Water Heater Size Calculator

Two ways to size a water heater

A storage tank and a tankless heater are sized by different questions. A tank holds hot water in reserve, so the question is how much hot water your home uses in its busiest hour. A tankless heater makes hot water on demand, so the question is how much flow it must heat at once, and by how many degrees. The calculator has a tab for each.

Both tabs print their working with your numbers so you can check it by hand. Metric mode takes litres and °C and still shows the gallons and °F that the maker labels use.

Storage tank: first hour rating

The DOE method adds up the hot water for every use in the busiest hour. The calculator lists showers, shaves, hand dishwashing or food prep, dishwasher loads and top-loading and front-loading washer loads, plus a box for other gallons. Each use has a gallons-per-use figure from the DOE worksheet, and you can edit it.

Those figures sit at the top of the sourced range. DOE counts 20 gallons a shower, while Ferguson uses 12 and Sobieski 10. If your showers are short, type your own number and the answer drops.

The result is a first hour rating (FHR), rounded up to the next gallon. FHR is printed on the EnergyGuide label, and it is how many gallons a full tank delivers in its first hour. Look for a heater whose FHR is at or above the answer. If you already know your heater’s FHR, type it in and the lamp turns green when it covers the demand and red when it does not.

Tankless: flow and temperature rise

Add the flow of every fixture that can run together. A standard shower is 2.5 GPM (EPA WaterSense labels showerheads at 2.0 GPM or less), a faucet is about 1.0, a dishwasher 1.5 and a clothes washer 2.5. Pick a fixture and its preset fills in, or choose Other and type the flow from the label.

The rise is the hot water temperature minus the cold water coming in. DOE suggests assuming 50 °F unless you know otherwise, or holding a thermometer under a cold tap. The heat needed is 500 times the GPM times the rise in °F, in BTU/h. The 500 is the weight of a gallon of water times 60 minutes, and HVAC School and MEP Academy both print it.

For an electric unit the calculator divides by 3,412.14 to get kilowatts and then works out the current from the unrounded kW. Size results round up: BTU/h to the next 1,000, kW to 0.1 and amps to the next whole amp.

Electric units are big

A whole-house electric tankless unit draws a lot. The largest we found are 36 kW at 240 V, which is 150 A. Both EcoSmart and Stiebel Eltron split that current over several breakers and ask for a service of 200 to 300 A, so the calculator does not give a single breaker or wire size. The unit’s manual does.

If your need is above 36 kW, the lamp turns red with a caution. Gas or two electric units may suit better. At 208 V a heater rated at 240 V delivers about 75% of its power, so compare against the 208 V rating on the label.

What it leaves out

This is an estimate. It has no heat pump water heaters, gas input or efficiency, recirculation, solar, gas line or venting, and it does not price running costs. For gas, the answer is a flow and a rise to look up on the maker’s own flow chart.

DOE also advises keeping the hot water setting at 120 °F for most uses, because a higher setting raises the risk of scalding. Your unit’s sizing chart and manual, your code edition and your local inspector decide the final choice.

Worked example

1. Inputs: 3 showers, 1 shave, 1 hand dishwashing, 1 dishwasher load and 1 front-load wash in the busiest hour.
2. Showers = 3 × 20 gal = 60 gal
3. Shaves = 1 × 2 gal = 2 gal
4. Hand dishwashing or food prep = 1 × 3 gal = 3 gal
5. Dishwasher loads = 1 × 7 gal = 7 gal
6. Front-loading washer loads = 1 × 15 gal = 15 gal
7. Busiest hour = 60 + 2 + 3 + 7 + 15 = 87 gal
8. First hour rating needed = 87 gal rounded up = 87 gal

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