A gas water heater heats a full tank in 30 to 40 minutes while electric models take 60 to 80 minutes. Recovery time depends on tank size burner BTU rating or element wattage and incoming water temperature. Tankless heaters provide instant hot water with no recovery wait. First-hour delivery rating indicates how much hot water your heater produces per hour. In cold climates heating takes longer due to lower incoming water temperatures. Proper sizing prevents running out of hot water.