What this calculator works out
This calculator estimates the heat needed to hold a greenhouse above the outside temperature, from its glazed surface area, the glazing's U-value and the temperature lift required.
It works from the glazed area rather than the floor area, because heat is lost through the glass — and a greenhouse has several times more glass than floor.
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Default unit rates are the Ofgem price cap average for Great Britain, Direct Debit, 1 July to 30 September 2026, including VAT. The cap is reviewed quarterly — use the rate from your own bill where it differs.
Before you size a heater
- Eaves height matters as much as footprint. A tall greenhouse has considerably more glass than a low one of the same floor area.
- The default U-value of 5.8 W/m²K is typical for single glazing. Bubble wrap lining roughly halves it; twin-wall polycarbonate is lower again.
- Temperature difference is the lift you need, not the outside temperature. Holding 5°C when it is −3°C outside is an 8°C lift.
- The figure is for a well-sealed structure. Draughts through gaps, vents and doors can add substantially.
Holding a greenhouse just above freezing costs a fraction of holding it at a growing temperature, because the heat loss is proportional to the temperature difference. Most overwintering only needs frost protection, and heating to 10°C when 2°C would do can multiply the running cost several times over.
How this calculator works
Surface area times U-value times temperature difference:
Wall area = 2 × (length × height) + 2 × (width × height)
Roof area ≈ length × width × 1.15
Heat needed in watts = total glazed area × U-value × temperature differenceThe roof factor of 1.15 approximates the extra area of a pitched roof over its plan footprint. It is an approximation — a steeply pitched roof has more.
The running cost assumes 25p per kilowatt-hour and continuous operation at full output. A thermostatically controlled heater cycles, so real consumption is lower and depends on how cold the winter is.
Worked example: a small greenhouse
Using the default figures — 2.4 m by 1.8 m, 2 m to the eaves, a 10°C lift, single glazing at 5.8 W/m²K:
- Glazed area: 21.8 m²
- Heat needed: 1,263 watts
- Running cost at full output: £0.33 an hour
Now line it with bubble wrap, halving the U-value to about 2.9. The requirement falls to 631 watts and 16p an hour. Bubble wrap costs a few pounds and halves the heating bill — which is a considerably better investment than a larger heater, and the first thing to do before sizing one.
Common mistakes
- Sizing from floor area. A greenhouse has several times more glass than floor.
- Heating to a growing temperature when frost-free would do. The cost difference is large.
- Ignoring draughts. Gaps around vents and doors can add substantially to the loss.
- Not insulating first. Bubble wrap lining halves the requirement for a few pounds.
- Using an unsuitable heater. Paraffin and gas heaters release water vapour, raising humidity and disease risk.
Frequently asked questions
What temperature does a greenhouse need?
It depends entirely on what is in it. Frost-free, meaning just above 0°C, protects most overwintering plants and is by far the cheapest option. 5°C suits tender perennials and citrus. 10°C or more is needed for propagation and tropical plants and costs several times as much to maintain.
What kind of heater is best?
Electric fan heaters with a thermostat are the usual choice — they respond quickly, add no moisture and are controllable. Electric tube heaters suit gentle background warmth. Paraffin and bottled gas heaters release water vapour and combustion products, which raises humidity and disease risk considerably, though they need no electricity supply.
How much does greenhouse heating cost?
It depends on the winter, the target temperature and the insulation. A small single-glazed greenhouse held frost-free through a UK winter might use a few hundred kilowatt-hours; held at 10°C it could be several times that. Insulating first, and heating only the space that needs it, both help substantially.
Can I heat only part of a greenhouse?
Yes, and it is often the sensible approach. Partitioning off a section with bubble wrap or fleece, or using a heated propagator for seedlings rather than warming the whole structure, cuts the heated volume dramatically. Horticultural fleece over plants adds a few degrees of protection on cold nights for almost nothing.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- Royal Horticultural Society — greenhouse management, insulation and overwintering
- Health and Safety Executive — electrical safety in outbuildings and safe use of fuel-burning heaters
Sources are checked at publication and can change — how I choose and check references.