Do Solar Greenhouse Heaters Work? The UK Winter Numbers

Search for a solar greenhouse heater and you will find small kits: a panel, sometimes a battery, and a 12-volt heater. Listings in Google’s results in October 2026 included 12-volt heaters rated at around 50 W and a heater kit built around a single 30 W panel. The question gardeners ask on forums is whether any of them will keep frost off in January. The honest way to answer is with two numbers — how much energy a panel makes in a UK winter, and how much heat a greenhouse loses — and they are a long way apart.

What a solar panel makes in a UK winter

PVGIS, the European Commission’s solar model, gives the average daily output of a south-facing panel tilted at 30° near Evesham. These averages include cloudy days; a run of dull weather gives less.

Panel	December, per day	January, per day	June, per day
30 W	0.029 kWh	0.033 kWh	0.12 kWh
50 W	0.048 kWh	0.056 kWh	0.21 kWh
100 W	0.097 kWh	0.111 kWh	0.41 kWh

PVGIS v5.3, near Evesham (52.09°N), south-facing, 30°, 14% system losses, queried 7 October 2026: 29.9 kWh per kW of panels for the whole of December, 34.5 for January and 123.2 for June.

A panel makes over four times as much on an average June day as in December, which is the opposite of when a greenhouse needs heat.

How much heat a small greenhouse loses

Take a common 6 by 8 foot greenhouse: 1.83 by 2.44 metres, 1.5 metres to the eaves and 2.1 metres to the ridge. Its walls, gables and roof add up to about 19 square metres of glass.

Glass loses heat fast. The government’s indicative U-value for single glazing in a wooden or PVC frame is 4.8 W per square metre for each degree of temperature difference (as tabulated by Pilkington from Building Regulations data). An aluminium greenhouse frame, gaps around the door and vents, and the floor all add to that, so treat what follows as a floor, not a ceiling.

• 19 square metres × 4.8 W = about 92 W for every degree the inside is kept above the outside.
• The RHS’s figure for frost-free is 4°C. On a night when it is −1°C outside, that is 5 degrees of difference: about 0.46 kW of heat, continuously.
• Over a ten-hour night: about 4.6 kWh.

That is nearly 48 December days of output from a 100 W panel, to get through one cold night. To supply it from solar on an average December day you would need roughly 4.8 kW of panels — about twelve 400 W panels — plus a battery holding 4.6 kWh, for a greenhouse with 19 square metres of glass.

What a small solar heater kit can actually do

A heater rated at 50 W puts out 50 W of heat. In the example above, that holds the greenhouse about half a degree above the outside temperature — not enough to matter on a frosty night. Running it for ten hours takes 0.5 kWh, about five December days of output from a 100 W panel, so even half a degree could only be bought on one night in five.

A kit with no battery can only run its heater while the sun shines — warming the greenhouse in the daytime, when the sun is already warming it, and doing nothing at night, when frost happens.

What keeps frost out instead

The RHS’s advice on heating a greenhouse efficiently points to the things that work:

• A frost-free target, not a warm one. Keeping the greenhouse at 4°C lets you overwinter tender plants without heating it to a growing temperature.
• Insulation. Lining the glass with bubble plastic cuts heat loss; the RHS notes it also cuts out about ten per cent of the light, which it considers usually a worthwhile trade-off.
• A thermostat. A thermostat-controlled heater runs only when it is needed.
• Mains electricity where possible. Paraffin and gas heaters produce water, ethylene and carbon monoxide, so the greenhouse has to be ventilated, which reduces their heating effect; the RHS says they are only really useful for plants that can withstand quite low temperatures.

Storing the sun’s daytime heat in water or masonry is the other route, and it does not need a panel at all: see how a passive solar greenhouse works.

Where solar does earn its place in a greenhouse

Solar’s problem with heating is timing, and that same timing makes it good at cooling. A greenhouse overheats when the sun is strongest, which is exactly when a panel produces most. The RHS warns that plant damage can occur once temperatures build up beyond about 27°C.

• Ventilation fans. A fan rated at 20 W running for eight hours uses 0.16 kWh, well within an average June day’s output from a 100 W panel.
• Automatic vent openers. These need no panel at all: a wax-filled cylinder expands as it warms and pushes the vent open.
• Watering pumps, sensors and controllers. Small loads that a panel and a modest battery handle through the growing season.

If you grow commercially, the same arithmetic applies at a larger scale. Our guide to solar panels for greenhouses covers where to put an array on a glasshouse site so that it does not shade the crop, which loads it can carry, and why winter heat has to come from somewhere else.