Calculator inputs
Large screens use 3 columns, smaller screens 2, mobile 1.
Example data table
Sample scenarios to show typical output ranges. Adjust inputs for your location, coverings, and wind.
| Scenario | Size (L×W×H) | Roof | Screens | Wind | ΔT | Heater target |
|---|---|---|---|---|---|---|
| Evening tea, partly screened | 4×3×2.4 m | Polycarbonate | 40% | Moderate | 10°C | ~9–12 kW |
| Dinner with wind breaks | 5×4×2.6 m | Insulated panel | 75% | Low | 12°C | ~6–9 kW |
| Open pergola, breezy night | 3×3×2.3 m | Open slats | 0% | High | 8°C | ~10–14 kW |
| Covered lounge, mild weather | 6×3×2.5 m | Fabric canopy | 60% | Moderate | 6°C | ~4–7 kW |
Formula used
1) Conduction through roof and floor
For each surface:
Q = U × A × ΔT
Q in Watts, U in W/m²·K, A in m², ΔT in °C (or K).
2) Ventilation / infiltration (drafts)
Approximated using air changes per hour (ACH):
Qvent = 0.33 × ACH × V × ΔT
V is volume in m³. Constant 0.33 ≈ (air density × heat capacity) / 3600.
ACH model: This calculator maps screen coverage to a draft rate, then adjusts by wind exposure.
Fully open pergolas can exceed 20 ACH during gusty conditions.
How to use this calculator
- Pick your unit system and enter pergola length, width, and average height.
- Set inside and outside temperatures for the time you use it.
- Select a roof type. Use Custom U-value only if you know it.
- Estimate side screen coverage. Include curtains, blinds, or wind breaks.
- Choose wind exposure based on your location and pergola orientation.
- Optionally include floor loss if your floor is very cold.
- Set a safety margin, then press Calculate.
- Use the heater target to compare heaters and plan placement.
- Download the CSV or PDF for sharing or records.
Practical notes
- Radiant heaters can feel warmer than air heaters at the same kW.
- Wind breaks often reduce heat loss more than roof changes.
- If you want “near-indoor” comfort, aim for higher screen coverage and lower ACH.
- For fuel heaters, set efficiency to match your heater’s rated performance.