Water Heater Recovery Rate Calculator

It is how many gallons the heater can raise to your set point in one hour of continuous firing. A higher input, a higher efficiency, or a smaller temperature rise all push the number up, which is why the same heater recovers faster in summer than in winter.

Recovery in gallons per hour equals the BTU input times efficiency, divided by 8.33 times the temperature rise. The 8.33 is the pounds in one US gallon of water, and one BTU raises one pound by one degree F, so 8.33 BTU covers a one degree rise for a gallon.

The menu offers three common values: Electric resistance at 0.98, high efficiency condensing gas at 0.95, and standard atmospheric gas at 0.80. Use the tag on your unit or its spec sheet if you have it, otherwise these are reasonable defaults for each fuel type.

Multiply watts by 3.412. A 4,500 watt element is about 15,357 BTU per hour, and a 5,500 watt element is about 18,767 BTU per hour. Enter that BTU figure and choose the Electric (0.98) efficiency.

Recovery is inversely proportional to the rise, so doubling the rise roughly halves the gallons per hour. Cold winter inlet water needs a bigger rise to reach the same set point, which is the main reason a heater feels slower in the cold months.

Yes. Use the F or C toggle above the inputs. The rise is a temperature difference, not an absolute reading, so the tool scales it by 9/5 with no 32 degree offset when you switch units.

It is the minutes to raise 100 gallons through your chosen rise at the calculated recovery rate, using 100 divided by the gallons per hour, times 60. It is a handy stand in for how long a fully drained tank of that size takes to come back to temperature.

No. Recovery rate is only the reheating capacity. A first hour rating adds the tank of hot water already on hand at the start, so it is larger. This tool models the steady recovery physics and does not include standby losses, mixing, or draw down, so treat the number as a clean theoretical estimate.

Yes. The calculation runs entirely in your browser using client side JavaScript. Nothing you type is uploaded or stored on a server, so you can use it offline once the page has loaded.