Solar Thermal Collector Daily Heat and Hot Water Calculator

How much heat does a solar collector produce?

A solar collector passes the sunlight falling on it to water. The usable heat it produces in a day is the product of three things:

Q = A × H × η

• A is the collector area (m²).
• H is the irradiation on the collector plane in one day (kWh/m²).
• η is the system efficiency: the share of that energy you actually get as hot water.

The volume of water this heat can warm is V = Q / (1.163 × ΔT), with V in litres, Q in watt hours and ΔT the difference between the target temperature and the mains temperature.

Example

With 4 m² of collector, a daily irradiation of 5 kWh/m² and a system efficiency of 40 percent, the daily output is 4 × 5 × 0.40 = 8 kWh. With mains water at 15 °C and a target of 45 °C that heats 8000 / (1.163 × 30) = 229 litres.

How many square metres of collector do I need?

Work backwards. First the daily heat demand: warming 200 litres from 15 °C to 45 °C takes 1.163 × 200 × 30 = 6977 Wh. Then A = Q / (H × η): with 5 kWh/m² a day and 40 percent efficiency that is 3.5 m², or two 2 m² collectors.

Do the same with winter irradiation and the area multiplies. That is why systems are usually sized to cover most of the year rather than all of it, with an electric or boiler fed backup heater for winter.

How big should the tank be?

The sun heats during the day while water is mostly used in the morning and evening. The tank bridges that gap. A common rule of thumb is 50 to 70 litres per square metre of collector. Too small a tank overheats and lowers collector efficiency. Too large a tank never gets hot enough.

What this tool does not cover

Solar panels that make electricity (photovoltaics) are a separate subject: use the solar energy yield calculator for those. Collector efficiency curves, hourly irradiation, shading, detailed storage and pipe losses, frost protection and pressure design are not included here.