Greenhouse Heating Cost Calculator

Getting an answer takes six quick steps. Measure the greenhouse, pick the glazing, set the temperatures, and the calculator works out the heat loss, the heater size and the monthly cost.

1. Type the length, width and wall height in feet. Measure the outside of the structure.
2. Pick the glazing material. The U-value for it is filled in for you.
3. Type the inside temperature you want and the coldest outside temperature you plan for.
4. Type the heater efficiency and pick the fuel, then type your fuel price.
5. Type the heating hours per day and the days per month.
6. Press Calculate, or press Enter. The monthly cost appears first and large, with the heat loss, the heater size and the step-by-step math below. Press Reset to clear the form and start over.

Four short formulas power this calculator. They follow the heat loss method taught by university extension services.

Surface area = 2 × (length × height + width × height) + 1.15 × length × width. The walls are the first part, and the roof is the plan area times 1.15, which approximates a pitched roof. Heat escapes through this whole envelope, not through the floor.

Conduction = U × surface area × temperature difference. U is the heat transfer coefficient of the glazing, in BTU per hour per square foot per degree F. Double polyethylene film has a U of 0.70, single polyethylene film has 1.15, and twin-wall polycarbonate has 0.60.

Infiltration = 0.02 × volume × temperature difference. This covers the heat carried out by air leaking through gaps and vents, at one air change per hour. A tight new house loses about 16 percent of its total this way, so it is worth counting.

Heater size = Total heat loss ÷ efficiency. Monthly cost = Heat loss × hours per day × days ÷ fuel energy × fuel price. The fuel energy contents are the standard figures of 91,500 BTU per gallon of propane, 100,000 BTU per therm of natural gas and 3,412 BTU per kWh of electricity.

You have a 20 by 12 foot greenhouse with 8 foot walls and double polyethylene film. You want 65 F inside when it is 25 F outside, your propane heater is 80 percent efficient, propane costs $2.50 a gallon, and the heater runs 12 hours a day for 30 days. Here is exactly what the calculator does.

1. Surface area: 2 × (20 × 8 + 12 × 8) + 1.15 × 20 × 12 = 788 sq ft.
2. Temperature difference: 65 − 25 = 40 F.
3. Conduction: 0.70 × 788 × 40 = 22,064 BTU/hr.
4. Infiltration: 0.02 × 1,920 cu ft × 40 = 1,536 BTU/hr.
5. Total heat loss: 22,064 + 1,536 = 23,600 BTU/hr.
6. Heater size: 23,600 ÷ 0.80 = 29,500 BTU/hr.
7. Monthly propane: 23,600 × 12 × 30 ÷ 91,500 = 92.85 gallons.
8. Monthly cost: 92.85 × $2.50 = $232.13.

Glazing is the single biggest lever on the heating bill, because the whole envelope is glazing. A U-value tells you how fast heat passes through one square foot for each degree of temperature difference, so a lower U means a lower bill.

Single polyethylene film, at a U of 1.15, loses about 64 percent more heat than double polyethylene film at 0.70. Switching from single to double film cuts the conduction part of the bill by about 39 percent, which is why most growers inflate a second layer of film over the first. Twin-wall polycarbonate, at 0.60, does better still, and it lasts years longer than film.

Single glass, at 1.13, behaves much like single film, double glass, at 0.65, sits close to double film, and fiberglass, at 1.00, sits between the two. If your glazing is old or torn, fix the cover before you buy a bigger heater. Our insulation calculator can help you plan extra insulation for the coldest walls.

The most common and most expensive mistake in a home-made heating spreadsheet is sizing from the floor area. Heat escapes through the walls and the roof, and a greenhouse has far more of that surface than floor. A 30 by 96 foot hoop house has about 5,230 square feet of cover over 2,880 square feet of floor, and the GrowGrid heating calculator notes that using the floor area understates the heat loss by 38 percent.

That gap shows up twice. First the heater is bought too small, so the greenhouse cannot hold temperature on the coldest night. Then the monthly cost comes in far above the estimate, because the real heat loss was never counted. This calculator uses the full glazed surface area, walls plus roof, so the heater size and the cost both come from the right number.

Sizing from the average temperature instead of the coldest night is the first mistake. The heater must hold temperature on the coldest night of the year, so the outside temperature you enter should be the coldest you plan for, not the monthly average.

Ignoring air leaks is the second. Gaps around doors, vents and the base let warm air out all winter, and the infiltration term in this calculator counts that at one air change per hour. Seal the gaps first, because a dollar of weatherstripping often saves more than a dollar of fuel.

Forgetting heater efficiency is the third. A propane heater rated at 30,000 BTU of input only delivers about 24,000 BTU of heat at 80 percent efficiency, so a heater picked from the heat loss alone runs short. Divide by the efficiency, as the calculator does, and the heater will actually hold the temperature.

Finally, decide the target temperature on purpose. Holding a greenhouse just above freezing costs a fraction of holding it at a growing temperature, because the heat loss grows in direct proportion to the temperature difference. Our soil calculator can help you plan the beds for the temperature you choose.

This calculator gives an estimate for planning and is for informational purposes only, not installation advice. Gas heaters need proper ventilation and a carbon monoxide alarm, and electric heaters must suit the circuit they run on. See our disclaimer.