Greenhouse Heater Size Calculator — BTU/Hr to Watts

A greenhouse loses heat through its skin, so the heater has to replace exactly what escapes. The surface-area method captures that with one equation: heat loss equals the exposed glazing area times a heat-loss coefficient for the covering times the temperature difference you want to hold between inside and outside. Match the heater's output to that loss and the greenhouse stays warm on the coldest design night.

Two inputs drive the answer. The exposed surface area is the total of the walls and roof — every square foot facing the cold — not the floor. The temperature difference is how far above the outside low you want to keep the interior; a greenhouse held at 50°F when it is 10°F out is working against a 40 degree swing. Glazing sets the coefficient: single glass or single poly leaks fastest, while double poly and twin-wall polycarbonate cut the loss by a third or more.

The result comes in BTU per hour, the unit heaters are rated in, alongside a kilowatt figure for electric units. Add a margin for wind, air leaks, and a safety cushion, and remember this is a rule-of-thumb estimate — a full heat-loss analysis is more precise for a large or costly build.