A purpose-built greenhouse fan heater is the better choice for most houses, because moving air distributes heat evenly and helps with the humidity and stagnant air that cause disease in winter. An oil-filled radiator is quieter, has no exposed element and produces a gentler radiant heat that suits a small propagation area, but it heats the air near it rather than the whole volume, and consumer radiators are not built for a damp environment.
1500 W output
About 5,120 BTU per hour either way
15 A continuous limit
About 1,440 W
Fan heater benefit
Distribution and air movement
Radiator benefit
No exposed element, quiet
Researched from published specifications and verified owner reviews · Updated 2026-08-16
Two 1500 watt heaters produce the same amount of heat, roughly 5,120 BTU per hour. Anything either manufacturer claims about efficiency is noise: electric resistance heating is essentially 100 per cent efficient at the point of use in both cases. What differs is where that heat ends up and what else the appliance does while producing it.
The comparison
Figure 1. Oil-filled radiator against greenhouse fan heater·8 rows
Figure 1. Oil-filled radiator against greenhouse fan heater
Measure
Oil-filled radiator
Greenhouse fan heater
Heat output at 1500 W
About 5,120 BTU/hr
About 5,120 BTU/hr
Distribution through the house
Poor, local
Good, air is moved
Air movement benefit against disease
None
Real
Noise
Silent
Audible
Exposed element
None
Enclosed, but check the model
Built for damp environments
Usually not
Purpose-built models are
Response to a temperature drop
Slow
Fast
Suits a single bench
Yes
Yes
Researched general information rather than professional advice. Greenhouse electrical work should meet local code.
Distribution is the real difference
A greenhouse is a long thin volume with cold glazing on every side, and heat that is not moved stratifies at the ridge while the crop at floor level stays cold. A Bio Green Palma 1500 W Greenhouse Heater with Digital Thermostat ($161.19) pushes warm air along the house, so the thermostat reading and the plant experience are closer together. That air movement is also worth something on its own account, because still damp air on cool leaf surfaces is what botrytis needs.
A PELONIS Oil-Filled Electric Radiator, 1500 W ($79.99) warms what is near it, gently and silently, and the far end of the house stays cold. In a small propagation area or under a bench of cuttings that is exactly the right behaviour. Across a 10 by 12 house it is not.
Damp is the thing consumer appliances are not built for
A greenhouse in winter is a humid, splash-prone environment with condensation running down every surface. Purpose-built greenhouse heaters have splash-resistant bodies and are designed for it. Household radiators and household fan heaters generally are not, and using one is a compromise to make deliberately rather than by accident, on a residual current protected circuit with the appliance kept off the floor and clear of splash.
A Dr. Heater DR218 1500 W Greenhouse Infrared Heater ($178.18) takes a third approach, warming surfaces and soil rather than only air, which suits benches of seedlings where root zone temperature is what limits germination.
One 1500 W heater fills a 15 amp circuit
A 1500 W heater draws about 12.5 amps, and a continuous load should not exceed 80 per cent of a circuit rating, which is about 1,440 W on a 15 amp circuit. That means one heater per circuit, no extension leads shared with anything else, and no daisy chaining. Greenhouse wiring should be on a residual current protected outdoor-rated circuit installed to local code.
Control matters more than the appliance
Built-in thermostats sit inside the appliance and measure the air the appliance has just heated, which is not where the plants are. An external controller such as an Inkbird ITC-308 Digital Temperature Controller ($36.00) with its probe at plant height gives a tighter differential and heats when the crop is cold rather than when the heater casing is. That single change usually saves more running cost than choosing between these two appliances does.
The decision, summarised
Heating a whole small greenhouse: a purpose-built greenhouse fan heater.
A propagation bench or a corner: an oil-filled radiator, quiet and gentle.
Germination limited by cold media rather than cold air: a radiant heater, or a heat mat under the trays.
Any of them: on an external controller with the probe at plant height.
Above about 50 sq ft in a cold climate: work out the BTU need first, because 1500 W may not be enough at all.
Three ways to put 1500 watts into a greenhouse
Fan, radiant and oil-filled, plus the controller that makes any of them cheaper to run.
+- Which type of electric heater is cheapest to run in a greenhouse?
None of them, meaningfully. Electric resistance heating converts essentially all the electricity into heat regardless of the format, so a 1500 W radiator and a 1500 W fan heater cost the same per hour to run. The savings come from the controller, from insulation, from sealing draughts and from setting a realistic minimum temperature, not from choosing between appliance types.
+- Is it safe to leave a greenhouse heater on unattended?
It is what a heated greenhouse requires, so the question is really about the installation. Use a purpose-built heater rated for damp conditions, on an outdoor-rated residual current protected circuit installed to local code, with nothing else on the circuit and no combustible material near the outlet. Fit a remote monitoring sensor so you know if the house goes cold, and check the appliance regularly.
+- What size heater does my greenhouse need?
It depends on the floor and glazing area, the glazing type, and the temperature difference you want to hold against the coldest expected night. Work it out with the heater BTU calculator rather than by rule of thumb, because the answer changes a great deal between single-layer film and twin-wall polycarbonate. If the answer exceeds what a 15 amp circuit can supply, the decision moves to a 240 volt circuit or a vented propane appliance.
+- Can I use a household fan heater in a greenhouse?
People do, and it is a compromise. Household appliances are not built for condensation, splashing and constant damp, and a bathroom-grade or greenhouse-grade appliance exists for that reason. If you use one, keep it off the floor, away from splash and on a protected circuit, and accept that it will not last as long as one designed for the environment.
Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the
paid version of these pages: 8 printable worksheets you fill in with your own
numbers, plus the full PDF, $29.
