Greenhouse Heater Cost Calculator
| BTU/h at 120 sq ft | ΔT 25° | ΔT 40° | ΔT 55° | ΔT 70° |
|---|---|---|---|---|
| single glass | 3,390 | 5,420 | 7,450 | 9,460 |
| single poly | 3,300 | 5,280 | 7,260 | 9,240 |
| polycarbonate | 2,250 | 3,600 | 4,950 | 6,300 |
| double poly | 1,800 | 2,880 | 3,960 | 5,040 |
Enter the floor area, what the walls are made of, the lowest temperature you promise your plants and the coldest night you design for. The calculator sizes the heater with the standard greenhouse heat-loss formula and prices a night, a month and the heating season at your fuel rate.
The framing that saves real money: heat loss scales with the temperature gap and the covering, not the heater. Virginia Tech's greenhouse energy guide pegs double-poly coverings at about half the heating cost of single layer - the covering decision moves the bill more than any heater swap, and dropping the night setpoint one more notch scales it down linearly.
How to use
- Floor area in square feet - heat loss runs off the covered surface, and the calculator's U-factors fold a typical shape's roof and walls into it.
- Covering U-factors: single poly about 1.10, double poly about 0.60, glass about 1.13, twin-wall polycarbonate about 0.75.
- Target minimum by crop: 35 just-above-freeze storage, 45 dormant tender plants, 55 steady growth, 65 germination. Outdoor design temperature is your typical coldest night, not the record.
- Fuel: electric enters dollars per kWh, gas dollars per therm, propane dollars per gallon - the calculator converts each to a heater hour.
Frequently asked questions
How many BTU does a small greenhouse need in winter?
A 120 sq ft hobby greenhouse holding 45 degrees on a 20-degree night asks about 1,900 BTU/h under double poly, 2,400 under polycarbonate, and 3,600 under single film or glass. Halve the gap - hold 40 instead of 45 - and every number drops 20 percent, which is the cheapest upgrade on the page.
Is a 1500-watt heater enough for a greenhouse?
For an insulated small hobby house on all but the coldest nights, yes - 5,118 BTU covers a double-poly 120 sq ft house down past zero degrees. It is not enough for glass or single poly of the same size, and the right mental model is backstop: the heater exists for the coldest nights, while the covering and a thermostat do the average-night work.
Does doubling the poly really halve the bill?
About, yes - Virginia Tech's energy guide pegs double polyethylene at roughly 50 percent lower heating cost than single, and the U-factor math here agrees (0.60 versus 1.10). An inflation fan between the layers costs a few watts and pays for itself in the first cold month.
What temperature should I keep a greenhouse in winter?
The lowest your plants accept, not the warmest they like: hardy greens tolerate the mid-20s, dormant tender plants hold at 40-45, actively growing crops want 55-plus. Every degree of setpoint is a straight percentage on the bill - the linear term in the formula is the whole argument for the cool house.
Should I heat all night or just the coldest hours?
Coldest hours plus a thermostat. The load formula prices a 14-hour night honestly - but a thermostat holds the heater off until the actual drop, and clear nights after sunny days can fall 15 degrees below the day's average, so run the calculator on your design temperature and let the thermostat collect the savings on the mild ones.