Tank heaters
The number that matters is the first-hour rating on the label: how much hot water the heater can deliver in an hour starting full. Compare it with the busiest hour of a typical day — back-to-back showers, a dishwasher and laundry.
- One or two people often manage with a smaller tank
- Three or four people usually need a mid-size tank
- Five or more, or a large soaking tub, usually need a larger tank or high recovery
Tankless heaters
A tankless unit is sized in gallons per minute at a given temperature rise. Colder incoming water in winter means a bigger rise and less flow, so the same unit delivers less in a cold climate.
Why sizing matters
Too small means running out of hot water. Too large means paying to keep water hot that is never used. A plumber can size a heater from how the household actually uses water.