Sunlight on a clear day at midday delivers roughly 1,000 watts of power to each square metre facing it, but what a heater earns over a whole day is energy, measured in kilowatt-hours, and it is always far less than peak power running all day. Solar heating works by catching that energy on a dark surface, trapping it behind glass and insulation, and storing it in water. Keep power (watts) and energy (watt-hours) separate in every calculation, or the sizing comes out wrong by a large factor.
Part of Solar Thermal Collectors
Sunlight arrives as a flow of power that varies through the day and the year; a solar heater is a device for turning that flow into stored heat.
Power Is A Rate, Energy Is A Total
Power is how fast energy arrives, in watts (W). Energy is the amount that has arrived, in watt-hours (Wh) or kilowatt-hours (kWh), or in joules (1 kWh = 3.6 MJ). A 1 kW heater running for 1 hour delivers 1 kWh. The sun’s peak of about 1,000 W/m² (the figure in the parent page, for a clear sky at noon with the surface facing the sun) is a power; the harvest of a day is an energy.
Treat the day as a number of full-strength hours. If a square metre receives 5 kWh over a day, that is the same as 5 hours at 1,000 W/m²: 1,000 W × 5 h = 5,000 Wh = 5 kWh. The sun is weaker in the morning and evening, so the real day is longer than 5 hours but equivalent to 5 full-strength ones.
Energy is also what a tank of water holds. Raising 1 kg of water by 1 K takes 4.18 kJ, so 1 kWh (3,600 kJ) warms 3,600 ÷ 4.18 = 861 kg by 1 K, or about 86 kg by 10 K. That one conversion links the sun’s energy to a tank.
A Solar Heater Does Three Jobs
- Catch. A matte black surface absorbs most of the sunlight that hits it (Absorption vs Reflection).
- Hold. Glass over the surface and insulation behind it slow the escape of heat (The Greenhouse Effect in Collectors).
- Store. Water in a tank keeps the heat for evening and night; without storage, the sun’s heat is gone by sunset.
Every design in this group is a different arrangement of those three jobs: a bare barrel, a glazed box with a tank inside it, or a flat collector feeding a separate tank.
What You Need
- A dark surface — flat black paint, stove paint or soot bound in oil
- Glass or clear plastic — to trap heat over the dark surface
- Insulation — dry material behind and around the dark surface
- Water in a container — the store of heat
- A thermometer, if you can find one — to measure what you actually gain
Worked Example: One Square Metre, One Day
Inputs, all assumed for the example: 5 kWh of sun on 1 m² over a day; a simple collector that turns 40% of it into stored heat; water warmed from 20°C to 50°C (a rise of 30 K).
- Heat stored = 5 kWh × 0.40 = 2.0 kWh = 2.0 × 3.6 = 7.2 MJ = 7,200 kJ.
- Water warmed by 30 K = 7,200 kJ ÷ (4.18 kJ/kg·K × 30 K) = 57 kg, about 57 L.
So a square metre of simple collector can warm about 57 L to bath-water temperature on a day like that. Two assumptions did the work: the sun amount and the 40%. Both change with place and weather (Insolation & Angle).
Common Mistakes
- Writing kW where kWh is meant. A “5 kW” day does not exist; a 5 kWh day does. The first is a rate and the second a total.
- Using the 1,000 W/m² peak as the daily average. It holds only near noon with a clear sky and the surface facing the sun.
- Assuming one sunshine figure for everywhere. Daily totals depend on latitude, season, cloud and tilt; see the insolation page for how to find your own.
Safety
Solar-heated water and surfaces burn quickly
A dark surface in the sun can pass 60°C easily, and water above about 60°C causes a deep scald in a few seconds. Treat every part of a solar heater as hot. Cool a burn at once with running cool water (Thermal Injury Care).
What’s Next
- Learn next: Insolation & Angle — how much sun reaches your surface and how to aim it
- Combine with: Heat Storage Systems — what to do with the heat once you have it