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 U-value is how many watts each square metre of glazing loses per °C of temperature difference. This calculator uses 5.7 W/m²/°C for single horticultural glass (CIBSE typical figure), 2.8 for sealed double glazing, 3.5 for twin-wall polycarbonate and 3.0 for bubble-wrap lining. The lower the U-value, the less heat escapes — twin-wall poly or bubble wrap roughly halves the loss of single glass.

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

External footprint length of the greenhouse

Width

External footprint width of the greenhouse

Height

Average height to the ridge (roughly midway between eaves and ridge)

Min Inside Temp

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

Coldest Outside Temp

UK typically -5 to -10°C in winter

Glazing MaterialSingle glass loses the most heat; twin-wall polycarbonate or a bubble-wrap lining roughly halves it

How We Calculate This

Formula: Heat loss (W) = glazed surface area (m²) × temperature difference (°C) × U-value (W/m²/°C).

Surface area is the four walls plus the roof and floor of the box. The U-value is a typical CIBSE figure for each glazing type — about 5.7 W/m²/°C for single horticultural glass, 2.8 for sealed double glazing, 3.5 for twin-wall polycarbonate and 3.0 for a bubble-wrap lining.

The result in watts is the minimum continuous heater output to hold your target temperature on the coldest night. BTU/hr conversion: 1 W = 3.412 BTU/hr. Add ~10-20% headroom and use a thermostat so the heater only runs when needed.

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.