Free greenhouse heater calculator: enter greenhouse size, glazing, and inside and outside temperatures to get heater size in BTU per hour and watts.
Your results will appear here.
Enter your values and press Calculate.
Enter the greenhouse length, width, and wall height in feet, choose the glazing material, and enter the inside temperature you want to hold and the coldest outside temperature you expect. Pick the wind exposure honestly: a greenhouse on an open hill loses far more heat than one sheltered by walls and hedges. The calculator works out the glazed area, applies the glazing U-value and the temperature difference for conduction loss, adds a wind and infiltration allowance, and gives the heater size in BTU per hour, kilowatts, and watts. Size for the coldest night, not the average night, or the heater will fall behind exactly when plants need it most.
Heat escapes through the glazing in proportion to three things: how much glazing there is, how easily it passes heat, and how big the temperature gap is. The formula is heat loss equals glazed area times U-value times temperature difference. Glazed area here is the two long walls plus the two end walls plus the roof, approximated as length times width. The U-value measures heat passed per square foot per degree: single glass is about 1.13, single polycarbonate about 1.20, double polycarbonate about 0.60, and double polyethylene about 0.70 BTU per hour per square foot per degree Fahrenheit. A lower U-value means better insulation, which is why double glazing roughly halves the heater size for the same greenhouse and temperatures.
This heating load calculation is the basis for greenhouse heater sizing: size the heater to the total heat loss, not to floor area alone. The watts to BTU conversion is watts times 3.412, and the BTU to kW conversion is BTU per hour divided by 3412, so a 15,100 BTU per hour load is 4.43 kW.
Measure length and width at the base, and wall height to the eaves, not to the ridge, since this calculator approximates the roof as a flat top. Choose the glazing that covers most of the greenhouse; if you have mixed glazing, pick the worse material, because heat escapes fastest through the weakest part. Enter the inside temperature your plants need at night, commonly about 55 degrees Fahrenheit for frost-free growing, and the coldest outside temperature your area reaches, not the average winter low. Choose wind exposure, press Calculate, and buy a heater at or slightly above the result: a heater exactly at the result has no margin for a colder night or a draughty door seal.
Conduction is heat passing straight through the glazing, and it is the larger share in most greenhouses. Infiltration is heat carried away by cold air leaking in through gaps, doors, and vents, plus wind stripping warm air from the surface. This calculator separates them: the conduction figure is area times U-value times temperature difference, and the wind allowance adds 0 percent sheltered, 15 percent moderate, or 30 percent exposed on top. Sealing gaps, adding draught strips to doors, and closing vents properly on cold nights can cut infiltration a lot, but no sealing makes a single glazed greenhouse behave like a double glazed one.
Air changes per hour measure how often the greenhouse air is replaced by outside air through gaps and vents: a draughty house can exceed one full change per hour on a windy night, which is why the wind allowance matters. The covering material sets the U-value, and a torn or loose cover leaks far more than its rated U-value suggests.
Double polycarbonate roughly halves heat loss against single glass, which is why it pays for itself in cold climates even though the sheets cost more. Double polyethylene with an inflated air gap between layers performs similarly. Bubble wrap insulation clipped inside single glazing in winter is the cheap version of the same idea: it adds an air layer and can cut heat loss noticeably, at the cost of some light. Insulating the north wall, which gets the least sun in northern latitudes, is another cheap win: a solid insulated north wall loses far less heat than glazing that faces away from the sun. None of these tricks change the formula, they change the U-value and the area, which is exactly what the calculator measures.
Electric fan heaters are the simplest choice for small greenhouses: they are cheap, thermostat controlled, and spread warm air evenly. Propane heaters suit larger greenhouses and places without enough electrical capacity, but they burn oxygen, produce moisture, and need ventilation, so never run an unvented gas heater in a sealed greenhouse. Always use a thermostat, and place it away from the heater and out of direct sun, or it will switch off while plants elsewhere freeze. Never leave a heater running unattended without checking it first: keep it clear of plants, pots, and flammable material, use a heater rated for greenhouse or outdoor use, and add a backup heater or a frost alarm if a single failure would lose valuable plants.
Greenhouse heater safety starts with clearance: keep the heater clear of plants, pots, and flammable material, and never drape anything over it. Condensation and humidity rise when a gas heater burns, because combustion adds water vapour to the air, so vent gas heaters and ventilate on mild days even in winter.
