Greenhouse Heating Calculator
Sizing a greenhouse heater correctly matters: too small and you risk losing plants to frost on a cold UK night, too large and you waste money running an oversized heater. UK winter lows regularly drop below freezing, and the wattage you need depends heavily on your greenhouse's size and how well its glazing holds heat. Enter your dimensions below to get an instant estimate for frost protection, a cool greenhouse, or a warm, tropical-growing environment.
Surface area: 29.5 m² · Design temperature difference: 7°C (from an assumed UK winter low of -5°C)
You'll need approximately 1,250W of heating
That points to roughly a 1kW–1.5kW heater for this greenhouse.
This is a simplified estimate for planning purposes only, based on a basic box-shaped model. It doesn't account for insulation upgrades, wind exposure, ventilation gaps or double-wall polycarbonate thickness — for exposed or unusually cold sites, size up.
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Frequently asked questions
What temperature should I keep my greenhouse at in winter?
It depends on what you’re growing. A frost-free greenhouse kept above 2°C will protect tender plants and pots from freezing. A cool greenhouse at a minimum of 7°C suits many overwintering vegetables and young plants. A warm or tropical greenhouse kept above 13°C is needed for exotic or heat-loving plants through a UK winter.
Is polycarbonate or glass better insulated?
Twin-wall polycarbonate traps air between its layers and insulates roughly twice as well as single-glazed glass, so a polycarbonate greenhouse typically needs a smaller heater than a glass one of the same size to hit the same target temperature.
How accurate is this calculator?
It gives a planning-level estimate using a simplified box-shaped model and an assumed UK winter low of -5°C. It doesn’t account for wind exposure, draughts, insulation upgrades like bubble wrap, or unusually cold microclimates, so treat the result as a starting point and size up if your site is exposed.
Can I reduce the heater size I need?
Yes. Lining a greenhouse with horticultural bubble wrap, sealing gaps and draught-proofing the door, using a windbreak, and grouping plants together can all cut heat loss and let you use a smaller heater than the raw calculation suggests.
How this calculator works
The calculator uses a simplified box-shaped model: it calculates the total surface area of your greenhouse (two walls × length × height, two walls × width × height, plus the roof area length × width), then multiplies that by a heat loss factor and a temperature difference. The heat loss factor is 6 W/m²/°C for single-glazed glass and 3.5 W/m²/°C for polycarbonate, reflecting the fact that polycarbonate traps air between its layers and insulates roughly twice as well as glass. The temperature difference is your target minimum temperature minus an assumed UK winter low of -5°C.
The result is rounded up to the nearest 50W and matched against standard UK greenhouse heater sizes (500W, 1kW, 1.5kW, 2kW, 3kW). This is a planning-level estimate only — it doesn't account for wind exposure, draughts, insulation upgrades like bubble wrap, or unusually cold microclimates. If your greenhouse is exposed, add 20-30% to the calculated wattage.
Greenhouse heating requirements by size and glazing
| Greenhouse size | Glass (frost-free) | Polycarbonate (frost-free) | Glass (cool, 7°C) | Polycarbonate (cool, 7°C) |
|---|---|---|---|---|
| 2m x 1.5m x 2m | ~500W | ~300W | ~700W | ~500W |
| 3m x 2m x 2m | ~1,000W | ~600W | ~1,500W | ~900W |
| 4m x 3m x 2m | ~1,500W | ~900W | ~2,000W | ~1,200W |
| 6m x 4m x 2.5m | ~2,500W | ~1,500W | ~3,500W | ~2,000W |
| 8m x 6m x 2.5m | ~4,000W | ~2,500W | ~5,500W | ~3,500W |
Worked example: A 3m x 2m x 2m glass greenhouse, frost-free
A greenhouse 3m long, 2m wide and 2m high has a surface area of 2 × (3 × 2) + 2 × (2 × 2) + (3 × 2) = 12 + 8 + 6 = 26m². For a frost-free target of 2°C, the temperature difference is 2 - (-5) = 7°C. Using glass (heat loss factor 6), the required wattage is 26 × 6 × 7 = 1,092W, rounded up to 1,100W. The calculator would suggest a 1kW heater as the minimum, with 1.5kW giving more headroom on very cold nights.
If the same greenhouse were made of polycarbonate (heat loss factor 3.5), the calculation would be 26 × 3.5 × 7 = 637W, rounded up to 650W — a 500W heater would be the minimum, with 1kW providing comfortable margin. This shows why glazing type matters: a polycarbonate greenhouse of the same size needs roughly half the heating power of a glass one to maintain the same temperature.
