A storage water heater has to hold enough hot water to get your household through its peak period— usually the morning rush — without running cold, then catch up between peaks. So size it by how many people use it and how they use it, not by the size of the house.
The per-person rule
Plan on roughly 13 gallons (50 L) of stored hot water per person per day for average use — showers, sinks and a kitchen. Light users (quick showers, no baths) get away with about 8 gallons (30 L) each; bath-heavy households or long showers push 18 gallons (70 L) or more per person.
| Household | Average use | Common tank |
|---|---|---|
| 1–2 people | ~25 gal (100 L) | 30 gal (120 L) |
| 3 people | ~40 gal (150 L) | 40 gal (150 L) |
| 4 people | ~50 gal (200 L) | 50 gal (200 L) |
| 5–6 people | ~70 gal (270 L) | 65–80 gal (250–300 L) |
Water Heater Size Calculator
Enter your household size and how you use hot water — it returns the tank size in gallons (or litres) and a typical element rating.
Why tank size matters even though it reheats
Recovery is slow. A 3 kW element heats a 40-gallon (150 L) tank by only about 30 °F (17 °C) per hour, so once the stored hot water is gone, you wait. The tank needs to cover the household’s peak demand in one go — the element is there to catch up between peaks, not to keep pace with three back-to-back showers.
What element size?
Most 25–40 gallon (100–150 L) tanks run a 2–3 kW element, and 50–65 gallon (200–250 L) tanks 3–4 kW (US electric heaters often use 4.5 kW). A bigger element shortens recovery but raises the instantaneous electrical load — check the circuit and breaker before upgrading one.
What about tankless (instant) heaters?
A tankless heater removes the storage question but must heat water as fast as you draw it: a decent shower needs roughly 24 kW of gas or electric instantaneous power. Electric units that size need serious supply capacity, which is why storage remains the default for whole-house electric hot water.
Can I save by going smaller?
A smaller tank trims standing losses, but undersizing costs comfort every day. The better savings come from the temperature and schedule: store at 120–140 °F (55–60 °C), fit a timer so it heats before peaks rather than all day, and jacket an older tank — or step up to a heat pump or solar water heater, which cut the energy per gallon by half or more.