Size a storage water heater from your peak-hour demand with the DOE/AHRI worksheet method, check the quick family-size rule, size a tankless unit by GPM and temperature rise, compare gas vs electric recovery, and estimate annual operating cost — in US or metric units.
Enter how many times each fixture is used during the busiest hour of the day. Gallons per use come from the AHRI worksheet reproduced by the U.S. DOE.
Rule-of-thumb bands (CenterPoint Energy / Blue Flame guide). Verify with the peak-hour tab for accuracy.
Enter fixtures that may run at the same time. Required heat: BTU/hr = GPM × 500 × ΔT.
Recovery at the standard 90°F rise. Gas: 40,000 BTU/hr @ 78% · Electric: 4,500 W @ 98%.
DOE Energy Saver formulas — gas: 365 × 0.4105 ÷ EF × price/therm · electric: 365 × 12.03 ÷ EF × price/kWh (based on 64.3 gal/day, 58°F inlet, 135°F).
The number that matters is not the tank's gallon size but its first-hour rating: how many gallons of hot water it can deliver during the busiest hour.
Example: a 40-gallon gas water heater holds about 28 usable gallons (0.7 × 40) and reheats about 41.6 GPH, for an FHR of about 69.6 gallons. DOE guidance: choose a heater whose FHR is at or just above your peak-hour demand.
Sources: AHRI Water Heater Calculator worksheet (ahrinet.org); U.S. DOE Energy Saver — “Sizing a New Water Heater” and “Estimating Costs and Efficiency of Storage, Demand, and Heat Pump Water Heaters” (2023); CenterPoint Energy residential water-heating sizing guide.
The U.S. Department of Energy (DOE) method sizes a storage water heater by the busiest hour of your day, not by tank gallons. An AHRI/DOE worksheet assigns hot-water gallons per use: shower 20, bath 20, shaving 2, hands and face washing 4, hair shampoo 4, hand dishwashing 4, automatic dishwasher 14, food preparation 5, and automatic clothes washer 32. Add up everything used during the peak hour. Example: 3 morning showers (60) + 1 shave (2) + 1 shampoo (4) + 1 hand dishwashing (4) = 70 gallons of peak-hour demand. You then choose a water heater whose first-hour rating (FHR) is at least 70 gallons.
First-hour rating is the gallons of hot water a heater can deliver during its busiest hour: usable stored water (about 70% of the tank, since cold water mixes in at the bottom) plus what it reheats in that hour. Example: a 40-gallon gas unit stores about 28 usable gallons and recovers about 41.6 gallons per hour (40,000 BTU/hr x 78% efficiency / (8.33 x 90°F rise)), giving an FHR of about 69.6 gallons. So the FHR — not the tank size — is the number that must cover your peak-hour demand; always compare FHR to your calculated demand.
Work the peak hour: a typical family-of-4 morning might be 2 showers (40 gallons) + 2 shaves (4) + 1 clothes-washer load (32) = 76 gallons of peak-hour demand. A 50-gallon gas water heater has an estimated FHR of about 76.6 gallons (0.7 x 50 + 41.6 recovery), which just covers it — so 50 gallons gas is the right call here, while a 50-gallon electric (FHR about 55) would run short. As a quick rule of thumb, 3–4 people need 40 gallons for regular usage or 50–75 gallons for high usage (long showers, frequent laundry).
Add the hot-water flow of every fixture that may run at once: shower 2.1 GPM, bathtub faucet 4.0, bathroom sink 0.5, kitchen sink 1.5, dishwasher 1.5, clothes washer 2.0 GPM. Example: 2 simultaneous showers (4.2) + dishwasher (1.5) = 5.7 GPM. Required heating power is BTU/hr = GPM x 500 x temperature rise. With 58°F incoming water and a 120°F setpoint, the rise is 62°F, so 5.7 x 500 x 62 = 176,700 BTU/hr — a 180,000 BTU unit fits. In a cold climate (40°F inlet, 80°F rise) the same demand needs 228,000 BTU/hr, which exceeds the largest common residential unit (199,000 BTU/hr), so two units in parallel are required.
Gas, by roughly 2 to 1. Recovery rate is GPH = burner input x efficiency / (8.33 x temperature rise). At a standard 90°F rise: a 40,000 BTU/hr gas burner at 78% efficiency recovers about 41.6 gallons per hour, while a 4,500-watt electric element (15,355 BTU/hr) at 98% efficiency recovers about 20.1 gallons per hour. That is why a 40-gallon gas tank (FHR about 69.6) can serve a peak demand that would require an 80-gallon electric tank (FHR about 76.1). Electric and heat-pump units compensate with lower standby losses and cheaper maintenance, not speed.
The DOE formulas are: gas/oil, 365 x 0.4105 therms/day / EF x fuel price per therm; electric (including heat pump), 365 x 12.03 kWh/day / EF x price per kWh — based on 64.3 gallons/day, 58°F inlet, 135°F setpoint. Worked examples: a gas unit at EF 0.58 with gas at $1.09/therm costs about $282/year; a standard electric at EF 0.92 with electricity at $0.17/kWh costs about $811/year; a heat-pump water heater at EF 4.0 at $0.1301/kWh costs about $143/year. This calculator lets you plug in your own energy factor and local prices.
Yes — switch to metric and every result converts: 70 gallons is 265.0 liters, 120°F is 48.9°C, and 199,000 BTU/hr is 58.3 kW. The sizing math is identical; only the units change. For the UK and Australia, enter your local prices (the calculator accepts any currency symbol). Note that UK gas is billed per kWh rather than per therm: 1 therm = 29.31 kWh, so $1.40/therm equals about $0.0478/kWh — use the per-kWh gas price option.
The DOE recommends 120°F (48.9°C): hot enough for showers, dishwashers and laundry, while limiting scald risk and standby energy loss. Every 10°F you lower the setpoint saves roughly 3–5% on water-heating energy. Avoid going below 120°F for stored hot water and avoid 140°F+ at the tap — water at 140°F can cause third-degree burns in seconds, especially for children and older adults.
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