How to Use the Greenhouse Heating Load Calculator
Sizing a greenhouse heater incorrectly is costly either way — too small and you risk cold damage on freezing nights, too large and you waste money on equipment and fuel. This calculator uses a standard agricultural engineering approach: enter your greenhouse area, indoor target temperature, outdoor low temperature, and covering material to get the required heating load in BTU per hour.
The formula is area × temperature difference (target minus outdoor) × the covering material's heat loss factor (U). This factor reflects how much heat passes through the covering — single-layer poly film is rated around 1.2 BTU/hr·ft²·°F, double poly film with an insulating air gap around 0.8, and glass around 1.0. As you can see, adding a second poly layer with an air gap meaningfully cuts heat loss compared to a single layer.
The result is a baseline steady-state heat loss, also converted to kW for convenience. Real-world demand can run higher on windy nights or in a loosely sealed greenhouse, so it's a good idea to choose a heater rated 10-20% above the calculated load rather than sizing it exactly to the minimum.
Frequently Asked Questions
The heat loss factor (U) reflects how much heat passes through the covering — a lower U means better insulation. Single-layer poly film loses heat quickly, while double poly with an air gap cuts heat loss nearly in half by trapping insulating air between the layers.
No — the calculated value is a baseline steady-state heat loss. Actual heat demand can be higher on windy nights or in a poorly sealed greenhouse, so it's safer to choose a heater rated 10-20% above the calculated load.