Water Heater (Geyser) Size Calculator

Plan on roughly 13 gallons (50 L) of stored hot water per person per day for average use — showers, sinks and a kitchen. A four-person household lands at about 50 gallons (200 L), the most common tank size. Light users (quick showers, no baths) get away with about 8 gallons (30 L) per person; bath-heavy households or long showers push 18 gallons (70 L) or more per person.

Recovery is slow: a 3 kW element heats a 40-gallon (150 L) tank by about 30 °F (17 °C) per hour, so once the stored hot water is gone you wait. The tank should hold enough for the household’s peak period — usually the morning rush — without running cold, with the element catching up between peaks.

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.

Instant heaters remove the storage question but must heat water as fast as you use it: a decent shower needs roughly 24 kW of gas or electric instantaneous power. Electric instant units that size need serious supply capacity, which is why storage remains the default for whole-house electric hot water.

A smaller tank saves standing losses, but undersizing costs comfort every day. The better savings come from the temperature and schedule: 120–140 °F (55–60 °C) storage, a timer so it heats before peaks rather than all day, an insulating jacket on older tanks — or stepping up to a heat pump or solar water heater, which cut the energy per gallon by half or more.