Size a solar hot-water collector and storage tank from daily demand, water temperatures and solar insolation.
About this tool
Solar Water Heater Sizing Calculator is a free, in-browser tool that works out the collector area and storage tank a solar hot-water system needs to meet a household's daily demand. Enter occupants or a daily volume, the cold and hot water temperatures and your local solar insolation, and it sizes the system locally.
The daily energy to heat the water comes from Q = m · c · ΔT, where mass is the water volume (1 litre ≈ 1 kg), c is 4.186 kJ/kg·°C (1 BTU/lb·°F) and ΔT is the rise from the incoming cold temperature to the target hot temperature. That energy divided by the daily solar insolation (kWh/m²/day) and the collector efficiency gives the required collector area, shown in both m² and ft². The tool works in metric or imperial units for volume and temperature.
Storage is sized at roughly 1.5 times the daily hot-water demand so the tank can bank a sunny day for later use. Use it to plan a domestic solar thermal system, check whether your roof has room for the collector, or right-size the tank for your family.
Frequently asked questions
How much hot water does each person use?
A common design figure is 40–60 litres (about 10–16 gallons) of hot water per person per day, so the default of 50 L suits a typical household. Adjust it up for baths and heavy use or down for efficient fixtures; you can also switch to entering a total daily volume directly.
What insolation value should I use?
Insolation is the daily solar energy on the collector in kWh/m²/day, similar to peak sun hours. Sunny climates run around 5–7, temperate ones 3–5, and it drops in winter. Use an annual or worst-month average for your location; a lower value sizes a larger, more reliable collector.
Why is collector efficiency less than 100%?
Real collectors lose heat to the surroundings and never capture all incoming sunlight. Flat-plate collectors run roughly 40–60% and evacuated tubes somewhat higher over a day, so the default 55% is a reasonable domestic figure. Lower efficiency increases the collector area the tool recommends.
How is the storage tank sized?
The tank is set at about 1.5 times daily demand so it can store a full sunny day's heated water for evening and morning use and ride through a cloudy spell. Systems in cloudier climates sometimes use a larger multiple; treat the figure as a sensible starting point for tank selection.