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Greenhouse heater BTU chart

The short answer

A 6 by 8 ft twin-wall polycarbonate greenhouse needs roughly 8,600 BTU per hour to hold a 40 degree Fahrenheit difference between inside and outside, which is beyond what a 1500 W electric heater can supply at 5,120 BTU per hour. The formula is BTU per hour = glazed surface area x U-factor x temperature difference, and it is surface area rather than floor area that governs.

Formula
A x U x deltaT
1500 W equals
5,120 BTU/hr
Sizes charted
12 footprints
Glazing assumed
4 mm twin-wall

Researched from published specifications and verified owner reviews · Updated 2026-08-16

Greenhouse heater sizing is one calculation, and almost every rule of thumb you will find online gets it wrong in the same way: by using floor area. Heat leaves through the glazing, not through the floor, and a long narrow house has substantially more glazed surface than a square one of identical floor area. Use the envelope.

The formula is the standard conduction form. BTU per hour equals glazed surface area in square feet, multiplied by the glazing U-factor, multiplied by the temperature difference in degrees Fahrenheit that you want to maintain. An infiltration allowance of 1.1 for a new tight structure, or up to 1.25 for an older film house with loose doors, accounts for air leaking through gaps.

The formulaA x U x dT

Glazed surface area in sq ft, multiplied by the glazing U-factor, multiplied by the inside-to-outside temperature difference in degrees F. Multiply the result by 1.1 for infiltration on a tight new structure.

The temperature difference is the part people get wrong second. It is not the difference between your target and the average winter temperature. It is the difference between your target and the coldest night you intend to protect against. If you want 45 degrees Fahrenheit inside and your design low is 5 degrees, your differential is 40, not 20.

Figure 1. Required heater output in BTU per hour, 4 mm twin-wall polycarbonate · 12 rows
Figure 1. Required heater output in BTU per hour, 4 mm twin-wall polycarbonate
SizeFloorEnvelope sq ft20 degF diff30 degF diff40 degF diff50 degF diff
4 x 6 ft 24 sq ft 158 2,300 3,400 4,500 5,700
6 x 4 ft 24 sq ft 156 2,200 3,400 4,500 5,600
6 x 6 ft 36 sq ft 194 2,800 4,200 5,500 6,900
6 x 8 ft 48 sq ft 231 3,300 5,000 6,600 8,300
6 x 10 ft 60 sq ft 269 3,800 5,800 7,700 9,600
8 x 8 ft 64 sq ft 271 3,900 5,800 7,800 9,700
8 x 10 ft 80 sq ft 312 4,500 6,700 8,900 11,200
8 x 12 ft 96 sq ft 353 5,100 7,600 10,100 12,600
10 x 12 ft 120 sq ft 401 5,700 8,600 11,500 14,300
10 x 16 ft 160 sq ft 490 7,000 10,500 14,000 17,500
10 x 18 ft 180 sq ft 534 7,600 11,500 15,300 19,100
12 x 20 ft 240 sq ft 642 9,200 13,800 18,400 23,000
Assumes a 5.5 ft eave, an 8 ft ridge and an infiltration allowance of 1.1. A 1500 W electric heater delivers 5,120 BTU per hour, so any figure above that needs a 240 V unit, a gas heater, better insulation or a lower target temperature.

What can a 1500 W heater actually hold?

A standard 120 volt circuit supports a 1500 watt heater and no more, which is 5,120 BTU per hour. Reading that figure across the chart above tells you the honest limit of single-circuit electric heating: a 6 by 6 house at a 30 degree differential, a 6 by 8 at about 25 degrees, an 8 by 10 at barely 15. The Bio Green Palma 1500 W Greenhouse Heater with Digital Thermostat is the purpose-built version of that heater, with a splash-resistant body and a digital thermostat, and it is the correct choice right up to the point where the arithmetic says it is not enough.

Past that limit there are three honest options and one dishonest one. The honest ones: run a 240 volt circuit and fit a heater like the Bio Green Phoenix 2.8 kW 240 V Greenhouse Heater at 9,553 BTU per hour, insulate the envelope so the required output drops below the limit, or lower the target temperature to frost protection rather than growing warmth. The dishonest one is plugging in a second 1500 W heater on the same circuit, which trips the breaker on the first cold night, usually at 3 am.

