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Temperature lift is how far above outside you hold the greenhouse: keeping frost-free (4-5C) on a -2C night needs a 6-7C lift. A thermostat means the heater only runs when needed, so real bills are usually lower than this steady-state estimate; bubble-wrap insulation in October is the classic money-saver.
What The Greenhouse Heater Cost Calculator Works Out
- Heat needed: glazed surface area x the glazing U-value (about 6 W/m2K for single horticultural glass, 4 with bubble wrap, 3.5 for twin-wall polycarbonate) x the temperature lift. A 6ft x 4ft house held 6 degrees above outside loses about 470 W, a few hundred watts, though a 2kW thermostatic fan heater is the standard buy for recovery speed after the door opens and air movement that fights botrytis.
- Cost per cold night: the watts x 12 hours, priced at the current Ofgem price cap unit rate. Each extra degree of lift is paid for across every square metre all night, which is why frost-free (holding 4 or 5 degrees on a minus two night) costs pounds a month for a small house while a heated orchid collection costs pounds a night.
- Per winter month: 30 cold nights. A thermostat only runs the heater when needed, so real bills are lower than this steady-state figure on milder nights.
- Unit rate, own area and hours (advanced): use your tariff, a measured glazed area (add up the roof and all four sides), and the hours the heater is live each night.
- Thermostat duty cycle and draughts (advanced): the share of the night the heater actually runs (often 50% to 70% on an average cold night) and an uplift for infiltration through broken panes and gappy doors, which can rival conduction losses.
- Whole winter (advanced): the monthly cost x months heated, typically four from November to February. The strategic dodge is reserving heat for a six-week propagation window in late winter.
- Heating a partitioned corner (advanced): heat a corner, not the greenhouse. A bubble-wrap-curtained section or a heated propagator bench shrinks the surface area term to a fraction, and the card shows the winter saving for the share you heat.
- Paraffin heater (advanced): the same kilowatt-hours from paraffin at 10.3 kWh per litre and your price per litre. Paraffin often costs less per kWh but pumps out water vapour, needs ventilation and has no thermostat; electric with a decent thermostat wins for control.
- Bubble wrap would save (advanced): the winter saving from lining single glass with horticultural bubble wrap in autumn, taking a third or so off heat loss for a few pounds at the price of some light. Most overwintering tender plants survive happily just above freezing, dry at the roots, and a max-min thermometer is cheap insurance that the thermostat means what it says on the coldest night.
Further reading: RHS: protecting plants in winter.