A greenhouse holds heat poorly compared to an insulated building, so the heater you choose has to keep pace on the coldest nights of your season. Oversize it and you waste energy; undersize it and you lose plants. This guide walks the decisions in order: how much heat you need, what fuel to use, how to size the unit, and what to add for seedlings.
How much heat do you actually need?
Heating capacity is measured in BTU per hour. The amount you need depends on the greenhouse’s surface area, how cold it gets outside, and how warm you want to keep it inside — the temperature difference the heater has to maintain.
A practical starting point for an uninsulated polycarbonate greenhouse is roughly 30–40 BTU per cubic foot of interior volume on a cold night. For a 10×16×8 ft house (about 1,280 cubic feet), that points to a heater in the 40,000–50,000 BTU/hr range for hard freezes, or far less if you only need to hold a few degrees above freezing.
Before you shop, settle three numbers: your greenhouse volume, your coldest expected outside temperature, and your target inside temperature.
Electric or gas?
Electric heaters are simple, safe to run unvented, and precise with a built-in thermostat — ideal for small greenhouses and for holding a modest temperature. Their ceiling is the circuit you can supply; large spaces may exceed what a standard outlet delivers.
Gas (propane) unit heaters produce far more heat per dollar and suit larger structures, but combustion heaters must be vented to the exterior — never run an unvented combustion heater around plants or people.
For most home and small-commercial greenhouses holding a moderate temperature, a thermostatically controlled electric heater is the straightforward choice:
1500W
Thermostatic electric option for small greenhouses
Sizing against your greenhouse volume
Match the heater’s rated output to the number you calculated above, then add margin for wind and single-layer glazing. It’s better to run a right-sized heater at a comfortable duty cycle than to cycle an oversized one on and off, which stresses the thermostat and creates temperature swings.
If your supply pressure or circuit is marginal, size down the space you heat — a smaller sealed growing zone within the greenhouse is cheaper to keep warm than the whole structure.
Bottom heat for seedlings
Air temperature isn’t the whole story. Seeds germinate on root-zone temperature, and a seedling heat mat warms the soil directly for a fraction of the energy it takes to raise the whole air volume. Pair a mat with a thermostat and you can start plants weeks earlier without heating the entire greenhouse to seedling temperatures.
10" · 20.75" · 1 Pack
Root-zone bottom heat for propagation
Safety considerations
- Vent any combustion heater to the outside, and never rely on an unvented gas heater in an occupied or planted space.
- Keep heaters clear of foliage, film, and flammable material, and use models with tip-over and overheat protection.
- Run wet-location circuits on GFCI protection, and use outdoor-rated cords sized for the load.
Where this fits
A heater is one stage of outfitting a greenhouse — it works alongside ventilation, irrigation, and monitoring. If you’re planning the whole build, start with the complete project guide, which sequences every decision and gathers the equipment into one checklist.
