I20 and 240 volts efficientcy
When the voltages doubles from 120 volts to 240 volts the aperage is cut in half and it is the amperage that determines the cost of electricity. But the work done also doubles so the cost would be the same. However 240 does a better job because of the recovery time it takes to re-heat the water replaced as it is used. A 240 duel element heater usually has an element at the upper that is 4000 watts and the lower will usually be 4500 watts. The upper element is hardly ever used until you have used all the hot water in the lower half of the heater. And only one element works at a time, both elements are never on at the same time. So you may consider the cost of wiring the 240 volt heater in making your decesion.