Water heater sizing guides
The reasoning behind the calculators, one number at a time. Each guide is short, carries its arithmetic in the open, and links back to the tool that uses it.
Running out of hot waterRunning out of hot water is either a tank too small for the hour or a fault inside one that used to be fine.
First hour rating calculatorApproximate first hour rating from tank size and fuel, then match it to your peak hour.
Gas vs electric recoveryA 40,000 BTU gas burner reheats about 43 gallons an hour; a 4,500 W element about 20.
Temperature rise and inlet waterWinter inlet water runs 42°F up north to 72°F on the Gulf.
Tank vs tankless sizingTanks are sized to the busiest hour in gallons; tankless to the worst instant in GPM.
Heat pump water heater sizingA heat pump water heater recovers about 8-15 gallons an hour in efficiency mode, a third of a gas burner.