Greenhouse heating in a UK winter: which heater, what size and what it costs

Heat is a means to an end, so start with what you want the greenhouse to do between November and March. The RHS guide to heating greenhouses draws the line at frost: kept frost-free at 4 °C, a greenhouse can carry tender plants through the winter. Anything warmer, for winter tomatoes or early cucumbers, multiplies the heat needed and the bill with it.

Hardy salads sown in late summer keep cropping through the winter without heat: our winter salads guide sets out the varieties and dates.

A heater only has to replace the heat flowing out, and most of it flows through the walls and roof. Three numbers set the flow: the square metres of panel, how well the panel insulates and how much warmer than outside you want it. Makers publish the second as a U-value, the watts lost through each square metre for each degree of difference.

Take a NORDIC 3 m long, with about 28 m² of walls and roof in 4 mm panels. Holding 4 °C inside on a −5 °C night means a 9 degree difference: 28 × 3.9 × 9 comes to about 980 W. Doors and joints leak as well, so add a margin; at 25 per cent that becomes about 1,230 W, so look for a heater of at least 1.3 kW. The margin is a planning allowance, not a measurement, so give an exposed or draughty site more.

The calculator uses the higher Brett Martin figures, so it errs towards a bigger heater. A glass greenhouse loses about half as much heat again as one in 4 mm twin-wall on the same night.

These are estimates for one cold night, worked from the drawings rather than measured, so treat them as the size to aim above. With 6 mm panels the figures drop by about five per cent. In a long greenhouse with only a few tender plants, the RHS suggests grouping them at one end behind a screen of bubble wrap and battens, and heating that part alone.

Whatever the type, a thermostat matches the heat to the plants. The RHS warns that thermostat dials can be a few degrees out, because they sit close to the heater. It suggests a maximum-minimum thermometer in the middle of the house, among the plants, to check what they actually get.

Products sold as solar greenhouse heaters range from small solar-powered fans to heat stores. A cold night needs heat after sunset, so ask how much energy a system can deliver overnight, in kWh. A 1 kW demand over 10 hours is 10 kWh, and a panel’s watt rating says nothing about that. Containers of water as a heat store are a gardening myth on the RHS’s reading, as their temperature lift is negligible.

We don’t sell heaters, so none of this is a product pitch.

The RHS is clear that the electrical connection to the greenhouse should be installed professionally, and that a fan heater should be one sold for greenhouses. Electrical Safety First advises a residual current device (RCD) with all outdoor electrical equipment. A new circuit to a garden building is work for an electrician, who designs, protects and certifies it. In England it falls under Approved Document P of the building regulations, and the electrician will tell you what needs notifying.

Give the electrician the heater’s rating before you buy it, and check that the heater and its controller are rated for a damp place. Plan the cable route while the base goes in, if you are building new: our guide to greenhouse bases covers the groundwork, and the steel arch assembly guide shows when the base has to be finished.

The ceiling is simple arithmetic. A Plot 3.14 6 m long, heated to 4 °C on a −5 °C night, needs about 2,060 W with the margin. Delivered for all 12 hours of the night, that is about 24.7 kWh; at an example rate of 25p per kWh, chosen for easy sums rather than as a quote, that comes to about £6.20 for the night. Your bill gives your real rate, and the calculator above takes it.

Most winter nights are milder than the coldest one, and a thermostat cycles the heater on and off, so a whole season costs far less than the number of nights times that ceiling. A plug-in energy monitor or your smart meter shows what the heater actually uses.

The RHS names draughts and the structure as the main routes for heat loss. Replace broken panes, check that vents and doors fit, and seal cracks with clear silicone. Re-used bubble wrap insulates well but cuts light by about ten per cent, so leave it off light-loving crops such as winter lettuce. Heavier grades of fleece over the plants give about 2 °C of frost protection on a cold snap.

Thicker panels help too. On the Plot 3.14 and NORDIC, 6 mm twin-wall is an option over the standard 4 mm, and it holds more heat. Our insulation guide goes through the materials in detail.