Common mistakes when heating a greenhouse
- Buying a heater based on the greenhouse's floor area alone. A 3m x 2m greenhouse has 6m² of floor but 26m² of surface area (walls and roof) that loses heat. Always size based on the total surface area, not just the footprint.
- Forgetting to insulate before heating. A layer of horticultural bubble wrap on the inside of the glass cuts heat loss by 30-40% and can reduce the required heater size by one tier. It's cheap, easy to apply in autumn, and removes in spring. Seal draughts around the door and vents too — even small gaps let out a surprising amount of heat.
- Running a heater without ventilation. Even in winter, a greenhouse needs some air exchange to prevent damp and disease. A small vent or window left open a crack, or an automatic vent opener, prevents the humid, stagnant air that causes botrytis and mildew.
- Using a heater without a thermostat. A greenhouse heater with a built-in thermostat (or a separate thermostat probe) turns on only when the temperature drops below your set point, saving electricity and preventing overheating on mild nights. A heater without a thermostat runs continuously and can push temperatures too high on sunny winter days.
What to look for when buying a greenhouse heater
Heater type is the first decision. Electric fan heaters are the most common for UK greenhouses — they're clean, easy to control with a thermostat, and available in standard wattages. Oil-filled radiators are safer around plants (no direct heat or fan) but slower to respond and heavier. Propane or paraffin heaters work without electricity but need ventilation and regular refilling, and they produce moisture as a byproduct, which can increase humidity.
Thermostat accuracy matters. A good greenhouse heater should maintain temperature within ±1°C of the set point. Look for a heater with a remote thermostat probe that you can place at plant height, not just near the heater itself. Some advanced models have dual probes — one for the heater and one for a fan or vent opener — to create a simple climate control system.
Finally, think about safety and power. A 2kW heater running continuously uses 48kWh per day, which costs roughly £7-£8 at typical UK electricity prices. A thermostat-controlled heater that only runs when needed might use 10-20kWh per day in winter, costing £1.50-£3. Make sure your outdoor socket is RCD-protected and rated for outdoor use, and consider a timer or smart plug to control the heater around the clock without leaving the house.
More questions answered
What temperature should I keep my greenhouse at in winter?
It depends on what you're growing. A frost-free greenhouse kept above 2°C will protect tender plants and pots from freezing. A cool greenhouse at a minimum of 7°C suits many overwintering vegetables and young plants. A warm or tropical greenhouse kept above 13°C is needed for exotic or heat-loving plants through a UK winter.
Is polycarbonate or glass better insulated?
Twin-wall polycarbonate traps air between its layers and insulates roughly twice as well as single-glazed glass, so a polycarbonate greenhouse typically needs a smaller heater than a glass one of the same size to hit the same target temperature.
How accurate is this calculator?
It gives a planning-level estimate using a simplified box-shaped model and an assumed UK winter low of -5°C. It doesn't account for wind exposure, draughts, insulation upgrades like bubble wrap, or unusually cold microclimates, so treat the result as a starting point and size up if your site is exposed.
Can I reduce the heater size I need?
Yes. Lining a greenhouse with horticultural bubble wrap, sealing gaps and draught-proofing the door, using a windbreak, and grouping plants together can all cut heat loss and let you use a smaller heater than the raw calculation suggests.
Should I heat my greenhouse all night?
For frost protection, yes — the coldest temperatures usually occur just before dawn, and turning the heater off at night risks losing plants to a sudden freeze. A thermostat-controlled heater will only draw power when the temperature drops below your set point, so the running cost is much lower than leaving a non-thermostatic heater on continuously.
What's the cheapest way to heat a greenhouse?
Insulation is the cheapest 'heating' method. Bubble wrap on the inside, draught-proofing around the door and vents, and a horticultural fleece over plants on very cold nights can cut heat loss by 50% or more. After that, a thermostatically controlled electric heater is the most efficient option — it only runs when needed. A 500-1kW heater with a thermostat typically costs £1-£3 per week in winter, compared to £10+ for an unthermostatted heater left on continuously.
Can I use a greenhouse heater for propagation?
Yes — many greenhouse heaters work well for propagation, but you'll need more precise temperature control. A propagator heater or heat mat with a thermostat set to 18-22°C is better for seed trays and cuttings than a full-size greenhouse heater, which is designed for larger air volumes. For a small propagator area within a larger greenhouse, a separate thermostatically controlled propagator heater gives better results.
Do I need to insulate my greenhouse for winter?
Insulating a greenhouse for winter can cut heat loss by 30-50% and significantly reduce your heating costs. The simplest method is horticultural bubble wrap (available from garden centres) fixed to the inside of the glass with wire or clips. It reduces light transmission by about 10%, which is acceptable in winter when light levels are low anyway. Remove it in spring to maximise light for summer crops.