Watch out

Frost-free and growing-warm are different products

Holding 35 to 40 degrees Fahrenheit, which keeps hardy plants alive and pipes intact, is a completely different heating bill from holding 60 degrees so tomatoes keep growing. On a 6 by 8 house at a 20 degree outside low, the first needs about 1,100 BTU per hour and the second about 8,600. Decide which one you are actually buying before you size anything.

How glazing changes the answer

The chart above assumes 4 mm twin-wall polycarbonate, which is what most hobby kits ship with. Better or worse glazing scales every figure in the table by the ratio of U-factors. Multiply by 1.77 for 6 mil single film, by 1.62 for single glass, by 0.89 for 6 mm twin-wall, by 0.74 for 8 mm triple-wall.

Figure 2. The same 8 x 12 ft house at a 40 degree F differential, by glazing · 12 rows
Figure 2. The same 8 x 12 ft house at a 40 degree F differential, by glazing
GlazingU-factorBTU per hourElectric equivalent
Single polyethylene film, 6 mil 1.15 17,900 5,250 W
Double polyethylene film, inflated 0.59 9,200 2,700 W
Single horticultural glass, 3 mm 1.05 16,300 4,800 W
Double insulated glass unit 0.50 7,800 2,300 W
Corrugated single-wall polycarbonate, 0.8 mm 1.20 18,600 5,450 W
Twin-wall polycarbonate, 4 mm 0.65 10,100 2,950 W
Twin-wall polycarbonate, 6 mm 0.58 9,000 2,650 W
Twin-wall polycarbonate, 8 mm 0.53 8,200 2,400 W
Triple-wall polycarbonate, 8 mm 0.48 7,500 2,200 W
Five-wall polycarbonate, 16 mm 0.40 6,200 1,800 W
Double-wall acrylic, 16 mm 0.50 7,800 2,300 W
Fiberglass reinforced panel 1.20 18,600 5,450 W
A 96 sq ft house has roughly 530 sq ft of envelope. Note that no glazing choice brings this footprint within reach of a single 120 V circuit at this differential.

Reducing the number before you buy the heater

Insulation is almost always cheaper per BTU saved than heat is per BTU supplied, and it goes on working for free every night thereafter. Three measures, in the order they pay back. First, reflective bubble insulation on the north wall and any knee wall: those surfaces contribute heat loss and almost no useful light, so insulating them is close to free. Second, seal the door. A hobby greenhouse door with a 6 mm gap around three sides leaks roughly as much heat as a missing panel. Third, thermal mass.

Thermal mass does not reduce total heat loss, it flattens the curve. Water stores about 8.34 BTU per gallon per degree Fahrenheit, so two 55 gallon barrels along the north wall hold roughly 920 BTU for every degree they swing. Painted black and placed where winter sun falls on them, they absorb during the day and release overnight, typically lifting the pre-dawn minimum by several degrees at zero running cost. The full treatment is in passive solar and thermal mass.

Heating equipment sized to the chart

Three heaters covering the three regions of the chart, plus the controller that makes any of them cycle on setpoint rather than run continuously.

Converting BTU to a heater you can actually buy

Heaters are sold in three different units and the conversions are worth memorizing. Electric heaters are rated in watts, and one watt is 3.412 BTU per hour, so 1500 W is 5,120 BTU per hour and a 2.8 kW unit is 9,553. Gas heaters are rated directly in BTU per hour, but check whether the figure is input or output: a vented gas heater at 80 percent efficiency with an 11,000 BTU input delivers about 8,800 BTU of usable heat. Paraffin and kerosene heaters are usually rated in BTU as well, and their combustion products, including a substantial amount of water vapor, all end up inside the house.

Figure 3. Common heater ratings in both units · 6 rows
Figure 3. Common heater ratings in both units
HeaterElectricalBTU per hourCircuit neededHolds a 6 x 8 house at
Small oil-filled radiator 700 W 2,388 120 V, 15 A about 11 degF above outside
Standard greenhouse fan heater 1,500 W 5,118 120 V, 15 A dedicated about 24 degF above outside
Two 1,500 W units 3,000 W 10,236 120 V, 30 A or two circuits about 48 degF above outside
240 V greenhouse heater 2,800 W 9,554 240 V, 20 A about 44 degF above outside
Larger 240 V unit 4,000 W 13,648 240 V, 30 A about 63 degF above outside
Direct-vent propane wall heater Fan only 11,000 input, about 8,800 output 120 V for the fan about 41 degF above outside
The final column assumes a 6 by 8 ft house glazed in 4 mm twin-wall with an infiltration allowance of 1.1. Two 1500 W heaters cannot share one 15 A circuit; they need either a 30 A circuit or two separate ones.

