What temperature should I heat my greenhouse to?

A frost-free greenhouse (5°C minimum) protects most overwintering plants. For tropical plants, maintain 10-15°C. Heating to 5°C uses far less energy than heating to 10°C.

Electric or gas greenhouse heater?

Electric fan heaters are the most popular for small greenhouses, easy to thermostat control, and produce no fumes. Gas heaters are cheaper to run for larger spaces but need ventilation.

How can I reduce greenhouse heating costs?

Insulate with bubble wrap (cuts heat loss 30-50%). Use a thermostat. Only heat the area you need. Consider a heated propagator mat instead of heating the whole greenhouse.

What does the U-value factor mean?

The calculation uses a heat loss factor of 6 W/m²/°C for single-glazed glass, which is standard for horticultural glass. Polycarbonate or bubble-wrap insulation reduces this significantly.

Do I need to heat my greenhouse all winter?

It depends what you are growing. Many hardy plants survive unheated. For tender plants or frost-free overwintering, heat from October to April in the UK.

GardenCalcs

Greenhouse Heating Calculator — What Size Heater Do I Need

Calculate heater wattage needed to maintain greenhouse temperature.

Length

Width

Height

Min Inside Temp

5°C = frost-free, 10°C = cool greenhouse

Coldest Outside Temp

UK typically -5 to -10°C in winter

How We Calculate This

Formula: Heat loss = Surface area x Temperature difference x U-value factor.

Surface area includes walls and roof. The U-value factor of 6 W/m²/°C is standard for single-glazed glass greenhouses.

The result in watts tells you the minimum heater size. BTU conversion: 1W = 3.412 BTU/hr.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.