Greenhouse heaters: electric vs paraffin running costs and the best models

The short comparison

The RHS puts it simply: electric heaters are easy to control and should have a thermostat, while gas and paraffin are for places with no electrical supply, such as allotments. More on keeping the temperature right in winter and summer is in our greenhouse heating and ventilation guides.

How much heat a 6x8 greenhouse needs

"Frost-free" means about 4°C to the RHS, which is warm enough to overwinter tender (H2) plants. An unheated greenhouse shelters half-hardy (H3) plants only.

Heat drains out through the glazing and through gaps. A standard way to size a heater is to multiply the area of each surface by its heat loss figure (U-value), add an allowance for air leaking out, then multiply by how many degrees you need to lift the temperature. Here it is for a typical 6x8:

1. Glazed area. We used the dimensions of the Vitavia Orion 6x8 (1.93m wide, 2.57m long, 1.24m to the eaves, 2.30m to the ridge) and treated it as straight-sided: sides 6.4m², ends 4.8m², gable triangles 2.0m², roof 7.4m². Total about 20.6m².
2. Heat loss through glass. Single glass loses 5.7W per m² for every °C of difference (the figure in the sizing method we followed; our polycarbonate guide quotes 5.8 from a greenhouse maker, under 2% different). 20.6 × 5.7 = 117W per °C.
3. Air leakage. Allowances run from 20% to 66% depending on how well sealed the structure is. Taking the lowest, 20%, gives 141W per °C. A draughty greenhouse needs more.
4. Temperature lift. To hold 4°C inside when it is −5°C outside is a lift of 9°C. 141 × 9 = about 1,270W.

So on a hard frost, a 6x8 glass greenhouse needs around 1.3kW of steady heat. That is why a 2kW thermostatic fan heater suits this size: it has headroom and cycles off when it is not needed.

If you have a twin-wall polycarbonate greenhouse, it loses noticeably less heat than glass; our polycarbonate vs glass guide has the figures. Polytunnels lose more on clear nights, which is one reason we suggest a greenhouse if you plan to heat (see polytunnel vs greenhouse).

Running cost worked example per winter

Prices used (checked 21 October 2026):

• Electricity: 26.32p per kWh, Ofgem's price cap average unit rate for direct debit customers on a default tariff, 1 October to 31 December 2026. Ofgem says there is no VAT on electricity from 1 October 2026 to 31 March 2027, so this is the rate you pay. January to March falls under the next quarter's cap, so redo the sum with your own tariff when it changes.
• Paraffin: £12.99 for 4 litres of Barrettine paraffin (BS 2869 Class C1, sold for greenhouse heaters) at Home Hardware Direct (click and collect), which is about £3.25 a litre.

Weather used: the Met Office UK average minimum temperatures for 1991–2020: November 3.6°C, December 1.4°C, January 1.2°C, February 1.1°C, March 2.2°C.

Electric heater with a thermostat set to 4°C

For each month, the lift needed is 4°C minus the average minimum (for example, January 4 − 1.2 = 2.8°C). Multiply by the days in the month and add the five months: 316 "degree-nights".

Assume each night needs 12 hours of heating at that lift:

141W × 316 × 12 hours = 534kWh, which at 26.32p is about £140 for November to March.

With horticultural bubble wrap cutting heat loss by 30% (the low end of a retailer's estimates, see below): 374kWh, about £98.

To redo this for your own greenhouse, change the area in step 1 and put in your own unit rate. For 6mm twin-wall polycarbonate use about 3.4–3.5 instead of 5.7: BC Greenhouses rates its insulating value at 1.62 against 0.96 for single glass (5.7 × 0.96 ÷ 1.62 is about 3.4), and our polycarbonate guide quotes 3.5.

Single nights, same greenhouse: held at 4°C for 12 hours, a −5°C night costs about £4.00 (15.2kWh); a 2°C night about 89p (3.4kWh). A run of hard frosts moves the bill far more than a mild month.

Paraffin heater (0.6kW, one-gallon tank)

This uses the listed specification of the Parasene Paraffin Midi: 0.6kW, a 1 gallon (about 4.5 litres) tank and about 3½ days of burning per fill. We could not confirm a current UK listing with a price or stock for it, so treat it as a worked example rather than a recommendation. Assuming the 3½ days is continuous burning at full output, that is 4.55 litres ÷ 84 hours = about 0.054 litres an hour, or about 17.6p an hour at £3.25 a litre (17.4p if you round the tank to 4.5 litres): about 29p per kWh of heat, a little more than electricity.

The bigger difference is control. The Midi's burners can be turned down by hand, but with no thermostat nothing adjusts them as the temperature changes. At both burners on full:

• Lit every night, 12 hours, November to March (151 nights): about 98 litres, about £319.
• Lit only on the nights with an air frost (the Met Office UK average is 46 between November and March): about 30 litres, about £97, if you can judge which nights those will be.

Lighting it only on frost nights costs less in total than the electric example (£97 against £140), but it also delivers far less heat, and as the note above shows, 0.6kW cannot hold 4°C in a glass 6x8 on a hard frost. Per unit of heat, electricity is slightly cheaper, and the thermostat means you only pay for heat when it is needed.

Thermostats and the frost-free setting

A thermostat is what makes electric heating cheap: the heater switches off as soon as the greenhouse reaches the set temperature. For frost protection, set it at about 4°C.

Two things the RHS points out:

• The dial is not exact. It sits close to the heater and can be a few degrees out. Hang a maximum/minimum thermometer in the middle of the greenhouse, near the plants, and adjust the setting until the overnight minimum is where you want it. The Bio Green Palma's thermostat, for example, is quoted at ±2°C.
• Fan heaters avoid cold corners by moving air around; tube heaters use less power and give gentle background heat.

