Greenhouse Heating Calculator
Calculate greenhouse BTU and watt heating requirements from dimensions, insulation R-value, and target temperature for accurate cold-night heater sizing.
Heating Requirement
Greenhouse heating requirements depend on the greenhouse surface area, the temperature difference between inside and outside, and the insulation quality of the covering material.
Basic heating formula:
BTU/hr = Surface Area (sq ft) × Temperature Difference (°F) × Heat Loss Factor
For metric:
Watts = Surface Area (m²) × Temperature Difference (°C) × U-value
Heat loss factors (U-values) by covering:
| Covering Material | Heat Loss Factor (BTU/hr/ft²/°F) | U-value (W/m²/°C) |
|---|---|---|
| Single-layer glass | 1.13 | 6.4 |
| Double-layer glass | 0.65 | 3.7 |
| Single-layer polyethylene | 1.15 | 6.5 |
| Double-layer polyethylene | 0.70 | 4.0 |
| Polycarbonate (twin-wall) | 0.58 | 3.3 |
| Polycarbonate (triple-wall) | 0.47 | 2.7 |
Surface area calculation for common shapes:
- A-frame/gable:
2 × (Length × Side Height) + 2 × (Width × Roof Slope) + 2 × Gable Triangles - Quonset/hoop:
Length × π × (Width/2)(semicircle profile) - Simplified:
Floor Area × 2.5(rough estimate for typical greenhouse)
Temperature guidelines for common plants:
| Plant Type | Minimum Night Temp | Ideal Day Temp |
|---|---|---|
| Cool-season crops | 40–45°F (4–7°C) | 55–65°F (13–18°C) |
| Warm-season crops | 55–60°F (13–16°C) | 70–80°F (21–27°C) |
| Tropical plants | 65–70°F (18–21°C) | 75–85°F (24–29°C) |
| Seed starting | 65–75°F (18–24°C) | 70–80°F (21–27°C) |
Conversion:
- 1 BTU/hr = 0.293 Watts
- 1 Watt = 3.412 BTU/hr
Practical tips:
- Use the coldest expected outside temperature for sizing.
- Add 10–15% capacity for wind exposure and aging insulation.
- Consider thermal mass (water barrels, stone floors) to buffer temperature swings.
- A thermal curtain or bubble wrap inner layer can reduce heat loss by 30–40%.