Greenhouse heater sizing: heat loss, output and running cost are different numbers
Define the calculation before selecting equipment
Our gable estimator follows the transmission, air-leakage and perimeter-loss structure in UGA Bulletin 792. It combines the enclosure area, volume, temperature difference and explicit user assumptions. It does not insert a hidden product-specific U-factor or automatically add a percentage and call the result a professionally sized heater. [Source]
A transparent worked scenario
Consider a simple 8 × 12 ft gable with 6 ft eaves and an 8 ft ridge. Its calculated roof-and-wall area is about 363.33 ft² and its enclosed volume is 672 ft³. With a U-factor of 0.5, a 30°F temperature difference, one air change per hour and a perimeter coefficient of 0.8, the model returns about 6,813 BTU/h, or 2.00 kW of useful heat. These are illustrative inputs and our arithmetic, not a specification for any listed kit.
Which assumption deserves your attention?
Change one input at a time. A 40°F rather than 30°F temperature difference increases every term in this particular model by one third. Doubling the leakage assumption doubles the leakage term, not the entire answer. This sensitivity check is more informative than reporting a result to several decimal places while the starting assumptions are uncertain. Use an appropriate local design condition supplied by a reliable source or professional, not an unusually mild day.
Thermal output does not automatically equal electrical input
The heater-sizing result describes estimated useful heat. The cost calculator instead asks how much electrical power the equipment actually draws and for how long. Do not transfer a gas burner’s output or a heat pump’s thermal output into the electrical-input field without knowing what that input represents. For a hypothetical 2 kW electrical load enabled for 12 hours per day over 30 days at 50% duty, use is 360 kWh. At an entered rate of 0.15 per kWh, the arithmetic energy charge is 54 in that currency.
What remains outside the model
The tool does not resolve warm-up recovery, humidity control, unusual leakage, power availability, emergency heat or a detailed envelope design. It cannot select a safe appliance location, circuit, fuel connection or combustion-air arrangement. Bring the documented envelope and load assumptions to an appropriately qualified installer. Equipment and local requirements control those decisions, even when a calculator’s number looks plausible.
Turn the guide into a buying decision.
Compare assumptions with the greenhouse heating benchmark and downloadable data. Its results describe hypothetical gable structures, not independent tests of the products in our catalog.
Use the tools
Greenhouse Heater Sizing Calculator →
Greenhouse Heating Cost Calculator →
Greenhouse Panel Area Calculator →
Model-specific records
Sources & verification
Manufacturer figures are reproduced as documented. Editorial comparisons and calculations are identified separately.
- University of Georgia Extension — Greenhouses: Heating, Ventilation and Cooling (B792) ↗ — checked 2026-09-19.
Source pages can change. The supplier’s current exact-model documentation and local requirements control final decisions. Report a correction.