Solar Water Heater Size Calculator
Size a solar water heater for your home: daily hot water by people, recommended tank in litres or gallons, number of evacuated tubes or flat-plate area, energy and LP gas saved per year.
How it works
daily hot water = people × litres per person · tank ≥ daily use · tubes ≈ daily litres ÷ 12 × climate factor · flat plate ≈ daily litres ÷ 60 L/m² × climate factor · heat = litres × 4.186 × 35 K ÷ 3,600
A solar water heater should store about one day of hot water. Households use roughly 40–60 litres (10–15 gallons) of hot water per person a day, so a family of four needs about 200 L (50–66 gal). For the common evacuated-tube heaters (58 × 1,800 mm tubes) sellers size about 12 litres of tank per tube in sunny climates; for flat-plate collectors the US Department of Energy rule is about 20 sq ft for the first two people plus 8 sq ft per extra person in the Sun Belt (≈ 1 m² per 60 L), and 12–14 sq ft per person further north. In cloudier climates add 25–50% more collector. A solar heater typically covers 50–80% of the yearly water-heating energy, with gas or electric backup for the rest.
Worked example
A family of 4 using a typical 50 L of hot water each needs 200 L a day, so the recommended tank is 200 L. In a sunny climate that is 200 ÷ 12 = 16.7 → an 18-tube evacuated-tube heater, or about 3.3 m² of flat-plate collector. Heating 200 L from 15 to 50 °C takes 8.1 kWh a day; covering 75% of the year saves about 2,228 kWh, or 232 kg of LP gas.
Sources: US DOE Energy Saver — Sizing a solar water heating system · CONUEE (Mexico) — Calentadores solares de agua
Frequently asked questions
What size solar water heater do I need for 4 people?
About 200 litres (50–66 gallons) of storage with 15–18 evacuated tubes, or roughly 3.3 m² (36 sq ft) of flat-plate collector in a sunny climate. Add 25–50% more collector in cloudy or cold regions.
How many tubes does a solar water heater need per person?
Roughly 4 tubes per person for typical use in sunny climates — each 58 × 1,800 mm tube heats about 12 litres a day. A 10-tube unit (about 120–130 L) suits 2–3 people and a 15–18-tube unit (180–200 L) suits 4–5.
How much hot water does one person use per day?
About 40–60 litres (10–15 gallons) at 40–50 °C for showering, washing up and laundry. Low-flow showers bring it down to 30–35 L; long showers or a bathtub push it to 70 L or more.
How much gas does a solar water heater save?
For a family of four, heating 200 L a day by 35 °C takes about 8 kWh. Covering 75% of that with sun saves around 2,200 kWh a year — roughly 230 kg of LP gas with a 75%-efficient gas heater.
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This calculator is for educational purposes. Double-check important results before acting on them.