Greenhouse Heating Calculator — What Size Heater Do I Need

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 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.

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.

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.

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.