Greenhouse Heater Size Calculator

How this is calculated

THE PHYSICS IS SIMPLE AND THE INPUTS ARE THE HARD PART. Heat loss through a surface is area x U-value x temperature difference. That is the whole calculation. What it needs from you is a U-value for your glazing and a design outside temperature - and both are specific to your greenhouse and your location in ways no calculator can guess. ⚠️ THE U-VALUE COMES FROM YOUR GLAZING MANUFACTURER. It differs substantially between single glass, twin-wall polycarbonate, triple-wall, horticultural film and old single film - and between products within each of those. It is published in the product's own data because it is a property of the material and its construction. A figure copied from a general table can easily be off by a factor of two, which is the difference between a heater that copes and one that does not. ⚠️ THE DESIGN TEMPERATURE IS A CHOICE, NOT A FACT. It is the coldest outside temperature you want to be able to hold your target against, and picking it is a decision about risk and money. Choosing your record low sizes a heater that is oversized almost every night of the year; choosing an average winter night sizes one that fails in a cold snap - which is the night it matters. Your meteorological service publishes design temperatures or percentile lows for your area, and those are the sensible basis. ⚠️ THIS IS A GLAZING-LOSS CALCULATION AND IT IS NOT THE WHOLE STORY. Three things it deliberately does not model, all of which push the real requirement UP: AIR LEAKAGE. Greenhouses leak, often substantially, through door gaps, vents, glazing laps and the joint at the base. On an older or poorly sealed structure infiltration can be a large share of total loss, and it is far more variable between structures than glazing loss is. GROUND AND PERIMETER LOSS, which depends on the base construction and the soil. WIND. Wind increases the effective heat loss of any surface, and an exposed site loses more than a sheltered one at the same air temperature. That is what the margin is for, and it is why erring generous is the sensible direction here. SOLAR GAIN IS THE OTHER SIDE AND IS NOT MODELLED EITHER. A greenhouse can overheat badly on a bright winter day, so ventilation is a separate design problem from heating and is not optional. The heater is sized for the worst night, not for the average day. THERMAL SCREENS AND BUBBLE INSULATION CHANGE THE ANSWER by changing the effective U-value - which is the honest way to model them here. If you are adding one, use the combined figure its manufacturer publishes rather than adjusting the result afterwards.

Common questions

Why won't it supply a U-value?

Because it is a property of your specific glazing and varies by a factor of two or more between materials - single glass, twin-wall polycarbonate, triple-wall, new horticultural film and tired old film are all substantially different, and products differ within each category. It is published in the manufacturer's own data because only they know how their product is constructed. Copying a figure from a general table is the one input error here that can double or halve the answer, which is the difference between a heater that copes on the coldest night and one that does not.

What outside temperature should I design for?

That is a decision about risk and cost rather than a fact to look up, which is why it is an input. Size for your all-time record low and you buy a heater that is oversized almost every night of its life. Size for an average winter night and it will fail during a cold snap - which is precisely the night the greenhouse needed it. Meteorological services publish design temperatures or percentile lows for exactly this purpose, and those are the sensible basis. Think about what is inside, too: a frost-free store and a propagation house have very different consequences for failure.

What is this calculation missing?

Three things, and all of them push the real requirement upward. AIR LEAKAGE through door gaps, vents, glazing laps and the base joint, which on an older or poorly sealed structure can be a large share of total loss and varies far more between greenhouses than glazing loss does. GROUND AND PERIMETER LOSS, which depends on the base construction. And WIND, which increases the effective loss of any surface, so an exposed site loses more than a sheltered one at the same air temperature. The margin exists for these, and erring generous is the sensible direction.

Why is the heat loss proportional to the temperature difference?

Because that is how heat flows through a surface - the driving force is the difference between the two sides, so doubling the gap doubles the loss. That has a practical consequence worth acting on: lowering your target by a few degrees saves proportionally, and it is often the cheapest change available. Holding a greenhouse at 45°F rather than 50°F against a 20°F night cuts the difference from 30 to 25, which is a sixth off the heating requirement for no capital cost at all. The loss-per-degree output is there to make that trade easy to look at.

Does it account for sunshine?

No, deliberately. A greenhouse can gain a great deal of heat on a bright day, including in winter, but a heater is sized for the worst NIGHT rather than the average day - the design case is the cold, still, dark hours when there is no gain at all. Solar gain matters enormously for the opposite problem: greenhouses overheat easily, and ventilation is a separate design question that is not optional. The two problems share a building and almost nothing else.

What about a thermal screen or bubble insulation?

The honest way to model those here is through the U-value, because that is what they change. If you are fitting a thermal screen or lining with bubble insulation, use the combined U-value that its manufacturer publishes for that assembly rather than calculating the bare glazing and discounting the answer afterwards. They can make a substantial difference - reducing the effective U-value directly reduces every figure on this page in proportion - and they are usually far cheaper than the extra heating capacity they save.

Should I round the heater size up?

Yes, and not only because of the omissions above. Heaters come in fixed sizes, a heater running flat out continuously has no reserve for a colder-than-design night, and thermostatic control means an oversized heater simply runs less rather than wasting its capacity. The exception worth noting is that a very oversized heater can cycle in a way that gives uneven temperatures, so round up to the next available size rather than doubling. For electric heaters, check the circuit can actually carry it - the kilowatt figure is what matters there.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Greenhouse Heater Size Calculator at https://www.bfcbrilliance.com/tools/greenhouse-heating-calculator.

What I entered:
- Length (ft): ___
- Width (ft): ___
- Height to the eaves (ft): ___
- Extra roof area over a flat top (%): ___
- U-value of your glazing (BTU/hr·ft²·°F): ___
- Temperature to hold inside (°F): ___
- Design outside low (°F): ___
- Margin for leakage, wind and ground loss (%): ___

What it calculated:
- Heater size to look for: ___
- Heat loss through the glazing alone: ___
- Same heater size in kilowatts: ___
- Glazed surface area: ___
- Temperature difference you are holding: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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