What It Costs to Heat a Greenhouse Each Month
Published: October 25, 2026 · 9 min read
A 20 x 12 greenhouse with twin-wall polycarbonate, held at 55°F through a Zone 6 winter, costs about $240 a month in propane. January alone runs $377. Natural gas brings the same house to $133 a month, a mini-split heat pump to $136, and resistance electric to $340.
Those numbers come from 992 square feet of envelope, a 0°F design low, and a 50,000 BTU per hour heater. Change the cover and the bill moves more than most people expect: the same frame in single-layer 6-mil film pays $2,565 a season instead of $1,450.
Four things set the monthly cost: envelope area times its U-value, the temperature inside, the climate outside, and what you burn. The rest walks them in order.
What One Month Costs, by Fuel
Season totals split across six heating months, October through April, for the 20 x 12 house above. Propane at $2.50 a gallon, gas at $1.50 a therm, power at 15 cents a kilowatt hour, and a heat pump at a coefficient of performance of 2.5.
| Heat source | Fuel used | Season | Monthly average | January |
|---|---|---|---|---|
| Natural gas, 87.5% AFUE | 531 therms | $796 | $133 | $207 |
| Heat pump, COP 2.5 | 5,439 kWh | $816 | $136 | $212 |
| Propane, 87.5% AFUE | 580 gal | $1,450 | $242 | $377 |
| Electric resistance | 13,607 kWh | $2,041 | $340 | $531 |
Cost per square foot of floor puts small and large houses on the same footing. This one runs $6.04 a square foot for the season, or about $1.01 a month. January is $1.57.
Why the Annual Number Is a Ceiling
Run the Greenhouse Heater Calculator on this house and it reports 39,010 BTU per hour and $3,654 a year on propane. That is the cost if the outdoors sat at 0°F for 3,000 hours, or 125 days.
No January does that. A design-day load times a heating-hour count is a sizing figure, not a forecast. Sizing needs the design low, because an undersized heater fails on the one night that matters. Budgeting needs degree days.
Zone 6 logs about 5,000 heating degree days against a 65°F base, near 3,000 against a 55°F setpoint. Multiply the 645 BTU per hour per degree of envelope loss by 3,000 and by 24: 46.4 million BTU for the season. The calculator said 117 million.
Budget 40 percent of the annual number and treat the rest as insurance. The rest is the 0°F worst case.
The Envelope Decides More Than the Fuel
Heat leaves through walls, roof, and floor at a rate of surface area times U-value times the inside-outside gap, plus 10 percent for wind and doors. A 20 x 12 x 8 house has 2 × (20×8 + 12×8 + 20×12) = 992 square feet of envelope.
Hold 55°F against a 0°F night and the gap is 55 degrees. Watch what the cover does to the load.
| Cover | U-value | BTU/hr at 0°F | Heater size | Propane season |
|---|---|---|---|---|
| 6-mil polyethylene, single layer | 1.15 | 69,000 | 80,000 | $2,565 |
| Polyethylene, double layer inflated | 0.70 | 42,000 | 50,000 | $1,561 |
| Polycarbonate twin-wall, 6mm | 0.65 | 39,000 | 50,000 | $1,450 |
| Polycarbonate twin-wall, 10mm | 0.55 | 33,000 | 50,000 | $1,227 |
| Polycarbonate triple-wall, 16mm | 0.45 | 27,000 | 30,000 | $1,003 |
Film to triple wall is 2.5 times the fuel and doubles the heater you buy. Over ten seasons that is $15,000 of propane against a cover costing $1,500 more up front. The people who regret a greenhouse almost always bought the cover last.
The formula treats the floor like glazing. Slab loss actually runs along the perimeter, which is why mulching the outside edge with straw pays for itself in a winter.
Dollars per Million BTU, Which Is the Only Fair Comparison
Gallons, therms, and kilowatt hours are not comparable. Convert each to the cost of a million BTU delivered and the ranking stops being a matter of opinion.
| Heat source | Unit price | Delivered efficiency | Cost per million BTU |
|---|---|---|---|
| Natural gas | $1.50/therm | 87.5% | $17.14 |
| Heat pump | $0.15/kWh | COP 2.5 | $17.58 |
| Firewood | $300/cord | 70% | $21.43 |
| Propane | $2.50/gal | 87.5% | $31.23 |
| Electric resistance | $0.15/kWh | 100% | $43.96 |
The heat pump line is worth sitting with. A cold-climate mini-split holds a coefficient of performance near 2 at 5°F and still heats at minus 13°F. A standard model loses capacity in the teens and falls back on resistance strips, which is how a heat pump ends up costing more than the propane it replaced.
Firewood at $300 a cord needs a hydronic boiler or forced-air furnace to go in. A woodstove in a greenhouse is a fire risk and a wet, cold morning.
