Calculator inputs
Example data table
| Scenario | Heater output | Inside / Outside | Cover | Wind | Buffer | Estimated coverage |
|---|---|---|---|---|---|---|
| Seedling bench in tunnel | 2,000 W @ 85% | 18°C / 2°C | Single-layer plastic | Moderate | 15% | ~ 12–18 m² |
| Greens row cover | 1.5 kW @ 90% | 12°C / 0°C | Double-layer plastic | Sheltered | 10% | ~ 14–22 m² |
| Small insulated shed | 5,000 BTU/hr @ 80% | 16°C / 4°C | Insulated shed | Sheltered | 20% | ~ 18–28 m² |
These examples are illustrative. Your sealing quality and air movement can shift results.
Formula used
The calculator uses a simple heat-loss model: Q = U × A × deltaT × k
- Q is required heat (Watts)
- U is heat transfer (W/m²·C) for your cover
- A is covered area (m²)
- deltaT is temperature difference (C)
- k is wind exposure factor
Rearranged to estimate area: A = (Heater_W × Efficiency) / (U × deltaT × k), then reduced by the safety buffer.
How to use this calculator
- Pick your unit system and enter heater output.
- Set your target inside temperature and outside temperature.
- Select the cover/structure that matches your setup.
- Choose wind exposure and add a buffer for cold snaps.
- Click Calculate coverage to see area and radius.
- Use the rectangle ratio to plan tunnel or bed layouts.
- Download CSV/PDF to share your plan or budget estimate.
Coverage planning notes
Heat coverage as a design number
Heat coverage is an estimate of how much footprint you can hold near a target temperature. It combines heater output, efficiency, and heat loss through the cover. As a quick rule, tighter structures can need 2–6× less heat than open air.
Inputs that change results the most
Temperature difference drives demand. A 10°C deltaT roughly halves coverage compared with a 5°C deltaT. U-values represent insulation quality. Typical planning U-values include: double plastic 3.5 W/m²·C, twin-wall polycarbonate 3.0, and insulated rooms near 2.0.
Reading area, radius, and rectangle suggestions
Area is the primary output. Radius is shown for a circular footprint, useful for spot heating or fan heaters. The rectangle suggestion converts the same area into a length and width using your chosen ratio, helping plan tunnels, benches, and bed lines without redoing math.
Energy and budget snapshots
Daily kWh uses nameplate power times hours per day, so it is a conservative maximum. Thermostats, cycling, and sun gain usually reduce real consumption. Use your local electricity rate to compare approaches like adding a second film layer versus upsizing the heater.
Example dataset for quick comparison
The table below shows sample input sets and typical planning outcomes. Results vary with sealing, ventilation, and moisture.
| Setup | Output | deltaT | Cover U | Wind | Buffer | Coverage (m²) |
|---|---|---|---|---|---|---|
| Single plastic tunnel | 2,000 W @ 85% | 16°C | 6.0 | Moderate | 15% | ~ 13.1 |
| Double plastic tunnel | 1.5 kW @ 90% | 12°C | 3.5 | Sheltered | 10% | ~ 29.0 |
| Insulated shed | 5,000 BTU/hr @ 80% | 12°C | 2.0 | Sheltered | 20% | ~ 31.8 |
FAQs
1) Is this coverage exact for my garden?
No. It is a planning estimate based on heat loss assumptions. Air leaks, wet soil, wind gusts, and heater placement can change results noticeably. Use a buffer and verify with a thermometer.
2) Which cover option should I pick?
Choose the closest match to your structure. If you have double film inflation, pick double plastic. For rigid panels, pick polycarbonate. If you know a tested U-value, use the custom option.
3) Why does wind reduce coverage so much?
Wind increases convective losses and drives infiltration through gaps. Even small drafts can move warm air out and cold air in, raising required heat. Sheltering the structure often improves coverage more than extra watts.
4) What efficiency should I use?
Electric resistance heaters are often near 95–100% at point of use, but distribution losses exist. Unvented gas can vary; vented systems lose more. If unsure, 80–90% is a cautious planning range.
5) How do I choose inside and outside temperatures?
Use the minimum inside temperature your crop can tolerate and a realistic cold-night outside temperature. Planning with worst-week averages or local design lows improves safety for seedlings and tender transplants.
6) Does sunlight or thermal mass matter?
Yes. Daytime sun gain can reduce runtime, while water barrels and soil mass slow temperature swings. This calculator uses a steady-state approach, so it does not credit solar gain or stored heat directly.
7) Can I export multiple scenarios?
The downloads export the most recent calculation. For multiple scenarios, run a case, download CSV or PDF, then adjust inputs and download again. Naming files by date keeps comparisons organized.