Size a tank or tankless water heater from peak household demand.
Tankless heaters are sized on instantaneous power for the peak simultaneous flow, while tanks are sized on stored volume plus recovery. Temperature rise is the dominant variable, so a cold winter inlet can halve tankless output. A tankless unit adequate in summer can fail to reach temperature in winter, because the same power produces a smaller rise as the inlet cools.
Water Heater Sizing
Power in kW = flow (L/min) × ΔT × 4.186 ÷ 60
Power in kW = flow (L/min) × ΔT × 4.186 ÷ 60 Tankless heaters are sized on instantaneous power for the peak simultaneous flow, while tanks are sized on stored volume plus recovery. Temperature rise is the dominant variable, so a cold winter inlet can halve tankless output.
A tankless unit adequate in summer can fail to reach temperature in winter, because the same power produces a smaller rise as the inlet cools.
This calculator takes 5 inputs: Occupants, Fixtures used at once, Flow per fixture, Cold inlet temperature, Delivered hot temperature. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.