Cold-weather planning
Greenhouse Heater Size Calculator
Estimate steady-state greenhouse heat loss from exposed surface area, glazing performance and the indoor-to-outdoor temperature difference.
How greenhouse heat loss is estimated
The calculator estimates exposed wall and roof area, multiplies that area by the entered U-factor and temperature difference, then adds the chosen capacity allowance. UGA Extension expresses conductive loss as surface area multiplied by temperature difference and divided by R-value. Because U-factor is the inverse of R-value, the two forms describe the same relationship.
The result does not fully model air leakage, heat lost at the perimeter, doors opening, wind or heat stored in the ground. Older or poorly sealed structures may need substantially more capacity than a conduction-only estimate suggests.
Choosing inputs
Use the cold outdoor design temperature you genuinely want the greenhouse to survive, not the annual average. Enter the overall U-factor for the complete glazing assembly. Double-wall polycarbonate and double inflated film generally lose less heat than a single layer, but the exact value should come from the assembly supplier.
Continue with the greenhouse heating cost calculator, or review greenhouse heater selection before choosing fuel type and controls.
Frequently asked questions
Can the result be converted to watts?
Yes. The calculator reports an equivalent electric output using 3,412 BTU per hour per kilowatt.
Should I size from the lowest temperature ever recorded?
Usually a local design temperature is more useful, but plant value and risk tolerance matter.
Is a larger heater always safer?
No. Severe oversizing can cause short cycling and uneven temperatures. Follow equipment and thermostat guidance.
Technical reference: UGA Cooperative Extension heat-loss equations and assembly R-values.