Solar Water Heater Sizing
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ℹ About This Calculator
A solar water heating system is sized around the daily hot-water demand and the energy the collector can capture. This calculator returns the collector area in m² and a recommended tank capacity (LPD) from the number of users, hot-water use per person, temperature rise, local solar irradiation and collector efficiency.
The daily energy needed is the mass of hot water times its temperature rise; the collector area follows from that energy divided by the useful solar gain (irradiation × efficiency). Flat-plate collectors run about 55-65% efficient and evacuated-tube around 50-60%. A common rule of thumb - roughly 1 m² and 100 L of tank per 2 people - falls straight out of this energy balance.
📐 Solar Water Heater Formula (Energy Balance)
NEC 690
Q = m × Cp × ΔT A = Q / (Irradiation × η) Q = daily energy demand, kWh m = daily hot-water mass ≈ litres/day Cp = 4.186 kJ/kg·K ΔT = hot − cold temperature rise, °C A = collector area, m² Irradiation = daily solar energy, kWh/m²·day η = collector efficiency
📝 Worked Calculation Example
Example: A family of 4 using 45 L of hot water each per day at 60 °C from 20 °C cold water needs 180 L/day. Energy Q = 180 × 4.186 × 40 / 3600 ≈ 8.4 kWh. At 5 kWh/m²·day irradiation and 60% efficiency, collector area A = 8.4 / (5 × 0.6) ≈ 2.79 m², matched to a standard 200 LPD system.