The thermal performance of a single-tank electric water heater is compared with that of a dual-tank electric water heater for the same average domestic hourly hot-water use pattern. Both water heaters have the same total power rating and volume capacity. However, the power rating and volume capacity of each tank in a dual-tank water heater are varied to find the best combination in terms of total hot-water output and energy saving. It is found that using two tanks connected in series provides about 10% move hot water at the required temperature while requiring about 4.5% less energy when the second tank has 10-30% of the total volume and 70-80% of the total power rating.