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How much heat does a greenhouse really need?
A greenhouse looks warm on a sunny day, but at night it loses heat fast through every pane of glass or plastic. A heater that is too small leaves your plants cold on the first frosty night, while a heater that is far too big wastes fuel and cycles on and off all night. The honest answer sits in the middle, and it comes from three numbers: the size of your greenhouse, how well its glazing holds heat, and how cold it gets outside compared with the temperature you want inside.
This calculator works out that answer for you. Enter the length, width, and height of your greenhouse, pick the glazing type, and enter the coldest outside temperature you expect and the lowest inside temperature your plants can accept. The tool estimates the heat loss through the walls and roof, adds a small safety margin, and gives you a heater size in BTU per hour and in kilowatts, plus how many standard 5,000 BTU heaters that equals.
How the heating math works
Heat escapes through the greenhouse skin, so the first step is to work out that skin area. The calculator adds the four wall areas, which are twice the height times the length plus the width, and then adds the roof area, estimated at the floor area times 1.15 to allow for a pitched roof. It then multiplies that area by the glazing U-value, which measures how fast heat passes through the material, and by the temperature gap between inside and outside.
Single glass loses heat fastest, with a U-value near 1.1, while double-wall polycarbonate holds heat better at about 0.7. Single plastic film sits between them at about 1.0. A safety factor of 1.15 covers wind, small air leaks, and colder nights than the average you entered. The result is an estimate, not a guarantee, because wind speed, gaps around doors, and thermal mass from water barrels and soil all change real heat loss.
A worked example
Take a greenhouse 20 feet long, 12 feet wide, and 8 feet tall, glazed with single glass. The wall area is 512 square feet and the roof allowance is 276 square feet, giving 788 square feet of glazing in total. If you want 65 degrees inside when it is 30 degrees outside, the gap is 35 degrees. Multiply 788 by 1.1 and by 35, add the safety margin, and you get about 34,900 BTU per hour, or roughly 10.2 kilowatts. That is seven 5,000 BTU heaters, or one larger heater of the same total size.
Change one input and the answer moves a lot. Switch that same greenhouse to double polycarbonate and the need falls to about 22,200 BTU per hour. Let the inside temperature drop to 50 degrees instead of 65 and it falls further still. This is why tender tropical plants cost far more to heat than hardy greens: every extra degree of inside warmth must be paid for all night long.
Tips for cutting heating costs
The cheapest heat is the heat you never lose. Seal gaps around doors and vents with weather stripping, and add bubble-wrap insulation inside the glazing in winter, which can cut heat loss noticeably. A water barrel painted black absorbs sun by day and releases it slowly at night, smoothing the coldest hours. A thermal curtain drawn at dusk does the same job as adding glazing, and it costs far less than a bigger heater.
Common mistakes
The most common mistake is sizing the heater from floor area alone and ignoring height. A tall greenhouse has far more wall area than a low one of the same footprint, and heat rises, so tall buildings lose more. Another mistake is using the average winter night instead of the coldest night you expect. Heaters are bought for the worst night, not the average one, so use a realistic cold snap temperature.
People also forget ventilation. A sealed greenhouse with a gas or propane heater can build up fumes and humidity, so heaters need proper venting and the building still needs some air exchange. Never run an unvented combustion heater in a sealed greenhouse, and fit a carbon monoxide alarm if you burn fuel inside.
Safety and limits
Greenhouse heating results are general guidance estimates only. Local climate, wind, and soil conditions govern, and a heater must be installed and vented exactly as its maker instructs. Never exceed a heater’s rated output, keep heaters clear of plants and plastic film, and use a thermostat with a low-temperature alarm so a heater failure does not go unnoticed on a freezing night. When in doubt, ask a local heating supplier who knows your climate.
Frequently asked questions
Should I buy one big heater or several small ones? Several small heaters spread the warmth more evenly and keep some heat going if one fails. One big heater is cheaper to buy but leaves the far end cold and the whole crop at risk if it stops.
Use the calculator above with your real dimensions and your coldest expected night, compare the result with the heater sizes you can actually buy, and insulate first before you pay for extra heat. A well-sealed greenhouse with the right heater keeps plants alive through winter without wasting money on warmth that escapes through the glass.
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