How it works: storage tank
The U.S. Department of Energy recommends estimating your household's peak-hour demand (the busiest hour of hot-water use) and choosing a heater whose first-hour rating (FHR) is within 1–2 gallons of it. FHR is on the EnergyGuide label; it is not the same as tank size.
| Use (gallons of hot water) | DOE worksheet | AHRI worksheet* |
|---|---|---|
| Shower | 10 | 20 |
| Bath | – | 20 |
| Shaving | 2 | 2 |
| Hand dishwashing / food prep | 4 | 4 |
| Automatic dishwasher | 6 | 14 |
| Clothes washer | 7 | 32 |
*AHRI's worksheet assumes no water-conservation fixtures, so it gives larger numbers. The DOE values reflect modern low-flow fixtures and efficient appliances.
How it works: tankless
Add up the flow of every fixture you might run at the same time, then work out the temperature rise:
Required flow (gpm) = Σ simultaneous fixture flows
Temperature rise (°F) = desired output − incoming water temperature
Heat output (Btu/h) = gpm × 60 × 8.34 × rise
kW = Btu/h ÷ 3,412
8.34 lb is the weight of a gallon of water and 1 Btu raises 1 lb of water by 1°F, so the formula is just the heat needed per hour. 3,412 Btu = 1 kWh. Federal maximums are 2.5 gpm for showerheads and 2.2 gpm for bathroom faucets; WaterSense-labeled models are 2.0 and 1.5 gpm or less.
Worked examples
Tank (DOE): three showers (3 × 10 = 30), one shave (2) and hand dishwashing (4) in the same morning hour = 36 gallons peak-hour demand, so look for an FHR of about 34–38 gallons. The same hour on the AHRI worksheet is 66 gallons.
Tankless (DOE example): one 2.5 gpm shower plus one 0.75 gpm faucet = 3.25 gpm. With 50°F incoming water and 120°F output, the rise is 70°F. Heat output = 3.25 × 60 × 8.34 × 70 ≈ 113,800 Btu/h ≈ 33.4 kW. As a cross-check, ACEEE notes it takes about 11 kW per gpm at a 77°F rise, and 11 × 3.25 × 70 ÷ 77 ≈ 32.5 kW.
Assumptions and limits
- Incoming water is colder in winter and in northern states; size tankless units for your coldest groundwater.
- DOE notes that tankless heaters typically deliver about 2–5 gpm, with gas models giving higher flow than electric, so large simultaneous demands may need more than one unit.
- The Btu/h result is heat delivered to the water. A gas heater's input rating is higher because of combustion losses. An electric unit's kW draws large current, so check your panel capacity.