Greenhouse Heating Calculator
Estimate the BTU/hr heating output needed to maintain a greenhouse temperature during cold nights.
How to use this tool
- Enter the greenhouse interior dimensions in feet.
- Enter your target minimum inside temperature and the coldest expected outside temperature.
- Select the glazing type (material covering the greenhouse).
- The result shows surface area, temperature difference, and required heater BTU/hr output.
Size your greenhouse heater correctly to keep plants alive on the coldest nights.
Formula
Surface area (ft²) = 2(L×W) + 2(L×H) + 2(W×H)
ΔT (°F) = Inside temp − Outside temp
BTU/hr = Surface area × ΔT × U-value (glazing coefficient)
Where the glazing U-value (heat loss coefficient) is selected by glazing type, e.g. 1.2 BTU/(hr·ft²·°F) for single-layer polyethylene film.
How it works
This calculator uses the steady-state heat-loss formula: power required equals the total glazed surface area multiplied by the inside-to-outside temperature difference and the glazing's U-value (thermal transmittance). It models the greenhouse as a simple rectangular box — two floors/ceilings, two long walls, and two end walls — which slightly overestimates surface area for gable-roofed structures.
The U-value encodes how quickly heat escapes through a given glazing material; single-layer polyethylene film loses heat faster (higher U) than double-wall polycarbonate (lower U). Results represent peak design load under coldest expected conditions, so actual average consumption will be lower. Infiltration losses and thermal mass are not included.
Worked example
Worked example
- Greenhouse: 20 ft long × 12 ft wide × 8 ft eave height. Inside 50 °F, outside 10 °F. Glazing U-value: 1.2.
- Surface area = 2(20×12) + 2(20×8) + 2(12×8) = 480 + 320 + 192 = 992 ft².
- ΔT = 50 − 10 = 40 °F.
- BTU/hr = 992 × 40 × 1.2 = 47,616 BTU/hr.
Heating required: 47,616 BTU/hr
Key terms
- BTU/hr
- British Thermal Units per hour — the standard US unit for heating capacity; one BTU raises one pound of water by 1 °F.
- U-value
- Thermal transmittance: the rate of heat flow through a material in BTU per hour per square foot per °F temperature difference. Higher U means more heat loss.
- ΔT (delta T)
- The temperature difference between inside and outside; the larger the gap, the more heating power is needed to maintain the set point.
- Design load
- The peak heating output sized for the coldest expected night; actual average energy use will be considerably lower throughout the season.
- Glazing
- The transparent or translucent covering of a greenhouse — glass, single-layer poly film, or double-wall polycarbonate — each with a different U-value.
Frequently asked questions
- How do I calculate greenhouse heating requirements?
- Multiply the greenhouse surface area × temperature difference (inside minus outside) × the glazing U-value. This gives BTU/hr required. Always add 20–25% as a safety margin.
- What temperature should I maintain in a greenhouse?
- For frost protection only, maintain 34–40°F. For cool-season crops (lettuce, kale), 45–55°F. For tomatoes and peppers, 60–70°F nights. Higher targets need significantly more heating.