Solar Water Heater Sizing Calculator - Collector & Tank

Free Online Solar Water Heater Sizing Calculator

Size a solar water heater: collector area and tank capacity from daily hot-water demand, temperature rise and solar irradiation. NEC 690 basis. Free online tool.

📐 Standard: NEC 690

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Solar Water Heater Sizing Calculator

Reference: NEC 690

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Size a solar water heater: collector area and tank capacity from daily hot-water demand, temperature rise and solar irradiation. NEC 690 basis. Free online tool.

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Calculation confidence: Planning-levelSuitable for early design, estimating and feasibility checks. Verify the final design against the governing code and a licensed engineer before construction or procurement.

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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 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.

Frequently Asked Questions

How do I size a solar water heater? +

Start from daily hot-water demand (litres per person × people), work out the energy to heat it through the required temperature rise, then divide by the useful solar gain (irradiation × collector efficiency) to get collector area. The tank is sized to hold roughly one day of hot water.

How much collector area do I need per person? +

As a rule of thumb about 1 m² of collector and 100 L of tank per two people in good sun, but it varies with location and water temperature. In lower-irradiation regions or for higher delivery temperatures the area per person increases.

What tank size goes with a solar water heater? +

The storage tank is usually sized to hold a full day's hot-water demand so the system can supply evening and morning draw-offs from daytime collection. A 180 L/day demand maps to a standard 200 LPD system.

Flat-plate vs evacuated-tube collector - which is better? +

Flat-plate collectors are robust and cost-effective in warm, sunny climates and run about 55-65% efficient. Evacuated-tube collectors perform better in cold or cloudy conditions and for higher temperatures. This tool lets you enter the efficiency of whichever type you choose.

Does solar irradiation affect the collector size? +

Strongly. The collector area is inversely proportional to daily irradiation, so a location receiving 4 kWh/m²·day needs about 25% more collector than one receiving 5 kWh/m²·day for the same hot-water demand.