  16. ^ Trend watch: Microinverters invade solar
  17. ^ Services and Solutions for Photovoltaic Systems
  18. ^ All About Maximum Power Point Tracking (MPPT)
  19. ^ Dan Fink, Charge Controller Buyer's Guide, January 2012
  20. ^ Residential Photovoltaic Metering and Interconnection Study
  21. ^ Integrating Variable Renewable Energy in Electric Power Markets
  22. ^ Smart PV Inverter Benefits for Utilities
  23. ^ People building their own solar systems from kits. Retrieved on 2012-04-23.
  24. ^ Example of diy PV system with pictures. (2007-11-05). Retrieved on 2012-04-23.
  25. ^ Graham, Michael. (2005-10-15) Low-cost PV solar kit preferred by diy-communities. Retrieved on 2012-04-23.
  26. ^ Ken Darrow and Mike Saxenian Appropriate Technology Sourcebook at the Wayback Machine (archived September 22, 2010).
  27. ^ "Alternative Energy Development: Michigan will be Nation’s Leader in Alternative Energy Technology, Jobs". State of Michigan, Office Of The Governor. Retrieved February 22, 2012. 
  28. ^ Energy
  29. ^ Solar Lamps
  30. ^ 'Pay bill for 4 months, get power for 25 years'
  31. ^ "Pumping Water with Sunshine". Retrieved 7 January 2014. 
  32. ^ "Solar Well Pumps". Retrieved 7 January 2014. 
  33. ^ How Solar Power Works
  34. ^ Photovoltaic... Cell, Module, String, Array
  35. ^ Innovative Electrical Concepts at the Wayback Machine (archived March 18, 2009). International Energy Agency (2001)
  36. ^ site7. Retrieved on 2012-04-23.
  37. ^ [2]
  38. ^ Feed in Tariffs. Retrieved on 2012-04-23.
  39. ^ FAQs on Solar Photovoltaic Panels at Swithenbanks, Solar Panels, Solar Hot Water, Water Turbines, Wind Turbines. Retrieved on 2012-04-23.
  40. ^ Santbergen, R; R.J.C. van Zolingen (22 October 2007). "The absorption factor of crystalline silicon PV cells: A numerical and experimental study". Solar Energy Materials & Solar Cells. 
  41. ^ El-Sharkawi, Mohamed A. (2005). Electric energy. CRC Press. pp. 87–88. ISBN 978-0-8493-3078-0. 
  42. ^ Optimum Tilt of Solar Panels
  43. ^ Stand Alone Photovoltaic Lighting Systems
  44. ^ Rob W. Andrews and Joshua M. Pearce, The effect of spectral albedo on amorphous silicon and crystalline silicon solar photovoltaic device performance, Solar Energy, 91,233–241 (2013). DOI:10.1016/j.solener.2013.01.030 open access
  45. ^ M.P. Brennan, A.L. Abramase, R.W. Andrews, J. M. Pearce, Effects of spectral albedo on solar photovoltaic devices, Solar Energy Materials and Solar Cells, 124, pp. 111-116,(2014). DOI:
  46. ^ "How much energy will my solar cells produce?". Retrieved 2012-05-30. 
  47. ^ Sahara's solar power potential underlined
  48. ^ Beginners' Guide to Solar Trackers: How to Increase Output for Your Home Solar Panel System
  49. ^ Insolation Levels (Europe)
  50. ^ 10 years Average Insolation Data
  51. ^ Utility Scale Solar Power Plants
  52. ^ Should You Install a Solar Tracker?
  53. ^ Kajihara, Atsushi, and A. T. Harakawa. "Model of photovoltaic cell circuits under partial shading." Industrial Technology, 2005. ICIT 2005. IEEE International Conference on. IEEE, 2005.
  54. ^ Drif, M., Perez, P. J., Aguilera, J., & Aguilar, J. D. (2008). A new estimation method of irradiance on a partially shaded PV generator in grid-connected photovoltaic systems. Renewable Energy, 33(9), 2048-2056.
  55. ^ VENTRE, JERRY AUTOR. Photovoltaic systems engineering. CRC press, 2004.
  56. ^ Ursula Eicker, Solar Technologies for Buildings, Wiley 2003, ISBN 0-471-48637-X, page 226
  57. ^ H. T. Nguyen and J. M. Pearce, Incorporating Shading Losses in Solar Photovoltaic Potential Assessment at the Municipal Scale, Solar Energy 86(5), pp. 1245–1260 (2012). Source:
  58. ^ Z. Dereli, C. Yücedağ and J. M. Pearce, Simple and Low-Cost Method of Planning for Tree Growth and Lifetime Effects on Solar Photovoltaic Systems Performance, Solar Energy, 95, pp.300-307 (2013). Available:
  59. ^ a b Rob Andrews and Joshua M. Pearce, “Prediction of Energy Effects on Photovoltaic Systems due to Snowfall Events” in: 2012 38th IEEE Photovoltaic Specialists Conference (PVSC). Presented at the 2012 38th IEEE Photovoltaic Specialists Conference (PVSC), pp. 003386 –003391. Available: DOI open access
  60. ^ Should you spring clean your solar panels?
  61. ^ a b Rob W. Andrews, Andrew Pollard, Joshua M. Pearce, “The Effects of Snowfall on Solar Photovoltaic Performance ”, Solar Energy 92, 8497 (2013). DOI: open access
  62. ^ "Solar Panel Comparison Table". Retrieved 2012-10-21. 
  63. ^ Andresen, Bjarne; R. Stephen Berry (May 1977). "Thermodynamics in finite time. I. The step-Carnot cycle". Physical Review A. 
