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.
First hour rating, explainedFHR is the tested hot gallons a tank delivers in 60 minutes.
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.