An oversized heater short cycles, switching on and off rapidly, which wastes energy and wears out controls, but it still holds temperature. An undersized heater runs constantly on the coldest nights and still loses ground, which is how crops are lost: the greenhouse slowly cools below the safe temperature while the heater does its best. Slightly oversizing is therefore safer than undersizing, within reason. Running cost follows from the kilowatt figure: multiply kilowatts by hours run per day and days per month, then by your electricity price per kilowatt hour. Electric heat is simple to price this way; propane needs the burner efficiency and the fuel price per gallon, so compare total monthly cost, not just heater price.
A heater size chart by greenhouse size is only a shortcut: an 8x10 greenhouse with single glass needs about 12,000 BTU per hour at a 35 degree difference with moderate wind, while the same house in double polycarbonate needs about half. Oversized heaters suffer short cycling, switching on and off rapidly, while undersized heaters cause crop loss on the coldest nights, so a small margin above the calculated load is the safe choice.
A 10 by 8 foot greenhouse, 7 foot walls, single glass, 55 degrees inside, 20 degrees outside, moderate wind: glazed area is 2 × 10 × 7 + 2 × 8 × 7 + 10 × 8 = 332 square feet. Temperature difference is 35 degrees. Conduction is 332 × 1.13 × 35 = 13,130.60 BTU per hour. With the moderate wind factor of 1.15, total heat loss is 15,100.19 BTU per hour, which is 4.43 kilowatts or about 4,425 watts. A 5 kilowatt electric heater, or two smaller heaters totalling at least 4.5 kilowatts, would cover this greenhouse with a small margin.
Multiply glazed area by the glazing U-value and the temperature difference, then add a wind allowance. A 10 by 8 foot single glass greenhouse holding 55 degrees against 20 degrees outside with moderate wind needs about 15,100 BTU per hour, or 4.43 kilowatts. Enter your dimensions above for your figure.
It depends on glazed area, glazing, and temperature difference. Small hobby greenhouses often need 5,000 to 20,000 BTU per hour, while large commercial houses need hundreds of thousands. The calculator gives your BTU figure from your own dimensions and temperatures.
Heat loss equals glazed area times U-value times temperature difference, times a wind factor. Glazed area is walls plus roof, U-value depends on glazing material, and temperature difference is inside minus outside temperature. The calculator shows each step.
Single glass is about 1.13, single polycarbonate about 1.20, double polycarbonate about 0.60, and double polyethylene about 0.70 BTU per hour per square foot per degree Fahrenheit. Lower is better: double glazing roughly halves heat loss against single glazing.
About 55 degrees Fahrenheit keeps most plants frost-free and growing slowly. Tender tropical plants may need 60 degrees or more, while hardy plants can survive near freezing. Higher inside temperatures raise the heater size directly, because the temperature difference grows.
Multiply the kilowatt result by hours run per day, days per month, and your electricity price per kilowatt hour. A 4.43 kilowatt heater running 10 hours a day for 30 days uses about 1,329 kilowatt hours, so at 20 cents per unit that is about 266 dollars a month. Real cost varies with weather and thermostat settings.
Only if you are growing through winter or protecting tender plants. Many gardeners heat only on frost nights, or only part of the greenhouse with a heated propagator, which costs far less than holding the whole house at growing temperature all winter.
Electric fan heaters suit small greenhouses: they are cheap, thermostat controlled, and spread heat evenly. Propane suits larger houses without enough electrical capacity, but needs ventilation and produces moisture. Choose by size, fuel availability, and ventilation, not by heater price alone.
Yes, with propane or paraffin heaters, thermal mass such as water barrels that store daytime heat, and heavy insulation including bubble wrap and an insulated north wall. None of these match an electric heater for control, so combine them with a frost alarm if plants are valuable.
Yes, if the heater is rated for greenhouse use, kept clear of plants and flammable material, and controlled by a thermostat placed away from the heater. Add a backup heater or a low temperature alarm if a single failure would lose valuable plants.
Yes, through glazed area. A tall greenhouse has more wall area per square foot of floor than a low one, and a long narrow house has more wall area than a square house of the same floor area. This calculator measures area from your actual dimensions, so shape is already included.
Wind strips warm air from the glazing and pushes cold air through gaps, raising heat loss. This calculator adds 0 percent for sheltered sites, 15 percent for moderate exposure, and 30 percent for exposed sites. Sealing gaps and adding windbreaks cut the same loss the wind factor adds.