Bubble wrap: how much it saves

Greenhouse Stores' own figures say 20mm horticultural bubble wrap cuts heat loss by up to 50%, and packaging bubble wrap by 30–35%; its running cost guide uses 30–40%. We used 30% in the example to be cautious.

The trade-off is light. The RHS says a layer of bubble wrap cuts out about 10% of the light (Greenhouse Stores puts horticultural wrap at 10–15%), so leave it off where you are growing light-hungry crops such as winter lettuce and alpines. Tips from both:

• Use large-bubble horticultural wrap, which traps more air and lets more light through.
• Clean the glass first, fit the bubble side to the glass, overlap sheets by at least 50mm, and keep at least one roof vent free to open.
• In a bigger greenhouse, screen off one end with bubble wrap on battens and heat only that part.
• Seal gaps, replace broken panes and make sure vents and doors close snugly: Greenhouse Stores puts draughts at roughly 15–25% of heat loss.

Safety: sockets, RCDs, fumes and damp

Electric heaters

• Electrical Safety First's advice is to use an RCD with all outdoor electrical equipment, and a plug-in RCD if your fusebox does not have one.
• NICEIC says all outdoor socket-outlets should be RCD protected, with a suitable weatherproof rating and covers that close tightly, and that outdoor equipment should be at least IP44.
• Avoid running a heater off a long extension lead: NICEIC warns it can overheat. If you must use a lead or cable reel, Electrical Safety First says it should be made for outdoor use, and a reel should be fully unwound and fitted with a thermal cut-out.
• The RHS recommends having the supply to the greenhouse installed professionally. In England and Wales some garden electrical work must be notified to building control, which a registered electrician can self-certify. If you are laying a new greenhouse base, plan the cable route at the same time.
• Use a heater sold for greenhouses, with a splash-proof rating (the Palma is IPX4).

Paraffin and gas heaters

• Burning paraffin or gas produces water, ethylene and carbon monoxide, so the RHS says the greenhouse must be ventilated to stop these building up. That extra water also means more damp, which encourages mould and rot: open doors and vents on mild, sunny days.
• Follow the maker's instructions for the wick. One owner on the Grow Fruit and Veg forum saw flames around the top of a paraffin heater after relighting it, with no cause found in the thread.
• Use paraffin sold for domestic and greenhouse heaters (the Barrettine fuel above is refined to BS 2869 Class C1) and follow the heater maker's fuel advice.

Our picks

How we chose: from maker and UK retailer specifications, and heat output against the sizing sums above. We have not tested these heaters ourselves. Prices are from the retailer pages we checked in October 2026. We could not confirm a current UK listing with a price or stock for a paraffin or gas heater whose output we could check, so we have not picked one.

Related: Polycarbonate vs glass greenhouses: which is better for a UK garden?

Questions people ask

What is the cheapest way to heat a small greenhouse in winter?

If you have mains power, a thermostatic electric heater, set to about 4°C, with the greenhouse lined in horticultural bubble wrap. In our worked example for a 6x8 glass greenhouse that costs about £100 for November to March at the October 2026 price cap. Paraffin costs slightly more per unit of heat, and the paraffin heater we checked lists no thermostat.

What size heater do I need for a 6x8 greenhouse?

About 1.3kW of steady heat to hold 4°C in a single-glazed 6x8 when it is −5°C outside, by the calculation in this guide. A 2kW thermostatic fan heater gives headroom. A 0.6kW paraffin heater lifts it only about 4°C.

Do paraffin greenhouse heaters need ventilation?

Yes. Burning paraffin gives off water, ethylene and carbon monoxide, so the RHS says the greenhouse must be ventilated. The water also adds to damp, so open up on mild days.

What temperature should I set a greenhouse heater to?

About 4°C keeps a greenhouse frost-free, according to the RHS. Check it with a max/min thermometer near the plants, because the thermostat dial can be a few degrees out.

Can I run a greenhouse heater from an extension lead?

It is better not to. NICEIC advises against long extension leads for mains equipment outdoors because they can overheat. Use an RCD-protected, weatherproof outdoor socket. If you must use a reel, Electrical Safety First says it should be suitable for outdoor use, fully unwound and fitted with a thermal cut-out, and to use a plug-in RCD if your fusebox has none.

Sources

1. Changes to energy price cap between 1 October and 31 December 2026, Ofgem
2. Barrettine 4 Litre Paraffin (WHL0120), Home Hardware Direct
3. How to heat a greenhouse efficiently, RHS
4. UK climate averages (1991–2020), Met Office
5. Electric heater size calculation, Tombling (via InspectApedia)
6. Vitavia Orion 3800 6x8 aluminium greenhouse, Birstall
7. The best way to insulate a greenhouse, Greenhouse Stores
8. How much does it cost to heat a greenhouse? UK running costs 2026, Greenhouse Stores
9. Electrical safety advice for the garden, Electrical Safety First
10. Gardens and electrics: what you need to know, NICEIC
11. Bio Green Palma 2.0kW Electric Greenhouse Heater, Manual Thermostat, Rhino Greenhouses
12. Bio Green Palma 2.0kW Electric Greenhouse Heater, Digital Thermostat, Rhino Greenhouses
13. Phoenix 2.8kW Electric Greenhouse Heater, First Tunnels
14. Hotbox Fan Heater, Allotment Garden
15. Paraffin Midi Heater, Allotment Garden
16. Bio Green Black Forest 3.5kW Propane Gas Heater, Allotment Garden
17. Greenhouse glazing guide, BC Greenhouses
18. Greenhouse heaters (owner question), Grow Fruit and Veg forum