Energy factor

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An energy factor is a metric used in the United States to compare the energy conversion efficiency of residential appliances and equipment. The energy factor is currently used for rating the efficiency of water heaters, dishwashers, clothes washers, and clothes dryers.[1] The term is used by the United States Department of Energy to develop and enforce minimum energy conservation standards under the Energy Conservation Program.[2]

Although the term energy factor is used to compare the relative efficiency of these appliances, the metric is defined differently for all four appliance categories. The energy factor is expressed in terms of site energy, which excludes losses through energy conversion. All of these efficiency metrics are defined by Department of Energy test procedures.[3]

Water heaters[edit]

The energy factor metric only applies to residential water heaters, which are currently defined by fuel, type, and input capacity.[4]

Residential water heaters
Fuel Type Input capacity
Natural Gas Storage ≤75 kBtu/h
Fuel Oil Storage ≤105 kBtu/h
Electric Storage ≤12 kW
Tabletop Storage ≤12 kW
Natural Gas Instantaneous <200 kBtu/hr
Electric Instantaneous ≤12 kW

Test procedure[edit]

The energy factor is determined using a stylized hot water use pattern. Hot water is drawn in six equal draws totaling 64.3 gallons, and a standby period of 18 hours follows.

The energy factor for residential water heaters is determined using the results from the 24-hour simulated use test. During the test 64.3±1.0 gallons of water are drawn from the water heater in six equally spaced draws that begin one hour apart. The hot water flow rate for each draw is 3.0±0.25 gallons per minute. After the beginning of the last draw a standby period of 18 hours follows. During the test, the test conditions must be operated at a specified value and accuracy. Heat pump water heaters (HPWHs) have different values specified for ambient air temperature and relative humidity.[5]

Specified test conditions
Measurement Required value and accuracy
Inlet water temperature 58°F±2°F
Outlet water temperature 135°F±5°F
Ambient air temperature 67.5°F±2.5°F
Ambient air temperature (HPWHs only) 67.5°F±1°F
Ambient relative humidity (HPWHs only) 50%±1%

From the standard test procedure, the energy factor is defined as

 EF=\sum\limits_{i=1}^6 \frac{M_i C_{pi}\left( 135^\circ F-58^\circ F\right)}{Q_{dm}},

where Q_{dm} is the modified daily water heating energy consumption (Btu), M_i is the mass withdrawn from the ith draw (lb), and C_{pi} is the specific heat of the water of the ith draw Btu/lb°F. The modified daily water heating energy consumption is defined as

 Q_{dm}=\sum\limits_{i=1}^6 \frac{M_i C_{pi}\left( \overline{T}_{del,i}-\overline{T}_{in,i}\right)}{\eta_r},

where \overline{T}_{del,i} is the average delivery temperature of the ith draw (°F) as measured during the test, \overline{T}_{in,i} is the average inlet temperature of the ith draw (°F) as measured during the test, and \eta_r is the recovery efficiency of the water heater.

Energy conservation standards[edit]

Minimum federal energy conservation standards are defined by fuel, type, and rated storage volume. All standards are calculated as a function of the rated storage volume V in gallons. The current conservation standards are less efficient than the standards that go into effect in 2015.[4][6][7]

Energy Conservation Standards
Fuel Type Rated Storage Volume Energy Factor (Effective April 16, 2015) Energy Factor
Natural Gas Storage ≥ 20 gal and ≤ 55 gal 0.675 − 0.0015V 0.67 − 0.0019V
Natural Gas Storage > 55 gal and ≤ 100 gal 0.8012 − 0.00078V 0.67 − 0.0019V
Fuel Oil Storage ≤ 50 gal 0.68 − 0.0019V 0.59 − 0.0019V
Electric Storage ≥ 20 gal and ≤ 55 gal 0.960 − 0.0003V 0.97 − 0.00132V
Electric Storage > 55 gal and ≤ 120 gal 2.057 − 0.00113V 0.97 − 0.00132V
Tabletop Storage ≥ 20 gal and ≤ 100 gal 0.93 − 0.00132V 0.93 − 0.00132V
Natural Gas Instantaneous < 2 gal 0.82 − 0.0019V 0.62 − 0.0019V
Electric Instantaneous < 2 gal 0.93 − 0.00132V 0.93 − 0.00132V

Dishwashers[edit]

The energy factor for dishwashers is defined as "the number of cycles per kWh of input power."[1]

Clothes washers[edit]

The energy factor for clothes washers, is defined as "the cubic foot capacity per kWh of input power per cycle."[1]

Clothes dryers[edit]

The energy factor for clothes dryers is defined as "the number of pounds of clothes dried per kWh of power consumed."[1]

See also[edit]

References[edit]

  1. ^ a b c d "Federal Tax Credits for Consumer Energy Efficiency: Definitions". energystar.gov. U.S. Environmental Protection Agency. Retrieved 26 March 2013. 
  2. ^ "Statutory Authorities and Rules". eere.energy.gov. U.S. Department of Energy. Retrieved 26 March 2013. 
  3. ^ "Standards and Test Procedures". eere.energy.gov. U.S. Department of Energy. Retrieved 26 March 2013. 
  4. ^ a b "Residential Water Heaters". eere.energy.gov. U.S. Department of Energy. Retrieved 26 March 2013. 
  5. ^ "10 CFR Part 430 Energy Conservation Program for Consumer Products: Test Procedure for Water Heaters; Final Rule". Federal Register 63 (90): 25995–26016. 11 May 1998. Retrieved 26 March 2013. 
  6. ^ "10 CFR Part 430 Energy Conservation Program: Energy Conservation Standards for Residential Water Heaters, Direct Heating Equipment, and Pool Heaters; Final Rule". Federal Register 75 (73): 20112-21981 [20113]. 16 April 2010. Retrieved 26 March 2013. 
  7. ^ "10 CFR Part 430 Energy Conservation Program for Consumer Products: Energy Conservation Program for Consumer Products: Energy Conservation Standards for Water Heaters; Final Rule". Federal Register 66 (11): 4474–4497 [4497]. 17 January 2001. Retrieved 26 March 2013.