Before anything else.
Paraffin and propane heaters burn fuel inside an enclosed space, consuming oxygen and producing carbon monoxide, which you cannot see or smell. Permanent ventilation and a carbon monoxide alarm are conditions of use, not optional extras. Electric heaters do not produce combustion products but carry their own requirement: a properly protected outdoor-rated supply, installed by a competent person.
The real question is not which is cheaper
Most comparisons of these two argue about running cost. That is the wrong starting point for the shoulder season, because in the six weeks either side of first frost you are heating on maybe ten to twenty nights, for a few hours each. The difference in fuel cost across a handful of nights is small either way.
What actually decides it is three practical constraints: whether you have power at the greenhouse, whether you need thermostatic control, and what happens to your plants when the supply fails.
| Paraffin / propane | Electric | |
|---|---|---|
| Needs power run | No | Yes — often the deciding factor |
| Thermostatic control | Limited or none | Precise and standard |
| Adds humidity | Yes, significantly | No |
| Combustion risk | Real; CO alarm mandatory | None |
| Works in a power cut | Yes | No |
| Attention needed | Fill, trim, light, extinguish | Set and forget |
| Upfront cost | Low to moderate | Low for the unit, high if wiring is needed |
| Best fit | Unwired greenhouse, occasional nights | Wired greenhouse, any duty cycle |
Constraint one: is there power out there?
This settles the argument for a large share of hobby greenhouses. Running a safe, permanent, properly protected electrical supply to a structure at the end of a garden is a real job with a real cost, usually involving armoured cable, a buried run and an electrician.
Extension leads are not the answer. A domestic extension lead run across wet grass to a metal-framed structure full of water is a genuine hazard, and it is how people get hurt. If there is no supply, the honest options are a fuel heater or no heater.
Given that, the fuel heater is often not a compromise at all — it is the appropriate tool for an unwired building used occasionally.
Constraint two: how precisely do you need to hold temperature?
Electric heaters do thermostatic control well, and it matters more than raw output. A thermostatically controlled heater runs only when the temperature drops to your setpoint, which for a "frost free" target means it may run for two hours in a night rather than eight.
A wick or burner heater has no such control. It burns at its rate from when you light it to when you extinguish it or it runs out. On a night that turns milder than forecast, it is still burning. That is fuel wasted, and in a small greenhouse it can push the temperature well above what you needed.
If you are holding a specific minimum through a long cold spell, electric with a thermostat is clearly better. If you are lighting something at dusk on a night that will definitely freeze, the lack of control matters much less.
Constraint three: the humidity nobody mentions
Burning paraffin produces water vapour in quantity. In a greenhouse you have just sealed and insulated for winter, that water condenses on every cold surface overnight, and still saturated air on plant surfaces is precisely what gray mold needs — see autumn condensation and gray mold.
This is a genuine mark against fuel heating and it is rarely mentioned in comparisons. It does not rule it out; it means a fuel heater and a deliberately open vent go together, always. Electric heat adds no moisture at all, which in a sealed autumn greenhouse is a real advantage.
The failure modes differ
An electric heater fails when the power does, which in many areas correlates with exactly the storm conditions that produce a cold night. If you are protecting something you cannot replace, that correlation matters.
A fuel heater fails when it runs out, sooners or later depending on the fill, or when a wick is fouled and it burns badly. Its failure is more predictable and more preventable, and it is unaffected by the grid.
Growers protecting genuinely valuable overwintering collections often keep a fuel heater as backup to an electric primary, precisely because the two fail for unrelated reasons.
The verdict
No power at the greenhouse: fuel heater, with a carbon monoxide alarm and a permanently open vent. This is most hobby greenhouses, and it is a correct answer rather than a compromise.
Power already run, correctly installed: electric with a thermostat. Better control, less fuel wasted on mild nights, no added humidity, nothing to fill or light.
Protecting something irreplaceable: electric primary, fuel backup, and a min/max thermometer to tell you whether either is actually working.
Before either: seal the structure, add thermal mass, cover the plants. In the shoulder season those three frequently remove the need to heat at all, and none of them can fail dangerously. The heater options themselves are covered in shoulder-season heaters.
What to buy either way
Carbon monoxide alarm
$ · Mandatory with any fuel heater · buy it before the heater
Battery powered, digital display, mounted at head height, tested at the start of the season. There is no version of fuel heating in an enclosed greenhouse where this is optional, and it costs a fraction of the heater it protects you from.
Carbon monoxide alarms on AmazonCarbon monoxide alarms on eBay
Min/max thermometer
$ · Both options · heater dials are frequently wrong
The only way to know whether your heating actually held the minimum you set is to measure what the house did overnight. Site it at plant height in the coldest corner, away from the door and out of direct sun, and set the heater by what it reports rather than by the dial.
Water barrels for thermal mass
$ to $$ · Before either heater · no fuel, no failure mode
Dark barrels of water along the north wall absorb daytime heat and release it overnight, which in September and October is often enough on its own. Free to run, cannot fail dangerously, and reduces how hard whichever heater you choose has to work.
Water storage barrels on AmazonWater storage barrels on eBay
Greenhouse Guide earns commission on qualifying purchases made through the links above, at no additional cost to you. We link to product categories rather than single items because stock and pricing in this category change week to week — compare current listings and buy what fits your structure.
Common questions
Is paraffin or electric heating cheaper for a greenhouse?
Across the ten or twenty nights of the shoulder season the difference is small either way. Over a full winter of sustained heating the calculation matters more, and thermostatic control — which electric does well and wick heaters do not at all — often saves more than the fuel price difference.
Can I run an extension lead to my greenhouse?
No. A domestic extension lead across wet ground to a metal structure full of water is a genuine hazard. A greenhouse supply needs appropriate outdoor-rated cable and protection, installed by a competent person. If that is not in place, a fuel heater is the correct option rather than a compromise.
Does a paraffin heater cause damp in a greenhouse?
Yes, substantially. Combustion produces water vapour, which condenses on every cold surface in a sealed house and creates the still, saturated conditions gray mold needs. A fuel heater and a deliberately open vent go together, always.
Which heater is better in a power cut?
The fuel heater, and the point matters because power failures often coincide with the storm conditions that produce a cold night. Growers protecting valuable overwintering plants often run electric as primary and keep a fuel heater as backup, because the two fail for unrelated reasons.
Do I need a heater at all in September and October?
Often not. Sealing the structure, adding thermal mass such as dark water barrels, and covering plants with frost cloth frequently carry a greenhouse through the shoulder season without any heat. Do those three first and buy a heater for the nights they do not cover.