Greenhouse Heater Calculator
Find the right heater size for your greenhouse in under a minute. Enter your greenhouse size, what it is glazed with and how cold your winters get. The calculator works out the heat your greenhouse loses on the coldest night, then shows matching heaters and what they cost to run.
How it works
How to calculate greenhouse heater size
A greenhouse loses heat through its glazing, its frame and through air leaks. The heater has to replace that heat as fast as it escapes on your coldest night. The standard formula used by growers and university extension services is:
BTU per hour = Surface area (sq ft) × Temperature difference (°F) × Heat loss factor (U)
Surface area is the total area of every wall and roof panel, not the floor area. A 10 x 12 ft greenhouse has 120 sq ft of floor but about 410 sq ft of walls and roof.
Temperature difference is the lowest temperature you want inside minus the coldest night outside. Keeping 45°F inside when it is 5°F outside means a 40°F difference.
Heat loss factor (U-value) is how fast heat passes through the glazing. Lower is better. We add 0 to 30% on top for wind and air leaks, depending on what you choose.
| Glazing | Heat loss factor (BTU/hr·sq ft·°F) |
|---|---|
| Single glass | 1.1 |
| Single polyethylene film | 1.2 |
| Single-wall polycarbonate or fiberglass | 1.0 |
| Double polyethylene film, inflated | 0.7 |
| Single glass with bubble insulation | about 0.7 |
| Twin-wall polycarbonate, 4 to 6 mm | 0.65 |
| Twin-wall polycarbonate, 8 to 10 mm | 0.58 |
| Triple-wall polycarbonate, 16 mm | 0.45 |
Worked example: a 10 x 12 greenhouse
A 10 x 12 ft freestanding greenhouse with 6 ft side walls and an 8 ft peak has about 413 sq ft of surface. In 8 mm twin-wall polycarbonate, held at 45°F on a 5°F night with average wind: 413 × 40 × 0.58 × 1.15 ≈ 11,000 BTU per hour, about 3,200 watts. The same greenhouse in single glass needs about 20,900 BTU per hour, nearly twice as much.
Choosing a heater
Electric or propane?
Electric heaters are clean, quiet and add no moisture or fumes. A standard 120V outlet tops out at about 1,500 watts, or 5,100 BTU, which suits small greenhouses in mild areas. Anything bigger needs a 240V circuit put in by an electrician.
Propane heaters give a lot of heat for the money and work where there is no power. Unvented heaters add moisture and use up oxygen, so pick one with an oxygen depletion sensor and leave a vent cracked. Vented heaters are the safest choice for heating every night.
Whichever you choose, use a thermostat, keep a fan moving air, and add a cheap min/max thermometer so you know what really happens overnight.
Save energy
Ways to cut your heating bill
- Line glass with bubble insulationHorticultural bubble wrap can cut heat loss through single glass by about a third. Take it down in spring to get full light back.
- Seal the gapsWeatherstrip doors and vents and check the seals along the base. Air leaks can add 15 to 30% to your heating load.
- Build on a knee wallA solid base wall loses far less heat than glass at ground level, where cold air settles.
- Add thermal massBlack water barrels or stone soak up heat during the day and give it back at night.
- Heat only what you needUse a thermostat, close off part of the greenhouse with a curtain, or use heat mats under seed trays.
- Choose better glazingTwin-wall and triple-wall polycarbonate hold heat far better than single glass. See glass vs polycarbonate.
Questions
Greenhouse heating FAQ
What size heater do I need for my greenhouse?
Multiply the surface area of the walls and roof by the difference between your inside and outside temperatures, then by the heat loss factor of your glazing. The calculator above does this for you and adds an allowance for wind and drafts.
How many BTUs does it take to heat a 10 x 12 greenhouse?
About 11,000 BTU per hour for a twin-wall polycarbonate greenhouse kept at 45°F on a 5°F night, and about 21,000 BTU per hour for single glass. Milder winters or a cooler target temperature need much less.
How many watts do I need to heat a greenhouse?
Divide the BTU figure by 3.412. For example, 11,000 BTU per hour equals about 3,200 watts. A plug-in 1,500 watt heater gives about 5,100 BTU per hour.
Can I use a regular space heater in my greenhouse?
Only one made for damp places. Greenhouses are wet, so use a heater rated for greenhouses or damp locations, plug it into a GFCI-protected outlet and keep it away from water and plants.
Why size for the coldest night instead of an average night?
Plants die on the one night the heater cannot keep up. Sizing for your coldest night means the heater can hold your target temperature when it matters. A thermostat makes sure it only runs as hard as each night needs.
Should I buy a bigger heater than the calculator says?
The result already includes an allowance for wind and drafts. If your choice falls between sizes, pick the next size up. For tropical plants or very cold areas, two smaller heaters give you a backup if one fails.
Want a second opinion?
Tell us your greenhouse and where you live, and we will recommend a heater, fan and thermostat setup that fits.
Call 678-408-8222 Send us a message