This water heater sizing guide uses only what ENERGY STAR publishes, checked on October 13, 2026. The short version: start from the tank you have now, and size a tankless unit by how much hot water you use at once. ENERGY STAR publishes no chart of tank size by household size, so this page has none.
How Do I Figure Out What Size Water Heater I Need?
Start with the gallon capacity of the storage water heater you have now. As of October 13, 2026, that is ENERGY STAR's advice, and the capacity is printed on the nameplate or the yellow EnergyGuide sticker.
ENERGY STAR gives that starting point on its heat pump water heater page. It then names the reasons to go larger:
- More people live in the home, or soon will.
- A new shower, faucet, dishwasher or washer will be in frequent use.
A contractor or a retail salesperson can help decide whether a larger model is needed, the same page says.
How Are Tankless Water Heaters Sized?
By flow, not by tank volume. Tankless models are rated in gallons of hot water per minute, and running more fixtures at once calls for a higher rating.
ENERGY STAR's tankless gas water heater page lists these average flows:
| Fixture | Average flow, gallons per minute |
|---|---|
| Shower and bathtub | 2.5 |
| Clothes washer | 3.3 |
| Kitchen and bathroom sink | 2.2 |
| Dishwasher | 1.3 |
Add up the fixtures likely to run at the same moment. ENERGY STAR says an experienced plumber should estimate the home's hot water demand, and the installer should confirm the right size.
What Size Heat Pump Water Heater Should I Get?
Heat pump water heaters typically come in 50-, 65- and 80-gallon tanks. ENERGY STAR suggests the same size as an electric tank that already meets your needs.
A heat pump water heater is the electric type that earns the ENERGY STAR label, and it needs access to air to run well. ENERGY STAR's water heater upgrade guide gives the sizing advice:
- Replacing an electric tank that meets your needs: consider the same size.
- Switching from a gas tank, or the family's needs have grown: move up to a larger tank.
- No room for a larger tank: consider a split system, which puts a small unit outside the house.
How Much Space Does a Heat Pump Water Heater Need?
ENERGY STAR gives two figures. Its product page says 1,000 cubic feet of air space. Its upgrade guide says most models need about 700.
The first figure is on the product page and the second in the upgrade guide. The upgrade guide describes 700 cubic feet as about a 10 ft by 10 ft room with a 7 ft ceiling, and says some models work with as little as 450 cubic feet. For a tight closet, it says a full louvered door solves the problem.
The two pages also describe the temperature of the space differently:
| ENERGY STAR page | What it says about the space |
|---|---|
| Product page | Interior spaces that stay between 40° and 90° F all year |
| Upgrade guide | Spaces that don't regularly get colder than 38° F |
The installer sets the final requirement from the manufacturer's specifications, according to the upgrade guide.
What Is a First Hour Rating?
It estimates how many gallons of hot water a storage water heater can supply in one hour, starting with a fully heated tank.
The definition is on ENERGY STAR's key product criteria page. The same page sets a minimum first hour rating for certified models:
| Certified water heater type | Minimum to earn the label |
|---|---|
| Heat pump water heater | First hour rating of 45 gallons per hour |
| Gas storage water heater | First hour rating of 51 gallons per hour |
| Gas tankless water heater | 2.8 gallons per minute over a 67° rise |
The first hour rating applies to storage water heaters. A tankless unit's gallons-per-minute figure is the hot water it can supply while raising the water temperature by a set amount.
Frequently Asked Questions
Is a 50-gallon water heater enough for 4 people?
ENERGY STAR publishes no rule by number of people. Its advice is to start from the size of the current tank and go larger if the household or its hot water use has grown.
Where do I find the size of my current water heater?
On its nameplate or on the yellow EnergyGuide sticker, according to ENERGY STAR.
Are there rebates for a new water heater in California?
They depend on the utility and the address.
The guide to heat pump water heater rebates in California covers the programs DuloCore has verified.
The eligibility check shows the programs for a specific address.
Replacing a water heater? See the verified rebates for your address with the free rebate calculator.