Clothes washers consume energy for both mechanical activities and water heating energy. The majority of the energy used is for water heating. Both machine energy and water heating energy are directly dependent upon the number of loads washed.
Home Energy Saver
The energy consumption of clothes washers is calculated using the method described in the Energy Data Sourcebook (Wenzel et al. 1997). Equation 1 calculates the total clothes washer energy. The machine energy is the electrical energy consumed by all the physical processes necessary to run a load of laundry (e.g. agitation, spin cycle), and is calculated using Equation 2. When ME and WE are in different units (e.g. for non-electric water heaters) the energy consumption for the clothes washer is calculated and stored separately for both fuels (e.g. 126 kWh and 23 therms).
EC = ME + WE Equation 1
ME = LE x loads x 52 Equation 2
where
EC = Annual energy consumption in utility units
ME = Machine energy (kWh/year)
WE = Water heating energy attributable to clothes washer in utility units (returned from water heater module)
LE = load energy (kWh/load)
loads = clothes washer loads/week
52 is weeks/year
HES professional and consuser use the estimate of energyer per load from the DOE Technical Support Document of 0.23 kWh/load (DOE 1990, Page 3-22 table 3.17).
In HES profession and consumer the daily usage (gallons) attributable to the clothes washer is calculated according to Equation 3 (Koomey et al. 1994).
CWHWday = (loads * HWuseload)/ 7 Equation 3
where
CWHWday = clothes washer hot water use (gallons/day)
Loads = number of loads per week,
HWUseload = hot water use for the average load (gallons/load)
7 = days per week
Energy consumed by the water heater in providing the necessary gallons of hot water for the clothes washer is calculated by the water heating model and incorporated into Equation 1 to arrive at the total energy consumption for the clothes washer.
Scoring Tool
The scoring tool method is similar in outline to that of the Home Energy Saver, however, it does not take an input for washer loads, instead loads are estimated based on number of (occupied) bedrooms, using the Building America calculation method (Parker, D., P. Fairey, and R. Hendron, 2010). In addition rather than building up from daily use, water heating energy is calculated on an annual basis. Consistent with the asset rating approach the hot water use per cycle is invariant in Scoring Tool. The gallons per cycle estimate is also from Parker, D., P. Fairey, and R. Hendron (ibid), and represents the usage of a standard efficiency clothes washer 2007 or later.
CWuse/yr = CWgal/cycle * CWcpy Equation 4.
CWcpy = 164 + (45.6 * Nbed) Equation 5.
where:
CWuse/yr = Clothes washer annual water use (gallons per year)
CWgal/cycle = Clothes washer water water use (gallons per cycle) = 9.62 * 0.5 = 4.81
CWcpy = Clothes washer cycles per year
Nbed = Number of bedrooms in home