Calculate heat loss and heater size needed to keep your greenhouse warm
Keeping a greenhouse warm through the British winter is essential for protecting tender plants, overwintering seedlings, and extending the growing season. However, heating a greenhouse can be expensive, and choosing the wrong heater size wastes both energy and money. The amount of heat a greenhouse loses depends on three factors: the total surface area of glass or polycarbonate, the U-value (thermal transmittance) of the glazing material, and the temperature difference between inside and outside. Single glass loses heat roughly twice as fast as double glazing or twin-wall polycarbonate. Our greenhouse heating calculator estimates the total heat loss in watts, recommends an appropriately sized heater in kilowatts, and calculates the overnight running cost based on current UK electricity prices. This helps you choose between electric fan heaters, tubular heaters, or paraffin heaters with confidence.
To calculate your greenhouse heating requirements: 1. Enter the greenhouse length in metres. Measure the external dimensions at ground level. 2. Enter the greenhouse width in metres. 3. Enter the greenhouse height in metres. For apex-roof greenhouses, use the average height (roughly eaves height plus one-third of the rise). 4. Select your glazing type. Single glass has a U-value of 6.0, double glass 3.0, and polycarbonate twin-wall 3.5 W/m2K. 5. Set your target temperature inside the greenhouse. A frost-free minimum is 2-4C, while tender plants need 7-10C or more. 6. Set the expected outside temperature for the coldest nights in your area. Typical UK winter lows range from 0C in mild regions to -10C in exposed areas. 7. Review the heat loss, recommended heater size, and estimated nightly running cost.
The greenhouse heating formula calculates heat loss through the glazing surfaces: Surface Area = (2 x Length x Height) + (2 x Width x Height) + (Length x Width) This includes the four walls and the roof. The floor is excluded as it does not lose heat through glazing. Heat Loss (watts) = U-value x Surface Area x Temperature Difference U-values used: - Single glass: 6.0 W/m2K - Double glass: 3.0 W/m2K - Polycarbonate twin-wall: 3.5 W/m2K Heater Size (kW) = Heat Loss / 1000, rounded up to the nearest 0.5 kW Running Cost per Night = Heater Size x 10 hours x 0.245 (GBP per kWh) The 10-hour period represents a typical winter night from dusk to dawn. The electricity rate of 24.5p per kWh is the current Ofgem price cap rate. Actual costs vary if you use a different tariff or heat source.
Inputs: Length: 2.4 m. Width: 1.8 m. Height: 2 m. Glazing: Single glass. Target: 5C. Outside: -3C.
Inputs: Length: 3 m. Width: 2.5 m. Height: 2.5 m. Glazing: Double glass. Target: 10C. Outside: -5C.
Inputs: Length: 4 m. Width: 3 m. Height: 2.5 m. Glazing: Polycarbonate. Target: 15C. Outside: -5C.