Tank water heaters: size by first hour rating, not gallons
A storage water heater's tank size (40, 50, 80 gallons) is not the number that matters most. What tells you whether it keeps up in your busiest hour is the first hour rating (FHR): the gallons of hot water it can deliver in one hour starting from a full, hot tank. It depends on the tank, the fuel and the size of the burner or element, and it is printed on the yellow EnergyGuide label as "Capacity (first hour rating)". The US Department of Energy recommends choosing a model whose FHR is at least your peak hour demand.
Peak hour demand = Σ (gallons per use × uses in your busiest hour)
Choose a tank heater with first hour rating ≥ peak hour demand
DOE hot water use per activity
| Use | Average gallons of hot water per use |
|---|---|
| Shower | 20 |
| Shaving (0.05 gallons per minute) | 2 |
| Hand dishwashing or food prep (2 gallons per minute) | 3 |
| Automatic dishwasher | 7 |
| Clothes washer, top-loader | 25 |
| Clothes washer, front-loader (H-axis) | 15 |
The worksheet estimates your single busiest hour, not total daily use. Think about the morning rush or the evening routine, whichever uses more hot water.
Worked example: a family's morning rush
- Two showers: 2 × 20 = 40 gallons.
- One shave: 2 gallons. One round of hand dishwashing: 3 gallons. One dishwasher load: 7 gallons.
- Peak hour demand = 40 + 2 + 3 + 7 = 52 gallons, so look for a first hour rating of at least 52 gallons on the EnergyGuide label.
Add a top-loading washer load in the same hour and demand climbs to 77 gallons, which is why many households simply move laundry to a quieter time of day.
Tankless water heaters: size by flow and temperature rise
A tankless (demand) water heater has no stored hot water, so it must heat everything that flows through it as it is used. It is rated by the temperature rise it can deliver at a given flow rate in gallons per minute (gpm). To size one, add up the flow of the fixtures you expect to run at the same time, then find the rise between your incoming water and the temperature you want.
Flow (gpm) = sum of fixtures used at once · Temperature rise = hot setting − incoming water
Heat output (BTU/h) ≈ gpm × rise (°F) × 500 · Electric kW = BTU/h ÷ 3,412
The 500 comes from water weighing about 8.34 lb per gallon × 60 minutes per hour; one BTU heats one pound of water by 1 °F. Gas models list their input rating, which is higher than the output by the heater's efficiency.
Worked example: a shower and a kitchen faucet
- DOE's example: one 2.5 gpm shower head and one 0.75 gpm faucet at once = 3.25 gpm.
- Incoming water 50 °F, hot water set to 120 °F: rise = 70 °F.
- Heat output ≈ 3.25 × 70 × 500 ≈ 114,000 BTU/h, or about 33 kW for an electric model, roughly 139 amps at 240 V.
DOE notes that a 70 °F rise is typically possible at about 5 gpm through gas-fired tankless heaters but only about 2 gpm through electric ones. That is why whole-house electric tankless heaters need very large electrical services, and why gas or a tank is more common for families.
Things that change the answer
- Incoming water temperature. Groundwater in northern states can be well below 50 °F in winter, which raises the rise a tankless heater must deliver.
- Low-flow fixtures. A 1.8 gpm shower head instead of 2.5 gpm cuts both peak flow and hot water use.
- Dishwashers without internal heaters may need 140 °F water, a 90 °F rise from 50 °F.
- Heat pump water heaters recover slowly in heat-pump-only mode; compare their first hour rating, not just tank size.
Common mistakes
- Buying by tank gallons alone. Two 50-gallon heaters can have very different first hour ratings.
- Sizing for daily use instead of the busiest hour.
- Forgetting winter when sizing a tankless heater: size for the coldest incoming water you will see.
- Ignoring the electrical service. Electric tankless units often need multiple large breakers; check with an electrician first.
Frequently asked questions
What size water heater do I need for a family of 4?
Size by the busiest hour, not the number of people. If that hour includes three showers, a shave and some dishwashing, demand is 3 × 20 + 2 + 3 = 65 gallons, so look for a tank with a first hour rating of 65 gallons or more.
What is a first hour rating?
It is the number of gallons of hot water a storage water heater can supply in one hour, starting with a full tank of hot water. It is printed on the EnergyGuide label.
How many gpm tankless water heater do I need?
Add up the fixtures you run at once. One 2.5 gpm shower and one 0.75 gpm faucet need 3.25 gpm at the temperature rise you need, often around 70 °F.
How many kW is an electric tankless water heater for a whole house?
Delivering 3.25 gpm with a 70 °F rise takes about 33 kW. Electric models typically manage around 2 gpm at a 70 °F rise, so whole-house electric units need very large electrical services.
What temperature should incoming water be assumed?
DOE suggests assuming 50 °F unless you know otherwise; in colder regions winter groundwater can be lower.
Sources
- US Department of Energy, Energy Saver: "Sizing a New Water Heater" (worksheet for estimating peak hour demand / first hour rating; tankless sizing by flow rate and temperature rise).
- Federal Trade Commission, EnergyGuide label for water heaters (Capacity / first hour rating).
- Physical constants: water ≈ 8.34 lb per US gallon; 1 kW = 3,412 BTU/h (NIST SP 811).
Last reviewed 31 October 2026. Results are estimates; see our disclaimer.