What Size Heater for a Greenhouse?

The calculation is one line

heat loss = area × U-value × temperature difference

That's it. What makes it hard isn't the arithmetic — it's that two of those three inputs are specific to your greenhouse and your location in ways no calculator can guess.

The area is the easy one. Enter Length, Width and Height to the eaves and the tool adds four walls plus a roof. Measure inside rather than out, and add a roof allowance if yours is pitched or hooped — a curved roof has meaningfully more surface than the footprint it covers. The floor isn't counted, because it loses heat to the ground rather than to the air, on a different and much smaller path.

The U-value has to come from your glazing

It varies by a factor of two or more between single glass, twin-wall polycarbonate, triple-wall, new horticultural film and tired old film — and between products within each category.

It's published in the manufacturer's own data, because only they know how their product is built.

This is the one input error that can double or halve your answer. Halving the U-value in the tool takes the loss from 12,883 to 6,442 BTU/hr — the difference between a heater that copes on the coldest night and one that doesn't.

The design low is a choice, not a lookup

Design outside low is the coldest temperature you want to hold your target against, and picking it is a decision about risk and money.

• Size for your record low and the heater is oversized almost every night of its life
• Size for an average winter night and it fails during a cold snap — precisely the night the greenhouse needed it

Meteorological services publish design temperatures or percentile lows for exactly this purpose. Think about what's inside too: a frost-free store and a propagation house have very different consequences for failure.

What this doesn't include

Three things, all pushing the real requirement up:

• Air leakage — door gaps, vents, glazing laps, the base joint. On an older or poorly sealed structure this can be a large share of total loss, and it varies far more between greenhouses than glazing does.
• Ground and perimeter loss, depending on base construction.
• Wind — an exposed site loses more than a sheltered one at the same air temperature.

That's what the margin is for, and it's why erring generous is right here.

Seal it before you size it. Sealing is nearly always cheaper than the heating capacity it saves, and it should happen before you choose the heater rather than after.

The cheapest saving available

Loss is directly proportional to the temperature gap. Halve the gap, halve the heat.

Holding 45°F instead of 50°F against a 20°F night takes the difference from 30 to 25 — a sixth off the requirement, for no capital cost whatsoever. The tool confirms it: 12,883 drops to 10,736 BTU/hr.

Heat loss per degree of difference is your greenhouse's own constant. Multiply it by any gap to see what a different target would cost, which makes that trade easy to look at before you spend money on capacity.

Ask what the plants actually need before assuming the round number.

Thermal screens work through the U-value

If you're fitting a thermal screen or lining with bubble insulation, use the combined U-value its manufacturer publishes for that assembly — don't calculate the bare glazing and discount the answer afterwards.

Reducing the effective U-value reduces every figure on the page in proportion, and screens are usually far cheaper than the extra heating capacity they save.

Sunshine is a different problem

The tool doesn't model solar gain, deliberately. A heater is sized for the worst night — the cold, still, dark hours when there's no gain at all.

Solar gain matters enormously for the opposite problem: greenhouses overheat easily, including in winter, and ventilation is a separate design question that isn't optional. The two problems share a building and almost nothing else.

Round up

Heaters come in fixed sizes, one 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 capacity.

Round to the next available size rather than doubling — a very oversized heater can cycle in a way that gives uneven temperatures. And for electric, check the circuit can carry it: Same heater size in kilowatts is the figure that matters there.