The base load on a grid is the minimum level of demand on an electrical grid over a span of time, for example, one week. This demand can be met by unvarying power plants or by a collection of smaller Intermittent energy sources, depending on which approach has the best mix of low cost and high reliability in any particular market. The remainder of demand, varying throughout a day, is met by Dispatchable generation, Load following power plants, and Peaking power plants, which can be turned up or down quickly, Operating reserve, Demand response and Energy storage.
When the cheapest power was from large coal and nuclear plants which could not be turned up or down quickly, they were used to generate base load, since it is constant, and they were called "baseload plants." Large standby reserves were needed in case of sudden failure of one of these large plants.  Unvarying power plants are no longer always the cheapest way to meet base load. The grid now includes many wind turbines which have such low marginal costs that they can bid lower prices than coal or nuclear, so they can provide some of the base load when the wind blows. Using wind turbines in areas with varying wind conditions, and supplementing them with solar in the day time, dispatchable generation and storage, handles the intermittency of individual wind sources.
Grid operators take long and short term bids to provide electricity over various time periods and balance supply and demand continuously. The detailed adjustments are known as the unit commitment problem in electrical power production.
While historically large power grids used unvarying power plants to meet the base load, there is no specific technical requirement for this to be so. The base load can equally well be met by the appropriate quantity of intermittent power sources and dispatchable generation. 
Unvarying power plants can be coal, nuclear, combined cycle plants, which may take several days to start up and shut down, hydroelectric, geothermal, biogas, biomass, solar thermal with storage and ocean thermal energy conversion.
Supply interruptions can affect all plants from breakdowns, and also hydroelectric plants from droughts, coal plants if their coal stockpiles freeze, and gas plants from pipeline leaks and closures.
The desirable attribute of dispatchability applies to some gas plants, wind (through blade pitch) and hydroelectricity. Grid operators also use curtailment to shut plants out of the grid when their energy is not needed. 
There are 195,000 megawatts of grid storage installed world-wide; 94% is pumped-storage hydroelectricity; 2% is in batteries. Pumped storage uses cheap power at times of low demand, usually night, to pump water from a lower reservoir to an upper reservoir, then lets it drop back through turbines during peak demand times, usually in the day. Availability of solar power in peak hours of the day can reduce the need for storage. The biggest storage facility in the world is on the Virginia-West Virginia border, with 50% more capacity than the Hoover dam.
Grid operators solicit bids to find the cheapest sources of electricity over short and long term buying periods.
Nuclear and coal plants have very high fixed costs, high plant load factor but very low marginal costs, though not as low as solar, wind, and hydroelectric. On the other hand, peak load generators, such as natural gas, have low fixed costs, low plant load factor and high marginal costs.
Coal and nuclear power plants do not change production to match power consumption demands since it is more economical to operate them at constant production levels. Use of higher cost combined-cycle plants or combustion turbines is thus minimized, and these plants can be cycled up and down to match more rapid fluctuations in consumption.
Nuclear power plants may take many hours, if not days, to change their power output, although modern stations, and those in France, can and do operate as load following power plants and alter their output to meet varying demands. Because they require a long period of time to heat up to operating temperature, these plants are only economic if their average cost for the time they are turned on is lower than the average cost of alternatives for that same period.
Different plants and technologies may have differing capacities to increase or decrease output on demand: nuclear plants are generally run at close to maximum output continuously (apart from maintenance, refueling and periodic refurbishment), while coal-fired plants may be cycled over the course of a day to meet demand. Plants with multiple generating units may be used as a group to improve the "fit" with demand, by turning units on and off.
According to National Grid plc chief executive officer Steve Holliday in 2015, baseload is "outdated", as microgrids would become the primary means of production, and large powerplants relegated to supply the remainder.
In 2016, Ambrose Evans-Pritchard of the Daily Telegraph wrote that, with advances in energy storage, 'there ceases to be much point in building costly "baseload" power plants' and goes on to argue 'Nuclear reactors cannot be switched on and off as need demands - unlike gas plants. They are useless as a back-up for the decentralized grid of the future, when wind, solar, hydro, and other renewables will dominate the power supply'.
- Capacity factor
- Energy demand management
- Grid energy storage
- Load balancing (electrical power)
- Smart grid
- Load following power plant
- Peaking power plant
This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (June 2009) (Learn how and when to remove this template message)
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