Inputs
Example data table
| Glazing | Area | Inside | Outside | ΔT | Heat loss | Energy (24h) |
|---|---|---|---|---|---|---|
| Double poly film | 20 m² | 22 °C | 5 °C | 17 K | 1,190 W | 28.6 kWh |
| Twinwall polycarbonate | 30 m² | 18 °C | 0 °C | 18 K | 1,620 W | 38.9 kWh |
| Low-E double glazing | 15 m² | 20 °C | 8 °C | 12 K | 288 W | 6.9 kWh |
Formula used
- Q = heat loss rate (Watts)
- U = overall heat transfer coefficient (W/m²·K)
- A = glazing area (m²)
- ΔT = temperature difference (K)
- E = energy (kWh)
- Cost = E × rate
- BTU/hr uses 1 W = 3.412 BTU/hr
How to use this calculator
- Measure the exposed glazing area, not the floor area.
- Choose your temperature unit, then enter inside and outside values.
- Select a glazing type, or enter a custom U or R value.
- Set the duration that the temperature difference applies.
- Enter your energy rate to estimate operating cost.
- Press Calculate to view results above the form.
- Use the download buttons to save CSV or PDF.
Practical notes for gardeners
- Air leaks matter: doors, vents, and gaps can dominate heat loss.
- Wind increases loss: exposed sites often need a safety margin.
- Night curtains help: thermal screens reduce effective U at night.
- Condensation reduces performance: keep glazing clean and dry.
- Use averages: for planning, use typical night temperatures.
Glazing selection drives nighttime heating demand
Glazing heat loss is governed by the U-value, exposed area, and the temperature difference. Lower U-value materials slow conduction and convection, helping soil and benches stay warmer when outside temperatures drop. Use the calculator to compare films, polycarbonate, and glass before committing to a retrofit.
Temperature difference and time set the real load
A greenhouse may only need aggressive heating for a few cold hours, while sunny afternoons rely more on solar gain. Enter realistic inside and outside temperatures for the period you care about, then set hours to match that window. This converts a snapshot heat rate into energy use, cost, and optional emissions.
Interpreting U-value and R-value for garden structures
U-value describes how many watts pass through each square meter per degree of difference. R-value is its inverse and is sometimes listed in product brochures. When you input R, the calculator converts imperial R to metric resistance, then applies U = 1/R for consistent comparisons. Remember that frames, joints, and fasteners can create thermal bridges.
Planning crops, heaters, and thermal screens
Once you know the expected watt loss, you can size heaters with a safety margin for wind and infiltration. Cold air leakage at doors and vents can rival glazing losses, so sealing is often the cheapest “upgrade.” Thermal curtains, inner liners, or double layers reduce effective U at night and can cut run time. Use BTU per hour if your heater is rated in BTU.
Using results to choose cost-effective retrofits
Compare scenarios by changing glazing type, area, and temperature target. A small drop in U-value can save significant kWh over a season, especially when you maintain high setpoints for tender crops. Use the cost output to estimate payback time and prioritize measures: stop leaks first, then add layers, then replace panels. Re-check after cleaning glazing and improving drainage to reduce condensation. Document assumptions so you can repeat the calculation later easily.
FAQs
Does this include air leakage losses?
No. It estimates conductive heat transfer through glazing only. Drafts from vents, doors, and gaps can add large losses, especially in wind. Use results as a baseline and add a safety margin for infiltration.
Which area should I enter?
Enter the total exposed glazing area: walls, roof panels, and end walls that face outdoor air. Exclude the floor. For arched houses, approximate using surface measurements or panel counts and sizes.
What U-value should I use for poly films?
Use manufacturer data when available. Typical single film is around 6 W/m²·K, while double inflated film is commonly near 3.5 W/m²·K. Aging, looseness, and moisture can worsen performance.
Why does lowering the inside temperature save so much?
Heat loss scales with ΔT. Dropping the setpoint by a few degrees reduces watt loss, kWh, and fuel use proportionally. Night setbacks and crop zoning are often easier than changing glazing.
Can I use this for daytime heating planning?
Yes, but daytime loads are often offset by solar gain. Use outside temperatures representative of cloudy periods, and shorter durations. For full energy modeling, also account for sunlight, thermal mass, and ventilation.
How accurate are the cost and emissions outputs?
They are planning estimates. Cost assumes your rate per kWh equivalent, and emissions use a simple factor you provide. Real results vary with heater efficiency, cycling, weather swings, and how tightly the structure is sealed.