The Shape of the Season
Heating cost is not a flat monthly line. A quarter of it lands in January, almost none in April.
| Month | Share of season | Propane | Natural gas | Electric |
|---|---|---|---|---|
| October | 4% | $58 | $32 | $82 |
| November | 12% | $174 | $96 | $245 |
| December | 23% | $334 | $183 | $469 |
| January | 26% | $377 | $207 | $531 |
| February | 20% | $290 | $159 | $408 |
| March | 12% | $174 | $96 | $245 |
| April | 3% | $44 | $24 | $61 |
January bills at 6.5 times October on the same house and fuel. That is why propane gets bought in August and overwintering plans get made in September.
How long a tank lasts
At design load the heater burns 39,010 ÷ 91,500 ÷ 0.875, or 0.49 gallons an hour. A 100-lb cylinder holds about 20 usable gallons. That is 41 hours, less than two days of a January cold snap, which is why anyone heating all winter runs a 250-gallon tank and buys on a summer contract.
What Actually Cuts the Bill
- An inner layer. Six-mil bubble wrap on the glazing pulls a single-film house toward double-film performance. On this 20 x 12 that is $2,565 down to $1,561, and the wrap costs $150 to $250. It pays back before February.
- A night screen. Commercial houses bank 30 to 40 percent of their heating energy from an automatic thermal screen. A hand-pulled sheet at dusk gets a share of that for the price of clips.
- Fifty degrees instead of 55. Each degree of setpoint is worth 2 to 3 percent of the bill. Five degrees is 9 percent on conduction math alone, and lettuce does not notice.
- Heat the bench, not the house. A floating row cover over seedlings buys 4 to 8°F. A hooped low tunnel inside the house buys another 6 to 10. In Zone 6 that is the difference between heating all winter and heating six weeks of it.
- Bottom heat. A 105-watt propagation mat holds soil at 70°F in a 40°F house for about $8 a month. Germination needs soil temperature, not air.
- Sealing. One air change per hour on 1,920 cubic feet is roughly 1,900 BTU per hour, about 5 percent of the load. A house with unsealed vents and a gap under the door runs two or three air changes and hands 15 percent of the bill outdoors.
Picking a Heater, and the Cost Nobody Quotes
An unvented propane heater releases about 0.85 gallons of water vapor per gallon burned, into the most humid space on the property. That is how a tomato crop gets botrytis in February with the heater running fine. Vented units cost more and are worth it above 30,000 BTU per hour.
Electric resistance is the cheapest install and the most expensive fuel. A 5,000-watt unit delivers 17,000 BTU per hour on a 240-volt circuit, which covers a small house in Zone 7 and nothing else.
If the service panel has room, a cold-climate mini-split is the best answer at these prices. Twelve thousand BTU per hour of rated output draws about 1,000 watts, needs no vent, and adds dehumidification in summer. The Firewood Requirement Calculator runs the same arithmetic for a wood-heated house, and how much firewood a winter takes pairs with it.
Size the heater before you buy one
Enter dimensions, glazing, inside temperature, and USDA zone. It returns BTU per hour, the next standard heater size, and an annual cost estimate.
Open the Greenhouse Heater Calculator →A chicken coop has the same problem at a tenth the scale, and the coop heating calculator covers the vented question there. The solar power calculator turns a monthly kilowatt-hour figure into panels.
Frequently Asked Questions
How much does it cost to heat a greenhouse per month?
About $240 a month in propane for a 20 x 12 twin-wall house held at 55 degrees through Zone 6, with January near $377. Natural gas lands near $133 and a mini-split heat pump near $136. Resistance electric is the expensive route at $340.
Is it cheaper to heat a greenhouse with propane or electricity?
Propane, unless the electricity runs a heat pump. At $2.50 a gallon and 87.5 percent efficiency, propane delivers a million BTU for about $31. Resistance electric at 15 cents a kilowatt hour costs $44. A heat pump at COP 2.5 cuts that to $17.58 and wins.
What temperature should I keep a greenhouse at night in winter?
Fifty to 55 degrees for lettuce, spinach, brassicas, and parsley. Tomatoes and peppers want 60 to 65 at night and are not worth heating through a northern winter. Every degree you drop the night setpoint saves 2 to 3 percent of the fuel bill.
Can a space heater heat a greenhouse?
Only in a small, well-sealed house in a mild climate. A 5,000 BTU heater cannot touch the 39,000 BTU per hour a 20 x 12 house in Zone 6 needs at 0 degrees. Match it to the calculated load or the house falls to outside temperature on the coldest night.
Two Numbers, One Bill
Budget 40 percent of the annual figure the Greenhouse Heater Calculator gives you, and expect January to take a quarter of the season. On a 20 x 12 twin-wall house in Zone 6 that is $1,450 in propane, $242 in an average month, $377 when the cold settles in.
The exception is a house you only heat to keep from freezing. Hold 35°F instead of 55°F and the load drops by a third, turning a $1,450 season into roughly $950. Lettuce and kale survive it. Tomatoes will not, and paying to pretend otherwise is how a greenhouse becomes an expensive shed.