A water heater that’s too small leaves the last person with a cold shower. One that’s too big wastes energy keeping extra water hot. Sizing depends on how much hot water your household uses at its busiest time, not just on how many people live there.
Tank water heaters: look at the first-hour rating
Tank size in gallons is only part of the story. The more useful number is the first-hour rating (FHR): how many gallons of hot water the heater can deliver in one hour, starting with a full tank. You’ll find it on the EnergyGuide label; see how to read an EnergyGuide label.
Estimate your peak hour
Think about your busiest hour, often the morning, and add up the hot water used. Typical figures vary with fixtures and habits, but for a rough estimate:
- A shower: roughly 10–20 gallons, depending on length and showerhead.
- A dishwasher load: roughly 4–6 gallons.
- A hot-water laundry load: often 7 gallons or more.
Choose a model with a first-hour rating at or slightly above your peak-hour total.
Rough tank sizes by household
| Household | Common tank size |
|---|---|
| 1–2 people | 30–40 gallons |
| 2–4 people | 40–50 gallons |
| 4–5 people | 50–60 gallons |
| 5+ people | 60–80 gallons |
These are starting points. A large soaking tub or several back-to-back showers can push you up a size.
Tankless water heaters: size by flow and temperature rise
Tankless units heat water as it flows, so they’re sized by gallons per minute (GPM). Add up the flow of fixtures likely to run at the same time, then check the unit’s GPM at your required temperature rise, the difference between incoming water and your target temperature. Colder climates need a bigger rise, so the same unit delivers fewer GPM.
Other things to check
- Fuel and power: electric tankless units can need large dedicated circuits; gas units need proper venting and gas supply.
- Space: measure the height and diameter available.
- Efficiency: heat pump water heaters can greatly reduce electricity use where they fit.
Water heater installation involves gas, venting, electrical and pressure-relief safety devices, and usually requires a permit. Have it installed by a licensed plumber or contractor.
