Greenhouse heating goes wrong in a predictable order. People choose a heater, install it, discover the circuit will not carry it or the bill is unacceptable, and then start looking at insulation. Reversing that order costs nothing and usually halves the problem, because the heat you do not lose is heat you never have to buy.
Assuming the envelope is as good as it is going to get, heating comes down to four decisions in this order: what minimum to hold, how much output that requires, what to burn or draw to produce it, and how to control it so it runs as little as possible.
Decide the minimum before anything else
This single number sets the size of the heater, the size of the circuit, and the running cost, and most growers set it higher than their crops require. Heat loss is directly proportional to the temperature difference, so every degree you decide not to hold is a proportional saving, every hour, all winter.
| Minimum held | What it protects | What it enables | Relative load |
|---|---|---|---|
| 33 to 35 degF | Hardy greens, dormant plants, stored bulbs | Frost-free storage and a slow winter salad harvest | Lowest |
| 40 degF | Tender perennials, overwintering pelargoniums, citrus in pots | Reliable overwintering of tender ornamentals | Moderate |
| 45 to 50 degF | Actively growing cool-season crops | Slow but genuine winter growth in greens and herbs | High |
| 55 to 60 degF | Warm-season propagation | Early transplant raising, weeks ahead of the season | Very high |
| 60 to 65 degF | Fruiting warm-season crops | Little in winter, because light rather than heat becomes the limit | Impractical at hobby scale |
The last row deserves its blunt phrasing. Holding 60 degrees through the darkest weeks does not produce winter tomatoes, because tomatoes want a daily light integral of 22 to 30 and a mid-latitude greenhouse in midwinter delivers a fraction of that. Winter fruiting is a lighting project with a heating component, not the reverse, and at hobby scale it is rarely worth it. Check what your own house delivers with the DLI calculator before committing to the heat.
A much better use of the same money is to heat a small zone rather than the whole house. A propagation bench held at 70 degrees under a low cover costs a fraction of holding an entire greenhouse there, and it does the same job for raising transplants.
Sizing the heater to the envelope
Heat loss is area multiplied by U-factor multiplied by temperature difference, with an infiltration allowance on top. The area in that expression is the glazed envelope, not the floor. This is the input people get wrong, and it matters because two houses with identical floor areas can lose heat at meaningfully different rates depending on their proportions.
Run the numbers in the heater BTU calculator, which builds the envelope properly from walls, gable ends and roof planes. Then read the answer against what the electrical supply can actually deliver, because that is a hard ceiling rather than a budget question.
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, which is 12 amps or about 1,440 watts. A 1500 watt heater draws 12.5 amps and is at or fractionally over that limit on its own. In BTU terms it delivers 5,120 BTU per hour. If the calculator returns much above that, the honest options are a lower target, better insulation, a 240 volt circuit, or a smaller house.
Electric, propane or neither
Electric resistance heat is 100 percent efficient at the point of use, adds no moisture and no combustion products to the air, needs no flue, and is trivially easy to control with a thermostat. It is also usually the most expensive per BTU, and it is capped by the circuit. A Bio Green Palma 1500 W Greenhouse Heater with Digital Thermostat ($161.19) with a built-in digital thermostat is the standard hobby answer, and a Bio Green Phoenix 2.8 kW 240 V Greenhouse Heater ($286.90) is the same idea at 240 volts for a larger house.
Propane is cheaper per BTU and independent of the electrical supply, which is why it is common in larger houses. It has three complications that matter in a small sealed space: it needs combustion air and a flue path, it produces about 1.6 pounds of water vapour per pound of propane burned, which lands directly in a house that already struggles with humidity, and unvented combustion in an enclosed space is a carbon monoxide risk. A direct-vent unit such as the Ashley Hearth 11,000 BTU Direct-Vent Propane Wall Heater ($449.99) takes combustion air from outside and exhausts outside, which addresses all three, and it needs proper installation to code.
Unvented tank-top radiant heaters such as the GasBRUH 18,000 BTU Tank-Top Propane Radiant Heater ($47.49) are widely used for occasional frost protection in ventilated tunnels. They are not appropriate as a continuous heat source in a sealed hobby greenhouse. The full comparison, with running costs, is in electric versus propane greenhouse heat.
Control is where the money is saved
An unregulated heater runs whenever it is switched on. A thermostatically controlled heater runs only when the house falls below the setpoint, and in a typical winter that is a small fraction of the hours. Control is therefore not a refinement, it is most of the saving.
A built-in heater thermostat is usually a bimetallic strip with a wide differential, which means the house swings several degrees either side of the target. 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, more usefully, it measures the temperature where the plants are rather than at the heater outlet, which is often several degrees warmer.
Two refinements are worth the trouble. Set the target lower at night than during the day if the crop tolerates it, because a deliberate day to night difference produces shorter, sturdier plants than a flat setpoint. And put a Roodike 5 in Clip-On Circulation Fans, 2-Pack ($35.99) on continuously, because a heated greenhouse stratifies badly: without circulation the ridge can be ten degrees warmer than the floor, so the thermostat calls for heat while the top of the house is already warm enough.
Zone the heat rather than raising it
The most efficient greenhouse heating decision available at hobby scale is not to heat the whole volume at all.
- Bottom heat under a propagation bench. Germination and rooting respond to root-zone temperature, not air temperature. A VIVOSUN 10 x 20.75 in Seedling Heat Mat and Digital Thermostat Set ($24.78) holds a tray at 75 degrees for a few watts, which would cost a large multiple of that to achieve in the air.
- A low cover over a bench. Fleece or bubble film draped over hoops on a bench at night traps a small volume around the plants and can hold several degrees above the house minimum for nothing.
- Row cover inside the house. A layer of Agfabric Frost Blanket, 0.55 oz, 10 x 100 ft ($32.61) over a winter greens bed is the classic double-cover method, and it is the difference between harvesting through a cold snap and losing the bed.
- Thermal mass on the sunlit side. Water barrels absorb heat during the day and release it overnight, raising the pre-dawn minimum at no running cost. Covered in passive solar and thermal mass.
Heating and control equipment
Before buying output, spend an hour on the envelope. The measures in insulating a greenhouse routinely cut the required heater size enough to keep a house inside a single 15 amp circuit, which is worth more than any efficiency difference between heaters.