Greenhouse Heater Calculator
Greenhouse heater calculator: glazing area × U-factor × temperature difference gives BTU/hr. A 200 sq ft single-poly house at 40°F needs 9,600 BTU/hr.
A greenhouse heater calculator sizes the heater from the heat that leaks out through the walls and roof. The rule is glazing area × U-factor × temperature difference = BTU per hour. A 200 sq ft single-poly greenhouse that must stay 40°F warmer than outdoors needs 200 × 1.2 × 40 = 9,600 BTU/hr.
The formula in plain words
Think of the glazing as a leaky wall. The bigger the wall, the more heat leaves. A poorer wall (higher U-factor) leaks faster. A colder night outside pushes more heat through.
So the calculator multiplies three things: how many square feet of glazing you have, how leaky that glazing is, and how many degrees apart inside and outside are. The U-factor is the heat that crosses one square foot for each degree of difference per hour. The calculator suggests 1.2 for single-layer poly and 0.7 for double-layer poly.
Worked examples
Example 1, the default case: 200 sq ft of glazing, single poly, 40°F difference.
- 200 × 1.2 × 40 = 9,600 BTU/hr
Same house with a second layer of poly blown in between two sheets:
- 200 × 0.7 × 40 = 5,600 BTU/hr
- The second layer removes 4,000 BTU/hr of load
Example 2, a bigger house in a colder place: 400 sq ft of double poly, holding 55°F inside when the coldest night is 5°F.
- Temperature difference: 55 − 5 = 50°F
- 400 × 0.7 × 50 = 14,000 BTU/hr
Heater size for 200 sq ft of glazing
| Temperature difference | Single poly (U 1.2) | Double poly (U 0.7) |
|---|---|---|
| 20°F | 4,800 BTU/hr | 2,800 BTU/hr |
| 30°F | 7,200 BTU/hr | 4,200 BTU/hr |
| 40°F | 9,600 BTU/hr | 5,600 BTU/hr |
| 50°F | 12,000 BTU/hr | 7,000 BTU/hr |
| 60°F | 14,400 BTU/hr | 8,400 BTU/hr |
How to measure glazing area
Add up every covered surface that lets heat out: both side walls, both end walls, and the roof. Do not count the floor. Multiply each surface’s width by its height, and for a curved or hoop roof, measure the arc length along the surface, not the straight line from side to side.
Subtract any wall that is solid and insulated, such as a wall shared with a heated building, or treat it with its own lower U-factor. Doors and vents count as glazing unless they are insulated.
Common mistakes
- Using floor area. Heat leaves through the covering, so use the surface area of the walls and roof.
- Using the average winter temperature. The heater has to carry the coldest night, so use a cold-night figure for the temperature difference.
- Forgetting the end walls, which can be a quarter of the total glazing in a small house.
- Treating the result as the whole load. Air leaks and floor loss are outside this estimate, so choose a heater with some headroom.
- Sizing a gas or propane heater without ventilation. Combustion heaters need fresh air and venting, so follow the heater manufacturer’s instructions and have a licensed professional connect the fuel line.
Ways to bring the number down
Every term in the formula is something you can change.
- Cut the area. Insulating a solid end wall removes that glazing from the total.
- Cut the U-factor. Going from single to double poly on 200 sq ft saves 4,000 BTU/hr at a 40°F difference.
- Cut the temperature difference. Holding the house 10°F cooler at night takes the load from 200 × 1.2 × 40 = 9,600 to 200 × 1.2 × 30 = 7,200 BTU/hr, a saving of 2,400.
Some crops tolerate cooler nights than others, so check what your plants need before you lower the set point. Sealing gaps around doors and vents helps too, though this calculator does not measure it.
Checking the heater you pick
Heater ratings list input and output separately. A gas unit rated at 10,000 BTU/hr input may deliver less to the greenhouse, so compare the output or efficiency on the label against the number you calculated. Electric resistance heaters turn nearly all their input into heat. Whatever the fuel, a thermostat set at the night temperature keeps the heater from running when it is not needed.
Related calculators
For a house or shop with more complex walls, the heat loss calculator works from different inputs. The BTU calculator converts the result to other units, and the furnace size calculator covers a heated building. To estimate running cost on an electric heater, use the electricity cost calculator.
Frequently asked questions
What size heater do I need for my greenhouse?
Multiply glazing area in square feet by the U-factor and by the temperature difference you must hold. A 200 sq ft house with single-layer poly (U = 1.2) kept 40°F above outdoors needs 200 × 1.2 × 40 = 9,600 BTU/hr. Pick a heater with a rated output at or above that figure, and leave some margin.
What's the difference between single-layer and double-layer poly glazing?
The calculator uses U = 1.2 for single poly and 0.7 for double poly. Double poly traps an air gap, so the same 200 sq ft house at a 40°F difference drops from 9,600 to 200 × 0.7 × 40 = 5,600 BTU/hr, a cut of 4,000 BTU/hr.
Does this calculator account for floor heat loss or air leaks?
No. It counts heat lost through the glazing only. Gaps around doors and vents, wind on the walls, and loss through the floor and foundation add to the real load. Adding a margin of 10% to 25% on top of the result is a common way to cover this, but a tight, sheltered house needs less.
How do I find my design temperature difference?
Subtract the coldest outdoor night temperature you expect from the indoor temperature you want to hold. Holding 55°F inside when it drops to 15°F outside gives a 40°F difference. Use a cold-night figure from local weather records, not the average winter temperature, or the heater will be undersized on the worst night.
How many watts is 9,600 BTU/hr for an electric heater?
One watt equals 3.412 BTU/hr, so 9,600 BTU/hr is about 9,600 ÷ 3.412 = 2,814 watts. A 120 V circuit carries 1,800 W at most on a 15 amp breaker, so a heater this size usually needs a 240 V circuit. Have a licensed electrician size the wiring.
Estimates only. Verify quantities with your supplier before purchasing.