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Size your daily hot water energy

Knowing how much energy your hot water needs each day lets you size collectors, tanks and backup. The physics is simple: energy equals litres x temperature rise x the specific heat of water.

How

1. Estimate litres of hot water per day from your water demand plan (showers, kitchen, laundry).
2. Note your cold inlet temperature (lowest in winter) and storage temperature.
3. Use the hot water calculator: kWh/day = litres x deltaT x 4.186 / 3600.
4. Use the same calculator for a first estimate of solar collector area and tank volume.
5. For a storage tank, the US DOE sizes by first hour rating: pick one at least equal to your busiest hour's hot water use.
6. For a tankless heater, DOE sizes by the total flow of outlets used at once and the temperature rise (outlet minus inlet).
7. Store hot water hot enough to control Legionella, as set out in thermal-23.
8. Cut demand first: low-flow showerheads and short pipe runs reduce both water and energy.

Do these first

• water-1

Sources

1. Solar Water Heaters (course R04-016). CED Engineering, accessed 2026-11
2. Sizing a new water heater. US Department of Energy, Energy Saver, accessed 2026-11
3. Managing legionella in hot and cold water systems. UK Health and Safety Executive, accessed 2026-11

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