Greenhouse Heater Size Calculator
Calculate the BTU heating requirements for your greenhouse based on dimensions, glazing type, and your local climate.
Enter Greenhouse Details
Heating Requirements
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Required BTU/hr
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Recommended Heater Size
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Surface Area (sq ft)
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Temperature Difference
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Est. Annual Heating Cost
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Est. Heating Hours/Year
How It Works
Greenhouse heat loss occurs through the walls, roof, and floor (collectively the "envelope"). The rate of heat loss depends on the surface area, the insulating quality of the glazing material (U-value), and the temperature difference between inside and outside.
Understanding U-Values
The U-value (or U-factor) measures how well a material conducts heat. Lower U-values mean better insulation and less heat loss:
- U = 0.45 — Polycarbonate triple-wall (best insulation)
- U = 0.55 — Polycarbonate twin-wall 10mm
- U = 0.65 — Polycarbonate twin-wall 6mm
- U = 0.70 — Double-pane glass or double-layer polyethylene
- U = 1.10 — Single-pane glass
- U = 1.15 — Single-layer polyethylene (least efficient)
Safety Factor
A 10% safety factor is applied to the calculation to account for air leaks, door openings, and windy conditions. This ensures your heater isn't undersized on the coldest nights.
Surface Area (sq ft) = 2 × (L×H + W×H) + L×W
(This accounts for walls and simplified roof area)
Temperature Difference (°F) = Inside temp - Outside temp
BTU/hr = Surface Area × U-value × Temperature Difference × 1.1
Annual Heating Cost = (BTU/hr × Heating Hours / BTU per unit) × Fuel cost per unit
Heating hours per year (by USDA Zone):
Zone 3-4: 4,000 hours
Zone 5-6: 3,000 hours
Zone 7-8: 2,000 hours
Zone 9: 1,000 hours
Standard heater sizes:
10,000 / 15,000 / 20,000 / 30,000 / 50,000 / 80,000 / 100,000 BTU/hr
(Round up to next available size)