Why you should not oversize the heater

The instinct is to buy the next size up for safety, and with greenhouse heating that instinct costs you twice. An oversized heater short-cycles: it reaches setpoint quickly, shuts off, and the house cools back through the thermostat deadband within minutes, so the unit spends its life starting and stopping. Fan bearings and relay contacts are what wear out from that, not heating elements.

The horticultural cost is worse. A large heater blasting a small volume creates a hot dry stream in front of it and a cold pocket at the far end, so a house nominally at 50 degrees Fahrenheit has a 70 degree zone by the outlet and a 40 degree zone at the opposite gable. Plants sitting in the airstream dry out and scorch while plants ten feet away are cold. Sizing close to the calculated requirement and adding internal circulation is the better answer than adding output.

Field tip

Two smaller heaters beat one large one

If the calculated requirement is above what one unit can supply and you have the circuits, two moderate heaters at opposite ends of the house give far more even temperature than one large unit at one end. They also give you redundancy: a single heater failure on the coldest night of the year is a total loss, whereas losing one of two is a survivable inconvenience.

Work your own figure rather than reading the nearest row: the heater BTU calculator takes your dimensions, glazing, ridge height, target and design low. Then check the running cost against reality in heating a greenhouse in winter, compare fuel types in electric versus propane, and see the safety requirements in greenhouse electrical and safety.

Related on this site

Common questions

6 answers

+ How many BTU do I need to heat a 10 by 12 greenhouse?

A 10 by 12 ft house glazed in 4 mm twin-wall polycarbonate has roughly 590 square feet of envelope. At a 30 degree Fahrenheit differential it needs about 12,700 BTU per hour, and at 40 degrees about 16,900. Both figures are far beyond a 1500 W electric heater. At this size you are looking at a 240 volt electric unit, a vented gas heater, or accepting frost protection rather than growing temperatures.

+ Can I heat a greenhouse with a 1500 W space heater?

Up to a point. 1500 W delivers 5,120 BTU per hour, which will hold a 6 by 6 house about 30 degrees Fahrenheit above outside, or a 6 by 8 about 25 degrees. Use a unit rated for damp locations with a proper thermostat rather than a living-room space heater, and put it on its own circuit with ground-fault protection. Two 1500 W heaters on one 120 volt circuit will trip the breaker.

+ Should I size the heater on floor area or surface area?

Surface area, always. Heat leaves through the glazing, so the glazed envelope is what governs. Floor area rules of thumb fail badly on long narrow houses, which have far more envelope per square foot of floor than square ones, and on tall houses, where the extra wall height adds loss that a floor-area rule never sees. A 6 by 10 and an 8 by 8 have nearly the same floor area and noticeably different envelopes.

+ What temperature should I keep a greenhouse at in winter?

It depends entirely on what is in it. Frost protection at 35 to 40 degrees Fahrenheit keeps hardy plants, overwintering perennials and stored tubers alive at a modest cost. A cool house at 45 to 50 degrees suits citrus, geraniums and winter salad. A warm house at 60 to 65 degrees keeps tomatoes and peppers growing but costs roughly eight times as much to run as frost protection on the same structure.

+ Does a bigger greenhouse cost more to heat per square foot?

No, less. Envelope area grows more slowly than floor area as a house gets larger, because the two gable ends are already paid for. A 6 by 6 house has about 8.7 square feet of envelope per square foot of floor; a 10 by 18 has about 5.1. That makes large houses proportionally cheaper to heat per growing square foot, which is a real argument for buying big that most buying advice misses.

+ How much does it cost to run a greenhouse heater?

Multiply the required BTU per hour by the hours the heater actually runs, which is far less than the total cold hours because the thermostat cycles. As a rough figure, a 1500 W heater cycling at 50 percent through a 14 hour night uses about 10.5 kilowatt hours, so at 16 cents per kilowatt hour that is roughly $1.70 a night. Insulation and thermal mass reduce the duty cycle rather than the rated output.

Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.