Calculator
Heater output required
How the calculation works
Greenhouse heat loss follows the standard conduction form used across building science: Q equals A times U times delta T. Area multiplied by the thermal transmittance of the envelope multiplied by the temperature difference across it. Everything else is refinement.
A is the glazed envelope area, not the floor area. This is the input people get wrong and it matters because two houses with the same floor can lose heat at meaningfully different rates. A long narrow 4 by 18 ft house has considerably more surface per square foot of floor than a 8 by 9 ft house of the same 72 square feet, because the perimeter is longer relative to the area enclosed. The calculator computes the envelope properly from walls, two triangular gable ends and two pitched roof planes rather than approximating.
U is the reciprocal of the glazing R-value. Four millimetre twin-wall polycarbonate at R-1.54 has a U-factor of about 0.65, meaning it passes 0.65 BTU per hour through each square foot for each degree F of difference. Single polyethylene film at R-0.87 has a U of about 1.15, so it loses nearly twice as fast through the same area.
Delta T is a decision, not a measurement. It is the difference between the minimum you intend to hold inside and the design low outside. Choosing this figure is the single largest lever you have over the answer, and most growers set it higher than their crops require.
Infiltration is applied as a multiplier, because air leakage through door gaps, vent seals and panel joints scales with the same envelope. Published greenhouse guidance runs from 1.0 for a new tight structure to about 1.25 for an older film house with loose doors. A hobby kit is realistically 1.1 to 1.2, and this is a genuine cost that no glazing upgrade addresses.
Reading the result against a real circuit
The number the calculator produces is only useful set against what you can actually supply, and for most hobby greenhouses that is a hard ceiling rather than a budget question.
A 120 V 15 A circuit supplies 1,800 W at full load. Continuous loads, which a heater is, should not exceed 80 percent of the rating, which is 12 A or about 1,440 W. A 1500 W heater draws 12.5 A and is at or fractionally over that limit on its own, which is why the heated builds on this site specify a dedicated circuit rather than an extension from the garage.
In BTU terms, 1500 W is 5,120 BTU per hour. That covers a 6 by 6 twin-wall house at a 40 degree F differential with a small margin, a 6 by 10 at around 30 degrees, and nothing much larger. If the calculator returns a figure above roughly 5,000 BTU per hour, the honest options are a 240 V circuit, a lower target temperature, better insulation, or a smaller house. Adding a second 1500 W heater on a second circuit is possible and is what the expert build does for redundancy, but two heaters on one 15 A circuit is not.
Outdoor electrical work is governed by local code, and a 240 V run to an outbuilding involves conductor sizing, burial depth, a disconnect, grounding and GFCI protection. This is researched general information rather than professional electrical advice, and that installation is a qualified electrician job. There is orientation in greenhouse electrical and safety.
Lowering the number before buying a bigger heater
Every term in the equation except the temperature difference is fixed by the building. The differential is a choice, and it is free to change.
Lower the target. Holding frost-free at 35 to 40 degrees F rather than a growing minimum of 55 roughly halves the differential in most climates, and halves the heating energy with it. For overwintering tender plants and growing hardy winter greens, the lower figure is also horticulturally correct: plants in low winter light cannot photosynthesize enough to use extra warmth, so heating them to 55 costs money and produces very little growth.
Reduce the area term selectively. Bubble insulation on the north wall and the gable ends effectively raises the R-value of that portion of the envelope, and the north wall contributes almost no useful winter light so the cost is negligible. Sealing door gaps addresses the infiltration multiplier, which is frequently a larger loss than growers assume.
Add thermal mass to flatten the peaks. Water barrels do not reduce the calculated steady-state loss, but they substantially reduce heater runtime around dawn by releasing stored daytime heat. Two 55 gallon barrels hold about 917 pounds of water, absorbing roughly 917 BTU per degree F of rise.
Zone the heat. This is the largest practical lever of all and the calculator cannot express it. Heating a 12 square foot propagation bench to 78 degrees F with mats on a thermostat costs a small fraction of heating an entire house to that temperature, and germination responds to soil temperature rather than air temperature anyway. Hold the house frost-free and heat the root zone.
What the calculator cannot tell you
Three things fall outside this arithmetic and all three matter.
Distribution. The equation gives total loss for the building. It says nothing about whether the heat reaches the far end. A house with ten degrees between one end and the other does not have a capacity problem, it has a circulation problem, and it is solved with a small fan rather than a larger heater. This misdiagnosis is common and expensive.
Wind. Heat loss rises with wind speed across the envelope, and an exposed site can lose substantially more than a sheltered one with identical glazing. A windbreak, or siting the house in the lee of a hedge or building, is a real reduction that no calculator input captures. See siting and orientation.
Solar gain. The calculation is a night-time worst case. During the day a greenhouse frequently needs no heat at all and often needs venting instead, even in winter. The heater sizing question is about the coldest hours before dawn, and the daily energy consumption is much lower than continuous operation at the calculated figure would suggest.
Using the result to choose a heater
Once you have a BTU figure, matching it to a product is straightforward, with one caveat about how outputs are stated.
Electric heater output is exact, because electric resistance heating converts essentially all input energy to heat. Multiply watts by 3.412 to get BTU per hour. A 1500 W Biogreen Palma delivers 5,120 BTU per hour, and there is no efficiency figure to apply because there are no combustion losses and no flue.
Combustion heater output is stated as input rating, and the useful heat delivered is lower. A direct-vent unit such as the 11,000 BTU propane heater loses a share of that up the flue, so the delivered figure is meaningfully below the rated one. Unvented units deliver nearly all the heat into the space and also deliver all the combustion products and water vapour, which is why they are the wrong choice for routine overnight use in a closed structure.
Aim to match rather than substantially exceed the calculated figure. A heater sized 20 percent above the requirement gives useful margin. One sized at double will short-cycle, swing the temperature and cost more in both purchase and standby. Selection by type is covered in best greenhouse heaters and the fuel comparison in electric versus propane.
