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.
Recommended greenhouses
- Polycarbonate
- Small gardens
- 10 Years
£349.99
Pros
- Shatterproof polycarbonate panels with over 90% light transmission
- Rust-resistant aluminium frame with galvanised steel base
- Sliding door and roof vent for airflow
Cons
- Assembly takes a full weekend for two people
- Wood
- Balcony/patio
- 1 Year
£79.99
Pros
- Wooden frame with polycarbonate glazing
- Great for hardening off seedlings
- Compact footprint for patios
Cons
- Not tall enough for mature plants
- Glass
- Large gardens
- 15 Years
£899.00
Pros
- Toughened safety glass throughout
- Smooth-action sliding door and air vent for airflow
- Curved eaves add extra headroom
Cons
- Higher price
- Requires a solid, level base
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.