Passive Solar Greenhouses: Storing the Sun's Heat in the UK
Published by SEO Dons · Last reviewed October 2026
In much of the world “solar greenhouse” does not mean a greenhouse with solar panels on it. It means a greenhouse built to collect the sun’s heat and keep it: a passive solar greenhouse. The idea is old, it needs no electricity, and it works best exactly where an electric solar heater fails — on a clear winter day followed by a cold night. Here is how the design works, what research on China’s solar greenhouses shows, and what a UK greenhouse can realistically expect.
The four parts of a passive solar greenhouse
- Glazing that faces the winter sun. The long side faces south, and the glazing is steep enough to catch a sun that, in a UK December, stays low in the southern sky.
- Thermal mass. Something heavy inside — a masonry or earth wall, or containers of water — absorbs heat while the sun shines and gives it back after dark.
- Insulation where the sun does not reach. The north wall, the ends and the north slope of the roof let in little winter sun but lose heat all night, so passive designs insulate them or build them solid.
- A night cover. An insulating blanket or screen drawn over the glazing at night keeps the stored heat in.
Each part matters. Glazing without mass overheats by day and is cold by night; mass without insulation loses its heat before dawn.
What China’s solar greenhouses show
The best-studied passive greenhouses are in northern China. A 2008 study in the International Journal of Agricultural and Biological Engineering (Tong and colleagues, Shenyang Agricultural University and Tsinghua University) describes them as relatively small, with a large thermal mass in the walls, a thin plastic film — usually about 0.1 mm — over the sloping south-facing roof by day, and a thermal blanket added at night to keep the heat inside. The authors say they let growers extend the season through northern China’s cold winters “with little or no additional heating”.
Their computer simulations, checked against measurements, also showed why the wall matters: a reinforced concrete wall gave smaller swings in air temperature than brick or aerated concrete because it stored more heat. The design is tuned to its climate, though. Shenyang, Beijing and Lanzhou — the three cities modelled — lie at roughly 36 to 42 degrees north, well south of England, and northern China’s winters are typically drier and sunnier than England’s.
How much winter sun a UK greenhouse receives
PVGIS, the European Commission’s solar model, gives the average sunlight falling on a south-facing surface near Evesham, including cloudy days:
| Surface facing south | December, per m² | January, per m² | June, per m² | Year, per m² |
|---|---|---|---|---|
| Flat (horizontal) | 18.9 kWh | 23.2 kWh | 156.3 kWh | 1,047 kWh |
| Tilted at 30° | 35.6 kWh | 40.8 kWh | 155.9 kWh | 1,216 kWh |
| Tilted at 60° | 45.0 kWh | 49.9 kWh | 130.0 kWh | 1,166 kWh |
| Vertical | 44.2 kWh | 48.1 kWh | 83.8 kWh | 899 kWh |
PVGIS v5.3 (SARAH3), in-plane global irradiation near Evesham (52.09°N), queried 7 October 2026.
In December a vertical south face receives more than twice the sunlight that falls on the same area of flat ground, while in June it receives about half as much. That is the case for steep south glazing: it collects most in the season that needs heat and sheds the high summer sun.
A worked example
Take a 6 by 8 foot greenhouse set with its long side facing south. Its south wall has about 3.7 square metres of glass and its south roof slope about 2.7 square metres at roughly 33°. On the PVGIS figures, that south-facing glass receives roughly 8 kWh of sunlight on an average December day.
Set that against the heat the same greenhouse loses on a frosty night. With about 19 square metres of single glazing at the government’s indicative 4.8 W per square metre per degree, holding 4°C inside on a −1°C night takes about 0.46 kW — some 4.6 kWh over ten hours (worked through in whether solar greenhouse heaters work).
So on an average December day the sun delivers more energy to the glass than the night takes out. The catches are that not all of it passes through the glass, the greenhouse loses heat all day as well, and an average hides the dull weeks when the sun delivers a fraction of it. Thermal mass is what turns a good day into a warmer night.
Storing the heat: water and masonry
Water is the easiest store for a garden greenhouse because it holds a lot of heat for its volume: warming a litre of water by one degree takes about 4.2 kJ. A thousand litres — a few large dark containers along the north wall — that cools by 5°C overnight gives back about 5.8 kWh, more than the 4.6 kWh the example greenhouse loses through its glass on a frosty night. To give it back, though, the water first has to be warmed by 5°C during the day, which takes a sunny day and a greenhouse that is not losing everything it gains.
Masonry works the same way with more weight: a solid north wall stores heat and insulates the coldest side at once, which is why it is the defining feature of the Chinese design.
What to expect in the UK
In the UK a passive solar greenhouse is a way to need less heat, not a way to do without it. What the design does well is cut the number of hours a heater has to run, smooth out the daily swing between a hot afternoon and a cold night, and, after a sunny day, carry stored heat into the night. For the coldest, dullest spells, the RHS’s frost-free target of 4°C, insulation and a thermostat-controlled heater on a mains supply remain the backstop.
Two practical points follow:
- Keep solar panels off the glazing. A passive greenhouse lives on the light and heat coming through its south glass; panels on it would take both. If you want electricity for fans or pumps, put a panel on a nearby shed.
- Ventilate in summer. Steep glazing, thermal mass and insulation that help in winter make overheating more likely in summer, and the RHS warns that plant damage can occur once temperatures build up beyond about 27°C.
For growers weighing panels for a glasshouse business, our guide to solar panels for greenhouses covers where an array goes without shading the crop. And if see-through panels sound like a way round the problem, see how transparent solar panels work for where that technology is today.