Window Energy Loss Calculator
How much heat are your windows losing — and what would it cost you? Count your windows, pick a size, choose your glazing type and heating tariff to see annual heat loss in kWh, the cost on your energy bill and how much you could save by upgrading to better glazing.
Quick Start
Tip: Walk through your home with a tape measure. Measure the glass — not the frame. Typical sizes: bathroom 60×80 cm, bedroom 90×120 cm, living room 120×150 cm, patio door 180×200 cm. Add one row per window size.
Settings
Your Windows
Give each row a name (e.g. "Bedroom"), enter the glass width and height in cm, and how many windows of that size you have. Measure the glass area only — not the frame.
Temperatures & Heating
Multi-year Projection
Window heat loss explained: U-values, glazing types and payback
How do windows lose heat?
Windows lose heat through three mechanisms: conduction through the glass and frame, convection of air within the glazing cavity, and radiation (infrared energy passing through the glass). The U-value combines all three into a single number expressing watts lost per square metre per degree Celsius of temperature difference. A lower U-value means better insulation.
U-values by glazing type
| Glazing type | Typical U-value (W/m²K) |
|---|---|
| Single glazed | 5.5–6.0 |
| Double glazed (pre-2000) | 2.5–3.0 |
| Double glazed (modern) | 1.4–1.8 |
| Double glazed Low-E | 0.9–1.2 |
| Triple glazed | 0.6–0.8 |
| Triple glazed Low-E | 0.4–0.6 |
How Low-E coatings work
Low-emissivity (Low-E) coatings are microscopically thin metallic layers applied to the glass surface. They reflect long-wave infrared radiation (heat) back into the room while allowing visible light to pass through. A standard double-glazed unit has a U-value around 1.6; adding a Low-E coating reduces this to 1.0–1.2 with no change to visible appearance.
Single to double glazing: the biggest improvement
Replacing single glazing with modern double Low-E units cuts window heat loss by approximately 80%. A home with 15 m² of single glazing in a 15°C temperature differential loses around 1,300 W continuously during heating season. The equivalent modern double Low-E window loses only 165 W — a saving of over 1 kW. At 2,400 heating hours per year and €0.28/kWh that is over €750 per year in heating cost.
Double to triple: diminishing returns
The jump from modern double (U 1.6) to triple glazing (U 0.7) is a real improvement, but smaller in percentage terms than single-to-double. Payback periods for double-to-triple upgrades are typically 20–30 years in temperate climates. The case is stronger in cold climates (Scandinavia, Canada, Alpine regions) where the temperature differential is much larger and heating seasons are longer.
Secondary glazing: a cost-effective compromise
Secondary glazing fits an additional pane inside the existing window frame, creating an air gap without replacing the original window. It achieves a combined U-value of around 1.8 W/m²K — significantly better than old double glazing and dramatically better than single glazing. Installed cost is typically €80–€150/m², making payback periods of 5–10 years realistic when replacing single glazing.