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A heater that’s too small won’t hold the temperature on the one night that matters; too big wastes money. Enter your greenhouse’s size, covering and the temperatures you care about to estimate the heater you need.
The formula
BTU per hour = covered surface area (sq ft) × temperature difference (°F) × U-value. The U-value is how much heat the covering loses per square foot per degree — lower is better insulated.
| Covering | Typical U-value |
|---|---|
| Single-layer polyethylene | 1.15 |
| Single glass | 1.1 |
| Double poly, air-inflated | 0.7 |
| Twin-wall polycarbonate | 0.65 |
| Triple-wall polycarbonate | 0.5 |
One watt of electric heat equals 3.412 BTU per hour, so a 1,500 W heater provides about 5,100 BTU/h.
FAQ
What’s the cheapest way to heat a greenhouse?
Reduce the heat loss first: bubble insulation on the walls, sealing gaps and thermal mass such as water barrels. Then size the heater to the smaller load.
Is propane or electric better?
Electric is simplest and safest in small greenhouses. Unvented propane or kerosene heaters produce moisture and fumes that can damage plants.
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