This decision is usually framed as a running cost question and usually decided by something else. For a small hobby house, the number of hours a heater actually runs is low enough that the fuel cost difference is modest, while the complications of combustion in a sealed structure are not. For a large house, electric simply cannot supply the output, and the question answers itself.
So the useful way to approach it is: work out the required output first, then see which options can supply it, then compare cost among whatever survives.
The comparison
| Measure | Electric resistance | Propane, direct vent | Propane, unvented |
|---|---|---|---|
| Efficiency at point of use | 100 percent | High, some heat up the flue | Effectively 100 percent, all products stay inside |
| Cost per BTU | Highest | Lower | Lower |
| Practical output ceiling | About 5,120 BTU/hr on a 15 A circuit | Tens of thousands of BTU/hr | Tens of thousands of BTU/hr |
| Moisture added to the air | None | None, it goes up the flue | About 1.6 lb of water per lb of propane |
| Oxygen consumed | None | None from inside, air is drawn from outside | Yes, from the greenhouse air |
| Carbon monoxide risk | None | Low with correct installation | Real in an enclosed space |
| Flue and combustion air | Not needed | Required, and a code matter | Requires substantial ventilation |
| Control precision | Excellent with an external thermostat | Good | Poor on most tank-top units |
| Power cut behaviour | Stops | Usually stops, most need power for controls | Continues |
| Installation | Plug in to a GFCI circuit | A qualified professional job to code | Portable, but not suitable as continuous heat in a sealed house |
The circuit is the real constraint on electric
A 120 volt 15 amp circuit supplies 1,800 watts at full load, and a continuous load should not exceed 80 percent of the rating, about 1,440 watts. A 1500 watt heater draws 12.5 amps and is at or fractionally over that limit on its own, delivering 5,120 BTU per hour.
That output heats a small house and nothing larger. A 6 by 6 foot twin-wall greenhouse holding 40 degrees Fahrenheit above outside needs roughly 4,900 BTU per hour, which a Bio Green Palma 1500 W Greenhouse Heater with Digital Thermostat ($161.19) just covers. A 6 by 10 at the same differential is beyond it. Work out where your own house sits with the heater BTU calculator before the fuel question even arises.
Beyond that ceiling, electric means a 240 volt circuit. A Bio Green Phoenix 2.8 kW 240 V Greenhouse Heater ($286.90) at 2.8 kW delivers about 9,550 BTU per hour, which covers a considerably larger house, at the cost of a trench, conduit, conductor sizing, a disconnect and an electrician. That installation cost is often the single largest line item in a heated greenhouse project, and it is worth setting against the whole propane option rather than against the heater price.
What propane does to the air
Burning propane produces carbon dioxide and water vapour. Roughly 1.6 pounds of water is produced per pound of propane burned, and in an unvented appliance all of it goes into the greenhouse.
In a structure that already struggles with humidity, particularly in winter when it is sealed and cool, that is a substantial addition. High humidity plus still air plus cool leaf surfaces is precisely the condition botrytis and downy mildew require, as set out in humidity and disease control. Unvented propane heat and a disease-free winter house are in tension.
The carbon dioxide is not a problem and is arguably a small benefit, since a sealed greenhouse on a bright morning can become carbon dioxide limited. What is a problem is that incomplete combustion produces carbon monoxide, and that oxygen is consumed from the same air a person is breathing when they walk in.
A direct-vent unit such as the Ashley Hearth 11,000 BTU Direct-Vent Propane Wall Heater ($449.99) addresses all three issues at once: combustion air comes from outside, exhaust including the water vapour goes outside, and nothing from the burner enters the growing space. It costs more and needs proper installation, and it is the version of propane heat that belongs in a greenhouse used through winter.
Running cost, honestly
Propane carries about 91,500 BTU per gallon. Electric resistance heat delivers 3,412 BTU per kilowatt-hour. Local prices for both vary enormously, so any general statement about which is cheaper is unreliable, but propane is usually cheaper per delivered BTU by a meaningful margin.
What that margin is worth depends entirely on how many hours the heater runs, and that is the figure people overestimate. A house held at a frost-free minimum runs its heater only on the coldest nights, which in a temperate climate is a modest number of hours. A house held at 50 degrees for active winter growing runs far more, and that is where the fuel difference starts to matter.
The glazing heat loss calculator gives a seasonal cost estimate for an electric house, which is a useful baseline to set the propane alternative against. And before comparing fuels at all, the measures in insulating a greenhouse usually reduce the load more cheaply than any fuel switch improves it.
Control and reliability
Electric wins on control. An external digital controller such as an Inkbird ITC-308 Digital Temperature Controller ($36.00) with its probe at plant height gives a tight differential and measures where the plants are, not where the heater is. Most propane appliances have coarser built-in control, and unvented tank-top units are effectively uncontrolled.
Propane wins on independence. It does not care whether the grid is up, which in a rural area with unreliable power is a genuine argument. In practice most direct-vent appliances still need power for their controls and fan, so the independence is partial unless the installation is specifically designed for it.
Both benefit from a monitoring sensor. A Govee H5179 WiFi Thermometer and Hygrometer ($33.99) reporting to a phone tells you the house went cold at 3 a.m., which is the only way anyone ever learns that a heater or a controller has failed.
The decision, summarised
- Under about 50 square feet, frost-free target, existing 15 A supply: electric, with no serious argument the other way.
- Under about 50 square feet, actively heated for winter growing: electric, and spend the difference on insulation rather than on a fuel switch.
- 50 to 150 square feet: the crossover. Compare the cost of a 240 V circuit against a direct-vent propane installation, including the electrician or the gas fitter in both cases.
- Above about 150 square feet, heated through a cold winter: propane, direct vent, professionally installed. Electric at this scale is neither practical nor affordable to run.
- No power at the site at all: propane, or accept that the house is a season extender rather than a heated one and use the methods in winter greens with no heat.
Heaters at both ends
Whatever the fuel, the cheapest heat is the heat you do not need. Work through insulating a greenhouse and passive solar and thermal mass before sizing anything.