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Greenhouse heater size calculator

This greenhouse heater size calculator works out how much heat your greenhouse loses on the coldest night you plan for, so you can buy a heater that keeps up. An 8x12 in 8 mm twin-wall polycarbonate needs about 7,701 Btu/h (2,257 W) to stay at 40 F when it is 10 F outside.

Greenhouse heater size calculator

Describe the greenhouse, the lowest temperature you want to keep inside and the coldest night you plan for. The calculator adds the heat lost through the glazing, through air leaks and along the base, the way UGA Extension's bulletin on greenhouse heating does it.

Heater output needed
7,701 Btu/h
In watts
2,257 W
1,500 W heaters
2
Night at full output
27.1 kWh

For a 30 F rise: 363.3 sq ft of glazing and 700 cu ft of air.

Where the heat goesArea or amountU or factorBtu/h
Roof glazing107.3 sq ft0.581,867
Wall and gable glazing256 sq ft0.584,454
Air leaks700 cu ft x 1/h0.02420
Base perimeter40 ft0.80960

Btu/h = area x U x temperature rise, plus 0.02 x volume x air changes x rise, plus 0.8 x perimeter x rise. U in Btu per hour per sq ft per F. A 1,500 W heater gives 5,118 Btu/h (1 W = 3.412 Btu/h). The night figure is the heater running flat out for 12 hours at the design temperature, an upper bound, not a forecast.

Open this greenhouse in the designer

Heater size for each plan

Our 11 free plans as drawn, with average wind. Frost protection is 40 F inside on a 10 F night; growing is 55 F inside on a 0 F night, the low end of UGA's 55 to 65 F for most greenhouse plants.

PlanGlazingFrost protectionGrowing1,500 W heaters (growing)
6x8 greenhouse8 mm twin-wall4,889 Btu/h8,963 Btu/h2
8x8 greenhouse8 mm twin-wall5,913 Btu/h10,841 Btu/h3
8x12 greenhouse8 mm twin-wall7,701 Btu/h14,119 Btu/h3
8x16 greenhouse8 mm twin-wall9,491 Btu/h17,401 Btu/h4
10x12 greenhouse8 mm twin-wall8,963 Btu/h16,432 Btu/h4
10x16 greenhouse8 mm twin-wall11,323 Btu/h20,759 Btu/h5
12x20 greenhouse8 mm twin-wall15,724 Btu/h28,827 Btu/h6
6x12 lean-to greenhouse8 mm twin-wall4,926 Btu/h9,031 Btu/h2
8x12 lean-to greenhouse8 mm twin-wall6,166 Btu/h11,304 Btu/h3
8x12 corrugated greenhouseCorrugated14,459 Btu/h26,508 Btu/h6
6x16 lean-to greenhouse8 mm twin-wall5,985 Btu/h10,972 Btu/h3

How the calculator works out heat loss

A heater has to replace the heat the greenhouse loses on a still, cloudy night, when the sun gives nothing back. UGA Extension's Bulletin 792 and the NRAES Greenhouse Engineering handbook split that loss three ways, and the calculator follows them line by line.

Through the glazing and walls

Each surface loses its area times its U-value times the temperature rise you want. The areas come from the same geometry the greenhouse designer draws: both roof slopes, the side and end walls, and the two gable triangles. For the 8x12 that is 363.3 sq ft of polycarbonate, and at a 30 F rise it accounts for 82% of the total. A lean-to leaves out the house wall, which is heated on the other side: the 6x12 lean-to needs 4,926 Btu/h on the same night.

Through air leaks

Cold air gets in around the door, the vent lids and the sheet edges. UGA and NRAES estimate it as 0.02 times the air volume times the number of air changes per hour times the rise. UGA's table gives 0.75 to 1 air change per hour for new glass or fiberglass houses, and its worked example uses 1. A sheltered, well sealed house can use 0.75; on a windy hill UGA adds about 20% to the whole figure.

Along the base

Heat also leaks into the ground along the edge of the floor: 0.8 Btu per hour per foot of perimeter per degree for an uninsulated base, from both sources. It is a small line, 960 Btu/h on the 8x12, but it is real.

The U-values we use

SurfaceUWhere it comes from
6 mm twin-wall polycarbonate0.62Palram lists 3.5 W/m2 K for 6 mm SUNLITE twin wall, which is 0.62 Btu per hour per sq ft per F (x 0.1761)
8 mm twin-wall polycarbonate0.58Palram lists 3.3 W/m2 K for 8 mm SUNLITE twin wall, which is 0.58 Btu per hour per sq ft per F (x 0.1761)
10 mm twin-wall polycarbonate0.51Palram lists 2.9 W/m2 K for 10 mm SUNLITE twin wall, which is 0.51 Btu per hour per sq ft per F (x 0.1761)
corrugated polycarbonate, one layer1.20No U-value is published for SUNTUF in the documents we read; we use UGA's R 0.83 for a single fiberglass reinforced panel (U 1.2), the same U that NRAES gives for single layer fiberglass and film
framed knee wall clad in T1-110.70UGA gives R 1.43 for 1/2 in plywood including surface coefficients (U 0.70); T1-11 is plywood siding, so we use that figure
Base perimeter, per foot0.80UGA and NRAES: 0.8 Btu per hour per foot of perimeter per F, uninsulated

Air leak settings: new, well sealed, sheltered from wind, UGA Table 2: new construction, glass or fiberglass, 0.75 to 1 air change per hour; low wind lowers it; new build, average wind, UGA Table 2 upper value for new construction, the rate UGA's worked example uses; exposed site, windy hill, UGA: about 20% more heat if the house is on a windy hill. NRAES gives 0.5 and Purdue's CEA program 0.55 Btu per hour per sq ft per F for double layer polycarbonate in general, and Purdue puts single polyethylene at about 1.2, the same U we use for one layer of corrugated sheet. We use Palram's own figures, 0.51 to 0.62 depending on thickness, because the designer specifies Palram SUNLITE sheet, and they sit on either side of the two general values. All of these U-values are modeling assumptions for the heat-loss arithmetic, not tested performance of the sheet in your greenhouse; the corrugated figure is a stand-in, because no U-value for SUNTUF is published in what we read.

