how long does it take a water heater to heat water?
While this is not exact, it is typically based on heating 1 full tank from 45F to 120F. Most 40 and 50 gallon units require 40 minutes to 1 hour. Between 30 minutes and 1 hour. Depending on amount of water used before and age of heater. This depends on age and how many gallons it is heating. If it is new & just filled or you ran out of hot water, About an hour. 3kW electric on a 50 gal tank (US) = 10,236 BtuH on 400 lb water. Since 1 BtuH = 1 lb water/1 degree F/Hr then 10,236 BtuH will heat 400 lb by about 25 deg F. in one hour. If you fill the tank completely with cold water at 40 deg F and want it at 140 deg F, then it will take about 4 Hr. Same tank on natural gas at 50,000 BtuH will take about 45 min.