Method, Ratings, and Assumptions
Design-night heat loss is transmission plus infiltration plus optional foundation-edge loss:
Qloss = (U × A × ΔT) + (0.02 × V × ACH × ΔT) + (F × P × ΔT)
Delivered heater output = heat loss × (1 + sizing margin). Appliance input rating = delivered output ÷ combustion/electric efficiency; for a heat pump, electrical input = delivered output ÷ COP. Heater labels may advertise either input or output, so confirm which rating the manufacturer uses and choose the next available standard size without excessive oversizing.
For monthly weather, the same building heat-loss coefficient is multiplied by each month's indoor-to-mean-outdoor temperature difference and hours. For HDD, energy = heat-loss coefficient × indoor-base degree days × 24. Fuel heat contents are 3,412.142 Btu/kWh, 100,000 Btu/therm, 91,452 Btu/US gal propane, and 138,500 Btu/US gal No. 2 oil.
Reproducible Examples
Each value and equation below is generated by the calculator's current geometry and heat-loss functions. Load an example to inspect or edit every assumption.
8 × 12 ft hobby house
20 × 48 ft hoop house
30 × 96 ft commercial house
The hoop example includes a fuel-cost comparison; the hobby example includes a single-film versus double-poly upgrade comparison.
Methodology, Validation, and Sources
Formula revision: GH-Heat 2.0, August 31, 2026. Responsible editorial team: Starlight Tools Agricultural Calculators Editorial Team. Preset groups were reviewed July–August 2026. U-values are planning whole-assembly estimates unless a manufacturer override is entered; panel-center ratings can omit frame and joint losses.
| Validation case | Independent hand result | Tool result | Difference |
|---|
Validation cases reproduce the extension-style U×A×ΔT, 0.02×V×ACH×ΔT, and perimeter equations; they are arithmetic checks, not field-performance certification.
- University of Georgia Extension: Greenhouses—Heating, Cooling and Ventilation — conduction, infiltration, R-values, and air-change ranges.
- Alabama Cooperative Extension: Controlling the Greenhouse Environment for Vegetable Crops — transmission and infiltration methods.
- University of Arkansas: Greenhouse Heating Systems — glazing coefficients and heating-load equations.
- U.S. Energy Information Administration: Energy Units and Calculators — energy conversions.
Known exclusions: solar and thermal-storage gains, detailed slab/soil heat flow, snow or condensation effects, humidity/latent load, fan and distribution losses, wind-pressure calculations, heat-pump defrost transients, capacity degradation, tariffs, and backup/redundancy design.
Greenhouse Heating FAQs
How many BTUs are needed to heat a greenhouse?
Multiply the greenhouse heat-loss coefficient by the indoor-to-design-outdoor temperature difference, then add a modest sizing margin. This calculator reports heat loss, required delivered output, and appliance input in both Btu/h and kW; select the next available standard size rather than greatly oversizing.
How much does it cost to heat a greenhouse through winter?
Winter cost depends mainly on covering U-value, exposed area, air leakage, indoor setpoint, local weather, fuel price, and heater efficiency. Use monthly mean temperatures or indoor-base heating degree days instead of applying the design-low load to every winter hour.
Is the greenhouse floor included in heat loss?
The covering area excludes the floor. The calculator instead includes an optional simplified foundation-perimeter loss. Uninsulated slabs, unusual foundations, and detailed ground heat storage require a site-specific model.
Which greenhouse heating fuel is cheapest?
There is no universal cheapest fuel. Compare local prices on delivered heat: the tool normalizes electric resistance, natural gas, propane, heating oil, and a cold-climate heat pump to cost per delivered million Btu and applies efficiency or COP.
What outdoor design temperature should I use?
Use the winter heating design temperature published for the nearest representative weather station, commonly the 99% annual heating design value. Do not use the all-time record low unless that is an intentional risk requirement.
Will a heat pump heat a greenhouse below freezing?
A properly selected cold-climate heat pump can operate below freezing, but capacity and COP fall as outdoor temperature drops and defrost cycles add load. Check the manufacturer capacity table at the design temperature and plan backup heat where crop loss risk is high.
How does wind change greenhouse heater size?
Wind increases infiltration through gaps and can worsen outside surface heat transfer. Use a higher ACH for exposed or leaky houses, and consider a wind-exposure allowance when finalizing equipment with a local professional.