Abstract: A domestic water heater 10 includes a water meter 14 to count the units of water used each day and has a timer 17 to time the heating cycle each day. A programmable means 18 stores the water usage and heating data for a sixty day period and using statistical techniques determines the time period required for heating the water in the tank 10. The means 18 also stores power load curve data from the power generator and matches the required heating time to an appropriate portion of a low in the power load curve. The tanks may be placed in any one of 9 groups and a tank in each group will by virtue of the program in means 18 center the mid point of the calculated heating period on the same time during the off peak period. Tanks fitted with such controllers can be allocated into one of two or more categories to enable a power utility to reallocate the water heater power load into low cost periods of the power load curve.