Tank gallons and first-hour rating
The Department of Energy Energy Saver guide sizes a storage water heater, including a heat-pump water heater with a tank, by first-hour rating, not by headcount alone. First-hour rating is the gallons of hot water the heater can supply in one hour, starting with a full tank of hot water. It depends on tank capacity and on the burner or element.
On a conventional storage heater, the EnergyGuide label lists that number in the top left as “Capacity (first hour rating).” ENERGY STAR defines first-hour rating the same way: the maximum gallons supplied in an hour that begins with the heater fully heated, measured at a 125°F outlet in the Uniform Energy Factor test. Shop for a model whose first-hour rating is within 1 or 2 gallons of your peak hour demand — the busiest single hour, not the whole day.
Department of Energy worksheet
Add up what the household uses in that one hour. Energy Saver publishes these averages per use:
| Use | Average gallons of hot water |
|---|---|
| Shower | 10 |
| Shaving | 2 |
| Hand dishwashing or food prep | 4 |
| Automatic dishwasher | 6 |
| Clothes washer | 7 |
If a shower in your house runs longer than that 10-gallon average, replace the 10 with your own count before you add. The worksheet is an estimate of one hour, not a promise that every home uses these amounts.
Household examples, using only those averages
These rows are examples of the worksheet. They are not a rule that a given household must buy a certain tank.
| Household example | Busiest hour | Peak hour demand | First-hour rating to look for |
|---|---|---|---|
| 1–2 people | 1 shower (10) + hand dishwashing (4) | 14 gallons | 12–16 gallons |
| 3–4 people | DOE example: 3 showers (30) + 1 shave (2) + hand dishwashing (4) | 36 gallons | 34–38 gallons |
| 5 or more people | 5 showers (50) + dishwasher (6) + clothes washer (7) | 63 gallons | 61–65 gallons |
The 3–4 person row is the worked example in the Energy Saver guide: peak hour demand of 36 gallons, so a first-hour rating of 34 to 38 gallons.
Where tank gallons fit
Tank gallons are the stored volume. First-hour rating is what you match to the busy hour. They are different numbers on the same product. ENERGY STAR’s explanation of uniform energy factor uses this illustration: a 40-gallon tank with a very high-powered heating element might produce 80 gallons in the first hour, while a lower-powered heater might need an 80-gallon tank to deliver the same 80 gallons. After the first hour, delivery drops until the tank reheats.
Read both numbers. Use first-hour rating to cover the busy hour. Use tank gallons to see how much water is stored and how much floor space the unit needs. A licensed plumber should confirm the model against the connections already in the house.
Tankless gallons per minute and temperature rise
ENERGY STAR sizes a whole-home gas tankless heater by flow, in gallons per minute, not by stored gallons. Add the fixtures that are likely to run together. ENERGY STAR publishes these average figures:
| Fixture | Average flow |
|---|---|
| Shower or bathtub | 2.5 GPM |
| Clothes washer | 3.3 GPM |
| Kitchen or bathroom sink | 2.2 GPM |
| Dishwasher | 1.3 GPM |
One shower and a kitchen sink at the same time is 2.5 + 2.2 = 4.7 GPM. A shower, a dishwasher, and a clothes washer is 2.5 + 1.3 + 3.3 = 7.1 GPM. That sum is the flow to compare with the unit’s listed gallons per minute.
Temperature rise is the hot-water setpoint minus the incoming water temperature. ENERGY STAR measures tankless gallons per minute over a 67°F rise, and its certification criteria for gas tankless models include a maximum flow of at least 2.8 GPM at that 67°F rise. The 2.8 GPM figure is a certification floor, not the flow a busy house needs.
If the rise your home needs is larger than the rise printed on the spec sheet, the heater delivers fewer gallons per minute at the temperature you want. Incoming temperature changes with the season and the water source, so use the rise on the model’s own sheet rather than a single statewide number. As a plain arithmetic example only: a 120°F setpoint minus a 60°F inlet is a 60°F rise. A colder inlet makes the rise larger.
What this page does not tell you to do
For a replacement already planned, start with the water heater replacement page or the homeowner guide. The equipment guide compares tank and tankless once the first-hour rating or the gallons-per-minute number is known.
Sizing questions
Is tank size the same as first-hour rating?
No. Tank gallons are stored volume. First-hour rating is hot water supplied in the hour that starts with a full hot tank. A smaller tank can have a high first-hour rating when the burner or element recovers quickly. ENERGY STAR’s 40-gallon versus 80-gallon illustration is the one to keep in mind.
How do I size a tank for my household?
Add the Energy Saver averages for the uses in your busiest hour, then look for a first-hour rating within 1 or 2 gallons of that total. Read tank gallons on the same label. The household table above is three examples of that worksheet, including the Department of Energy’s own 36-gallon example.
How do I size a tankless unit?
Add the ENERGY STAR flows for fixtures that run together, then compare that total with the unit’s gallons per minute at your temperature rise. Temperature rise is setpoint minus incoming water temperature. The rating method uses a 67°F rise.
Who checks the gas line, vent, and circuit?
A licensed professional. ENERGY STAR tells shoppers to have an installer confirm gas supply, venting, and electricity for forced-draft models. We'll connect you with a licensed plumber fast. Call or text (918) 417-8981.quote visit.
Sources
- U.S. Department of Energy, Energy Saver, “Sizing a New Water Heater.” First-hour rating, the EnergyGuide “Capacity (first hour rating)” label, the within-1-or-2-gallons rule, the per-use averages, and the 36-gallon worked example.
- ENERGY STAR, “What is Uniform Energy Factor and Why Does It Matter?” First-hour delivery definition and the 40-gallon / 80-gallon illustration. ENERGY STAR UEF explanation.
- ENERGY STAR, residential water heater key product criteria. First-hour rating measured at a 125°F outlet, and tankless gallons per minute over a 67°F rise, including the 2.8 GPM certification minimum. Key product criteria.
- ENERGY STAR, whole-home tankless gas water heaters. Fixture flows of 2.5, 3.3, 2.2, and 1.3 GPM, and the note to have an installer confirm gas, venting, and electricity. Tankless buying notes.