Calculator Inputs
Formula Used
The main conductive heat loss formula is:
Q = U × A × ΔT
Here, Q is heat loss in watts. U is the window U value. A is total glass area. ΔT is the indoor and outdoor temperature difference.
The adjusted formula used by this calculator is:
Adjusted Q = (U × A × ΔT × frame factor × safety factor × curtain factor) − solar gain
Energy is calculated as:
Energy = Q ÷ 1000 × hours
Cost is calculated as:
Cost = energy × price per kWh
How to Use This Calculator
- Enter the window width and height.
- Add the number of similar windows.
- Enter a custom area if you already know it.
- Add the window U value from the product label.
- Enter indoor and outdoor temperatures.
- Set the time period for energy loss.
- Add electricity, gas, or heating cost per kWh.
- Use optional correction factors for frames, curtains, and solar gain.
- Press the calculate button to view heat loss results.
Example Data Table
| Window Type | Area | U Value | Temperature Difference | Heat Loss |
|---|---|---|---|---|
| Single glazing | 2.00 m² | 5.70 W/m²·K | 20 K | 228 W |
| Double glazing | 2.00 m² | 2.80 W/m²·K | 20 K | 112 W |
| Low-e double glazing | 2.00 m² | 1.60 W/m²·K | 20 K | 64 W |
| Triple glazing | 2.00 m² | 0.90 W/m²·K | 20 K | 36 W |
Understanding Heat Loss Through Windows
Why Windows Lose Heat
Windows are weak points in many building envelopes. Walls often contain thick insulation. Glass is thinner and conducts heat more easily. Frames, seals, spacers, and air gaps also affect total performance. A room can feel cold even when the heater is running. One reason is radiant cooling near poor windows. Another reason is warm indoor air touching cold glass.
Role of U Value
The U value describes heat flow through one square meter for each degree of temperature difference. A high U value means faster heat loss. A low U value means stronger insulation. Single glazing usually has a high U value. Double glazing performs better. Low-e coatings and gas fills can reduce heat movement. Triple glazing often gives the lowest heat loss.
Area and Temperature Difference
Area matters because larger windows expose more surface. A small bathroom window may lose little heat. A large living room window can lose much more. Temperature difference is also important. Heat loss rises when outdoor air gets colder. The same window loses more heat on a freezing night than on a mild evening. This calculator uses absolute temperature difference.
Advanced Adjustments
Real buildings are not perfect laboratory cases. Frames may conduct extra heat. Edge spacers can increase loss near glass borders. Curtains can reduce heat transfer when they trap still air. Sunlight can offset part of the loss during daytime. A safety factor can cover uncertain product data, aging seals, installation gaps, or thermal bridges around the opening.
Energy and Cost Planning
Power loss in watts shows the instant heating load. Energy in kilowatt-hours shows total loss over time. Cost converts that energy into money. These results help compare glazing upgrades, curtains, storm windows, and insulation improvements. The calculator gives an estimate, not a certified audit. Still, it is useful for quick design checks and practical home energy planning.
FAQs
What is window heat loss?
Window heat loss is thermal energy moving from warm indoor air through glass, frames, and edges to colder outdoor air.
What U value should I enter?
Use the rated U value from the window label, specification sheet, or energy certificate. Lower U values mean better insulation.
Can I use Fahrenheit?
Yes. Select Fahrenheit in the temperature unit field. The calculator converts the difference to Kelvin for the heat loss formula.
What does frame factor mean?
Frame factor adjusts the result for frames, spacers, and edge losses. Use 1.00 when you do not want an adjustment.
How does curtain reduction work?
Curtain reduction lowers heat loss by a chosen percentage. It represents insulation from curtains, blinds, shutters, or trapped air layers.
Why is solar gain subtracted?
Solar gain adds heat through the window. During sunny periods, it can offset some conductive loss, so the calculator subtracts it.
Is this suitable for double glazing?
Yes. Enter the correct U value for the double glazed unit. The same formula works for single, double, and triple glazing.
Is the result exact?
No. It is an estimate. Air leaks, wind, installation quality, humidity, curtains, and room layout can change real heat loss.