Calculating Water Heating Energy

In both Home Energy Saver and Home Energy Scoring Tool, the gallons of hot water used by the clothes washer is sent to the water heating model, which calculates the energy consumed by the water heater to supply this amount of hot water to the clothes washer.

Home Energy Saver

In HESaver professional and consumer versions the daily usage (gallons) attributable to the clothes washer is calculated according to Equation 1 (Koomey et al. 1994).

Useday = (loadweek * useload)/ 7                                         Equation 1


Useday = hot water use (gallons/day)

Loadweek = number of loads per week,

Useload = 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 12 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).

CWuse/yr = CWgal/cycle * CWcpy                                    Equation 2.

CWcpy = 164 + (45.6 * Nbed)                                          Equation 3.


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

Daily hot water use is passed to the hot water method, and is calculated as Annual clothes washer hot water use/365