Heater for Small Greenhouse | Sizing, Types & Safe Picks

Choosing a heater for a small greenhouse comes down to size, power source, and how much frost protection your plants need.

Picking the right heater for a small greenhouse starts with square footage, not wattage. For most hobby greenhouses, a 1500-watt electric fan heater with a thermostat is the most practical choice. It handles spaces up to roughly 150–200 sq. ft., keeps temperatures stable, and runs on a standard outlet. The main decision is whether you have power at the greenhouse. If you don’t, your options narrow quickly to vented propane or paraffin units, which carry real safety trade-offs.

This guide covers sizing, the heater types that actually work, safety in damp conditions, and the mistakes that waste money. If you’re ready to compare specific tested models, our roundup of the best mini greenhouse heaters breaks down the top options side by side.

What Size Heater Does A Small Greenhouse Need?

The rule of thumb for small greenhouses is 5–10 watts per square foot.

Coverage claims vary by manufacturer, but a 1500W heater is the workhorse for most small greenhouses.

Before buying, measure the greenhouse floor space and, if a product lists coverage by volume, multiply length × width × height. A heater sized too small runs constantly; one sized too large short-cycles, which causes temperature swings that stress plants.

Electric Options: Fan, Tubular, And Radiant Heaters

Electric heaters are the best fit for small to medium greenhouses when power is available. They’re clean, quiet, and need no ventilation beyond normal airflow.

Fan Heaters

A fan heater with a built-in thermostat is the default recommendation.

Look for thermostats specifically. A heater that runs full-blast until you unplug it will overshoot your target temperature and waste electricity. You want a unit that cycles on and off to hold the setpoint.

Tubular Heaters

For very small greenhouses or just frost protection, a low-wattage tubular heater (around 80W) runs along the base of the bench and provides gentle, localized warmth. These are ideal for keeping a small space just above freezing, not for raising temperatures significantly.

Radiant / Infrared Heaters

Infrared and radiant heaters warm objects and plants directly rather than the air. They work well in small, well-sealed spaces and can be efficient, but they don’t move air, so cold spots and damp pockets can persist.

Off-Grid Options: Paraffin And Propane Heaters

For greenhouses without electricity, paraffin (kerosene) heaters are the common workaround. However, they produce combustion byproducts, including moisture and carbon monoxide, so ventilation is critical.

A greenhouse heated with paraffin must have vents open to exchange air, which defeats some of the heating effect. You’re trading heating efficiency for off-grid capability. Propane heaters are similarly positioned for larger structures or where ventilation can be managed.

For most small greenhouses, the practical advice is to run an extension cord or have a dedicated outlet installed by an electrician, then use a thermostatically-controlled electric heater. The cost of a professional outlet install is often less than the hassle and risk of combustion heaters in a damp, confined space.

Greenhouse Heater Safety And Common Mistakes

Greenhouses are damp environments, and that changes every safety calculation. Indoor-rated space heaters must stay away from direct water spray, misting lines, and wet floors. Look for weather-resistant or IPX4-rated units designed for humid use, especially near watering benches.

For an electric heater in a greenhouse setting, use a dedicated supply with RCD-protected sockets (the UK term for GFCI protection). This is not optional—it’s the difference between a tripped breaker and a serious shock risk on a wet floor.

The most common mistakes are easy to avoid:

• Buying by wattage alone. Match the heater to your greenhouse’s square footage, insulation, and local low temperatures.
• Oversizing. A heater too big for the space short-cycles, wastes energy, and creates unstable temperatures that stress plants.
• Using indoor heaters without checking splash resistance. Water and electricity don’t mix; check the rating before placing the unit anywhere near a watering can.
• Expecting small radiant units or heat mats to heat a whole greenhouse. These work for mini greenhouses or localized root-zone warmth, not for keeping a whole structure above freezing.

When you set the thermostat, aim for the lowest safe temperature for your plants. For frost protection in a general mixed greenhouse, that’s often just above freezing—around 37–40°F. There’s no point heating a frost-free zone to 70°F.

If your greenhouse is under about 150–200 sq. ft., a 1500W thermostatically-controlled fan heater from a reputable brand is the safest, most effective starting point.