Heater selection is the one greenhouse decision where getting it wrong is not recoverable by adjustment. An undersized heater runs continuously and still loses the house on the coldest night of the year, which is the only night it needed to work. An oversized one short-cycles, swings the temperature and costs more to buy and run. The sizing calculation is straightforward and doing it first changes which heater, and frequently which greenhouse, you should buy.
For most hobby houses the answer is the Biogreen Palma 1500 W at around $199, which is built for a wet, corrosive environment that kills domestic fan heaters within a season or two.
Best for most greenhouses
Sizing, before anything else
Heat loss in BTU per hour equals the glazing surface area multiplied by the temperature differential you want to hold, divided by the R-value of the glazing. That is the whole calculation, and the only input people commonly get wrong is the first one: it is surface area of the envelope, not floor area.
Worked through on a 6 by 6 house: roughly 190 square feet of glazing surface once four walls and the roof planes are counted. At 4 mm twin-wall, R-1.54. To hold 40 degrees F above outside, that is 190 times 40 divided by 1.54, which is about 4,900 BTU per hour. A 1500 W heater delivers 5,120, so it covers it with a small margin.
Now double the footprint. A 10 by 18 house has roughly 460 square feet of surface, and the same arithmetic gives about 12,000 BTU per hour, which is 3.5 kW. That is more than twice what any 120 V circuit can supply. This is why greenhouse size and heater choice are the same decision rather than two separate ones, and why the sizing should happen before the structure is bought. The heater BTU calculator runs it for your own numbers.
| House size | Glazing surface | At 20 F differential | At 30 F differential | At 40 F differential |
|---|---|---|---|---|
| 4 x 6 ft, 24 sq ft | About 140 sq ft | 1,800 BTU/hr | 2,700 BTU/hr | 3,600 BTU/hr |
| 6 x 6 ft, 36 sq ft | About 190 sq ft | 2,470 BTU/hr | 3,700 BTU/hr | 4,900 BTU/hr |
| 6 x 8 ft, 48 sq ft | About 230 sq ft | 2,990 BTU/hr | 4,480 BTU/hr | 5,970 BTU/hr |
| 6 x 10 ft, 60 sq ft | About 270 sq ft | 3,510 BTU/hr | 5,260 BTU/hr | 7,010 BTU/hr |
| 8 x 12 ft, 96 sq ft | About 350 sq ft | 4,550 BTU/hr | 6,820 BTU/hr | 9,090 BTU/hr |
| 10 x 12 ft, 120 sq ft | About 390 sq ft | 5,060 BTU/hr | 7,600 BTU/hr | 10,130 BTU/hr |
| 10 x 18 ft, 180 sq ft | About 460 sq ft | 5,970 BTU/hr | 8,960 BTU/hr | 11,950 BTU/hr |
Read that table against the 5,120 BTU per hour a 1500 W heater delivers and the boundary is obvious. On a single household circuit you can hold a 6 by 6 house at a 40 degree F differential, a 6 by 10 at 30, and a 10 by 12 at not much over 20. Everything beyond that is a 240 V conversation.
The three tiers
Greenhouse heaters in three tiers
Beginner
Buy once
Expert
Electric versus propane
The two practical fuel options for a hobby greenhouse behave very differently, and the deciding factor is usually not running cost.
Electric is clean, silent, precisely controllable and produces no combustion products at all. It adds no moisture to the house, which matters enormously in winter when humidity is already the limiting factor on disease. It is also entirely limited by the circuit available, and in most regions it costs more per delivered BTU than propane.
Propane, such as the direct-vent 11,000 BTU unit, delivers far more heat for the money and needs no electrical supply to the structure. The two costs are less obvious than the price per BTU suggests. First, unvented propane combustion produces roughly a gallon of water vapour for every gallon of fuel burned, which in a sealed winter greenhouse is a serious disease problem. Second, it produces carbon monoxide and consumes oxygen, which is why a direct-vent unit that draws combustion air from outside and exhausts outside is the correct specification rather than an open tank-top heater.