  64. ^ Fraunhofer Institute for Solar Energy Systems. "World Record Solar Cell with 44.7% Efficiency". Fraunhofer ISE. 
  65. ^ "Concentrix Solar: Concentrator Modules". Retrieved 2008-12-03. 
  66. ^ CPV Solar Cell Reach 27% System Efficiency
  67. ^ Enphase Technology
  68. ^ Solar Irradiance Measurements
  69. ^ Pearce, Joshua. M; Adegboyega Babasola; Rob Andrews (2012). "Open Solar Photovoltaic Systems Optimization". Proceedings of the 16th Annual National Collegiate Inventors and Innovators Alliance Conference (NCIIA): 1–7. 
  70. ^ Open Solar Photovoltaic Systems Optimization
  71. ^ Fraunhofer Center for Sustainable Energy Systems Announces Opening of Albuquerque Outdoor Solar Test Field
  72. ^ CSI—Metering and Performance Monitoring
  73. ^ Solar Energy
  74. ^ SolarGuard
  75. ^ Rob Andrews, Andrew Pollard, Joshua M. Pearce, “Improved parametric empirical determination of module short circuit current for modelling and optimization of solar photovoltaic systems”, Solar Energy 86, 2240-2254 (2012). DOI, open access
  76. ^ Rob W. Andrews, Andrew Pollard, Joshua M. Pearce, A new method to determine the effects of hydrodynamic surface coatings on the snow shedding effectiveness of solar photovoltaic modules. Solar Energy Materials and Solar Cells 113 (2013) 71–78. open access
  77. ^ "Solar Power (Photovoltaic, PV)". Agriculture and Agri-Food Canada. Retrieved 5 February 2010. 
  78. ^ The Worst Metric in Renewables: 'The Payback Period'. Renewable Energy World (2010-04-19). Retrieved on 2012-10-01.
  79. ^ It's payback time for home generation. BBC News (2010-06-22). Retrieved on 2012-04-23.
  80. ^ PV resources website, Hybrid power station accessed 10 Feb 08
  81. ^ Daten und Fakten at the Wayback Machine (archived July 19, 2011). Pellworm island website (in German)
  82. ^ Darul’a, Ivan; Stefan Marko (2007). "Large scale integration of renewable electricity production into the grids" (PDF). Journal of Eelectrical Engineering 58 (1): 58–60. ISSN 1335-3632. Retrieved 2008-02-10. 
  83. ^ J. M. Pearce (2009). "Expanding Photovoltaic Penetration with Residential Distributed Generation from Hybrid Solar Photovoltaic + Combined Heat and Power Systems". Energy 34: 1947–1954. doi:10.1016/ 
  84. ^ P. Derewonko and J.M. Pearce, “Optimizing Design of Household Scale Hybrid Solar Photovoltaic + Combined Heat and Power Systems for Ontario”, Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE, pp.1274–1279, 7–12 June 2009.
  85. ^ M. Mostofi, A. H. Nosrat, and J. M. Pearce, “Institutional-Scale Operational Symbiosis of Photovoltaic and Cogeneration Energy Systems” International Journal of Environmental Science and Technology 8(1), pp. 31–44, 2011. Available open access: [3]
  86. ^ Regan Arndt and Dr. Ing Robert Puto. Basic Understanding of IEC Standard Testing For Photovoltaic Panels. Available:
  87. ^ Average turn-key prices for roof-top PV systems up to 100 kWp. Sources: for data since 2009, pv-preisindex , displaying for each year average price of month of January. Data source for previous years (2006-2008), see Bundesverband Solarwirtschaft e.V. (BSW-Solar), September 2009, page 4, quarterly figures from EUPD-Research.
  88. ^ John Quiggin (January 3, 2012). "The End of the Nuclear Renaissance |". National Interest. 
  89. ^ "Solar photovoltaics: Competing in the energy sector". European Photovoltaic Industry Association (EPIA). 2011-09-01. Retrieved 2014-04-05. 
  90. ^ A Comparison of PV Technologies
  91. ^ p = \frac{I\cdot (1+i)+\sum_{t=0}^T \frac{I \cdot b}{(1+i)^t}}{\sum_{t=0}^T\frac{E\cdot (1-v)^t}{(1+i)^t}}
  92. ^ What is Levelized Cost?
  93. ^ IRENA: Renewable Power Generation Costs (German Photovoltaic System Costs: USD 2200/kW; Page 9)
  94. ^ PV operation and maintenance costs. (PDF) . Retrieved on 2012-04-23.
  95. ^ Solar PV warranties
  96. ^ Understanding Solar Panel Warrantees
  97. ^ Jasenek, J; U. Rau (17 April 2001). "Defect generation in Cu(In,Ga)Se2 heterojunction solar cells by high-energy electron and proton irradiation". Journal of Applied Physics 90 (650): 9. doi:10.1063/1.1379348. 
  98. ^ Brooks, Bill (February–March 2011). "The Bakersfield Fire: A Lesson in Ground-Fault Protection". Solar Pro (4.2). 
  99. ^ Solar Panels. Planning Portal. Retrieved on 2013-07-17.
  100. ^ "Requirements for Solar Installations". 2011. Retrieved March 31, 2011. 
  101. ^ "California Solar Rights Act". Retrieved February 25, 2012. 
  102. ^ Hunt, Tam. "Spain and Portugal Lead the Way on Renewable Energy Transformation". Renewable Energy World. 
  103. ^ Phillips Erb, Kelly (8/19/13). "Out Of Ideas And In Debt, Spain Sets Sights On Taxing The Sun". Forbes. Retrieved 10/4/13. 

External links[edit]