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Greenhouse Heating Cost Calculator

Enter your greenhouse details to estimate the heating cost for the season.

How it is calculated

Three steps. First the heat loss rate: Q = U × A × ΔT in watts, with U the U-value in W/(m²·K), A the covering surface area in m² and ΔT the inside-minus-outside temperature difference in degrees. Then the energy: kWh = Q × hours ÷ 1000. Then the money: cost = kWh × price, where price is per kWh. The tool assumes the same ΔT for every heating hour, so it is a simplified seasonal average, not a day-by-day simulation, and it assumes the heater converts every kWh of input into heat (no efficiency factor, so a gas boiler or heat pump needs the price adjusted). Area and U-value must be above zero; temperature difference, hours and price may be zero but not negative. The peak figure is the heat load Q in kW at that ΔT.

Worked example

A greenhouse with 150 m² of covering, U-value 4 W/(m²·K), held 18 degrees above outdoors for 1,500 heating hours at $0.15 per kWh. Heat loss: Q = 4 × 150 × 18 = 10,800 W, which is the 10.80 kW peak. Energy: 10,800 × 1,500 ÷ 1000 = 16,200 kWh. Cost: 16,200 × 0.15 = $2,430.00. Halving the U-value to 2 (a better covering) would halve both the energy and the bill.

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Common mistakes

  • Entering the floor area instead of the surface area of walls and roof. Heat escapes through the covering, so the area should be the glazed or film-covered surface.
  • Typing the outdoor temperature in the ΔT field. It must be the difference between the inside setpoint and the outside design temperature (for example 18 for 20 inside and 2 outside).
  • Using the U-value in the wrong unit, such as BTU/(h·ft²·°F). The formula expects W/(m²·K); a BTU-based value would overstate the loss.
  • Entering the number of heating days where the hours field is expected. Multiply days by 24 first, and remember that the tool applies the full ΔT to every hour.
  • Entering a gas, oil or heat-pump price per kWh without adjusting for efficiency. The tool assumes electric resistance heating, where each kWh bought becomes one kWh of heat.

Sources

About this calculator

Heat loss from a greenhouse follows Q = U × A × ΔT, where U is the structure's U-value (heat transfer coefficient, reflecting glazing material — single-layer poly film loses heat far faster than double-wall polycarbonate), A is the surface area losing heat, and ΔT is the temperature difference between inside and the outside design temperature. This calculator converts that heat-loss rate into energy consumed over a heating season and multiplies by your electricity price to estimate the seasonal heating bill.

Because U-value differences between glazing types can be dramatic — double-wall polycarbonate typically has roughly half the U-value (and heat loss) of single-layer film — the choice of covering material often has as much impact on heating cost as the heater's efficiency itself. Commercial growers, hobby gardeners planning a greenhouse build, and horticultural consultants use this calculation to compare glazing options and size a heating system correctly before committing to a structure.

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Comments (1)

  • Editorial team

    Run this tool twice with two different jobs in mind. For the seasonal bill, use an average temperature difference over your heating hours. For sizing the heater, rerun it with the difference on your coldest design night and read the kW line instead. With the defaults, 15 degrees gives 10.80 kW, but a 30 degree night needs 21.60 kW for the same covering. What covering and U-value are you working with?

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