A tank-top radiant heater has its place as emergency frost protection on an unusual night, in a ventilated structure, with the grower present. It is not a solution for routine overnight heating in a closed house. The full comparison including running cost arithmetic is in electric versus propane.
The circuit is the real constraint
A 15 A 120 V circuit supplies 1,800 W at full load. Continuous loads, which a heater is, should not exceed 80 percent of the circuit rating, which is 12 A or about 1,440 W. A 1500 W heater draws 12.5 A. It is at or fractionally over that limit on its own.
Three practical consequences. The heater needs its own dedicated circuit rather than sharing with lights, a fan and a propagation mat. The supply should be GFCI protected, because it is a wet outdoor environment, which is what the GFCI cord and GFCI extension are for in the build lists. And the cable run to the greenhouse needs sizing for the distance, because voltage drop over a long run reduces delivered wattage exactly when you need it most.
Outdoor electrical work is governed by local code and this is the section of a greenhouse project most worth handing to a qualified electrician. There is orientation rather than instruction in greenhouse electrical and safety.
Reducing the load before adding output
Every dollar spent reducing heat loss reduces the heating bill permanently. Every dollar spent on heater capacity raises it every night the heater runs. In that order.
Lower the target. This is free and it is the largest single lever. Holding frost-free at 35 to 40 F rather than a growing minimum of 55 F roughly halves the differential and the energy with it. For overwintering and winter greens, the lower figure is also horticulturally correct: hardy crops in low winter light do not benefit from warmth they cannot photosynthesize to use.
Seal and insulate. Bubble insulation on the north wall and gables adds roughly R-0.5 to R-1.0 and seals air leakage at the same time, for a small fraction of a heater upgrade. Door gaps are frequently a larger loss path than the glazing specification. Covered in insulating a greenhouse.
Add thermal mass. Water barrels inside the house absorb daytime heat and give it back overnight. Two 55 gallon barrels hold about 917 pounds of water, which absorbs roughly 917 BTU per degree F of rise. Over a 15 degree daily swing that is on the order of 13,000 BTU cycled at no running cost. See passive solar and thermal mass.
Zone the heat. Heating 12 square feet of propagation bench to 78 F with mats costs a small fraction of heating 180 square feet of air to the same figure, and germination responds to soil temperature rather than air temperature anyway. This is the single most effective idea in greenhouse heating at hobby scale.
How we chose
We did not run these heaters through a winter, and any site claiming hands-on testing of six greenhouse heaters across a heating season is telling you a story. We compared published output ratings, stated environmental specification and ingress protection where given, element and housing material, thermostat type and sensor placement, and verified owner reviews weighted toward the second and third season.
In this category the second-season filter is unusually decisive, because the failure that defines a greenhouse heater is corrosion rather than output. A heater delivers its rated BTU on day one regardless of build quality, since wattage is wattage. What separates the units is whether the element, the fan bearing and the thermostat contacts survive an environment that is close to saturated for months. Almost all negative reports in this category cluster in the second winter.
We also weighted heavily against any unvented combustion heater marketed for routine overnight use in a closed structure, regardless of its reviews, because the moisture and combustion product issues are inherent to the approach rather than to the product.
What goes with the heater
The four items a heater needs around it
Running cost, and what actually drives it
A 1500 W heater running at a 50 percent duty cycle for 12 hours overnight uses about 9 kWh, which at typical residential rates is on the order of a dollar to two dollars a night. Across the genuinely cold part of a temperate winter that is a real number and it is the entire economic question of a heated greenhouse.
The duty cycle is what you control, and it responds to the setpoint, the envelope and the thermal mass far more than to which heater you bought. Two growers with identical equipment, one holding 40 F in an insulated house with water barrels and one holding 55 F in a bare one, will see bills that differ by a factor of two or more. The heater is not the variable.
Oil-filled radiators like the Pelonis unit deserve a note here because they are frequently suggested and frequently misunderstood. They are exactly as efficient as any other electric resistance heater, because all of them convert electricity to heat at essentially 100 percent. What differs is delivery: a radiator heats gently by convection with no fan, which suits a small enclosed space and a plant collection that dislikes moving dry air, but distributes poorly in a larger house without a separate circulation fan.