Which temperatures to type in

Inside: what your plants need

UGA Extension puts the minimum for most situations at 40 F and calls 55 to 65 F at night adequate for most greenhouse plants. Set 40 F if all you want is to carry plants through frost; set the night temperature your crop needs if you grow through winter. Going from 40 F to 55 F inside on the same 10 F night takes the 8x12 from 7,701 to 11,552 Btu/h.

Outside: the design night, not the record

UGA's rule for sizing a heater is your average daily minimum temperature in January minus 15 F. If your January lows average 25 F, type 10 F. NRAES uses a similar margin, 10 F below the average minimum. Then check that the heater still keeps plants from freezing in your record cold: at 0 F outside, the 8x12 set to 55 F needs 14,119 Btu/h.

Choosing the heater

Electric

A plug-in 1,500 W heater gives 5,118 Btu/h. UGA notes that electric heat can be very costly and is usually used only in very small greenhouses; the calculator's night figure (watts times 12 hours, in kWh) lets you price a cold night at your own electricity rate. Any greenhouse circuit, like the fan circuit in our plans, is work for an electrician.

Gas, oil or kerosene

UGA warns that a gas, oil or coal heater needs a fresh air supply and an unobstructed chimney so carbon monoxide does not build up, and that all greenhouse heaters should be vented for plant and human health. Clemson Extension does not recommend unvented propane or kerosene heaters. Buy a vented unit rated at or above the calculator's figure.

Two units and a fan

UGA prefers two or more small heaters to one big one, so a failure does not freeze the house, and suggests an alarm for heater faults and power cuts. Clemson adds small circulating fans with a combined capacity of about one quarter of the greenhouse air volume per minute, so warm air does not sit at the ridge: about 175 cfm for the 8x12.

How to need a smaller heater

The glazing decides most of the load, so the cheapest heat is the heat you do not lose. On the same 8x12 and the same 40 F inside, 10 F outside night:

  • 8 mm twin wall: 7,701 Btu/h. 10 mm twin wall: 6,938. 6 mm: 8,137.
  • Single-layer corrugated polycarbonate: 14,459 Btu/h. NRAES notes that a second layer cuts conduction loss by about half.
  • A plain 24 in plywood knee wall: 7,989 Btu/h, slightly more, because uninsulated plywood (U 0.70) loses more than twin wall. Foam board in its stud bays turns it into a saving: NRAES lists 0.12 for softwood plus 2 in of urethane board.
  • A lean-to against a heated house wall, and a site out of the wind.

Read the full guide to keeping a greenhouse warm in winter, or size the summer side with the greenhouse ventilation calculator.

More free tools

Questions people ask

About 8,963 Btu/h, or 2,627 W, to hold 40 F inside on a 10 F night, for our 10x12 gable in 8 mm twin-wall polycarbonate on a timber base. Two 1,500 W electric heaters give 10,236 Btu/h. Change the walls, glazing or temperatures in the calculator and the figure follows.

Need less heat first. In the 8x12 plan, 82% of the loss goes through the glazing, so twin-wall over single-layer panels matters most: 14,459 Btu/h with corrugated sheet against 7,701 with 8 mm twin wall. Then keep the night setting as low as your plants allow and seal the vents and door.

Extension guides size summer exhaust fans to change the air 1 to 1.5 times a minute, and at least 8 cfm per sq ft of floor. That is a cooling question, separate from the heater: the ventilation calculator works it out for your size, with the intake shutter the fan needs.

UGA Extension says a night temperature of 55 to 65 F is adequate for most greenhouse plants, and 40 F if you only want frost protection. Every degree you hold above the outside temperature costs heat, so the calculator lets you compare both settings for your design.

UGA Extension prefers two or more small heating units to one large one, because two units still protect the plants if one fails, and suggests a temperature alarm as insurance against a heater fault or a power cut.

Sources

The rules in this guide that do not come from our own greenhouse designer were checked against these 6 pages and documents. Extension publications come first; builders' guides are marked as such and are only used where no extension source gives a number.

  1. Clemson HGIC 1704Clemson HGIC 1704, 06/99Read
  2. NRAESGreenhouse Engineering, NRAES-33 (R. A. Aldrich and J. W. Bartok Jr., 1994)Read
  3. PalramSUNLITE Technical Guide (07.2019)Read
  4. Purdue University Controlled Environment AgricultureCalculating Greenhouse Heating Requirements (A. Miller and K. Nemali)Read
  5. UGA Cooperative ExtensionGreenhouses: Heating, Ventilation, and Cooling, Bulletin 792 (J. W. Worley, revised by R. S. Ferrarezi, 2025)Read
  6. UGA ExtensionUGA Extension, B 910, 2023-11-17Read