Question: how much electricity does it take to heat a gallon of water?
Answer: A typical 50-gallon electric tank runs at 4500 watts. From above, heating a gallon of water by 1°F takes 8.33 BTU. Heating from 68°F to 104°F would by a 36°F rise, or 36 x 8.33 = 300 BTU to heat 1 gallon of water.