Water heaters running on 120-volt power have a long recovery time, which is impractical for most households; 120-volt elements consume 1125 watts, while residential 240-volt heaters use 4,500 watts. If a heater has two elements, people sometimes assume that they should add their wattage to get the total power consumption of the heater. This isn't correct, because both elements don't come on at the same time. A heater with two elements and a label that specifies a power usage of 4,500 watts for each element uses a maximum of 4,500 -- not 9,000 -- watts.