The value of Wd for the above equations should be the average water draw for the period under consideration (T). As shown in Table 2, for Thigh-Tlow=2Δ and with Δ = 2.5 °F, ton is always smaller than 1h what allows the use of the hourly ASHRAE Wd schedule for validating Eq. (4). By comparing the values of ton obtained with eq. (4) with those of the simulations one sees that the error were smaller than 0.01%. On the other hand, toff varies more with Wd and can be larger than 1 h, usually for low values of Wd. In this case, an average value for Wd valid for that duration needs to be considered.
The values of D obtained from (6), which are equivalent to PEWH in pu, are shown in Table 3 for different values of Wd and Td. There one sees that PEWH at Td = 140 °F is around twice that at Td = 100 °F for all values of Wd. Besides, operation at low values of Wd, limits significantly the variation of PEWH one can get by varying Td. Thus, in these cases, the EWH will be less effective as a means for balancing active power in the electric system.