Formula Used
The calculator uses steady-state heat transfer through a surface.
Heat flow: Q = U × A × ΔT
Adjusted heat flow: Qadj = Q × adjustment factor
Thermal energy: Eth = Qadj × hours ÷ 1000
Site energy: Es = Eth ÷ COP
Cost: Cost = Es × energy price
Emissions: CO2e = Es × emission factor
U is the thermal transmittance. A is area. ΔT is temperature difference. COP adjusts the delivered thermal load into energy purchased.
How to Use This Calculator
- Enter the surface name, such as window, wall, roof, or skylight.
- Add the total surface area and choose the correct area unit.
- Select whether you are entering U factor or R value.
- Enter indoor and outdoor temperatures.
- Choose heating, cooling, or absolute heat transfer mode.
- Add season hours, system COP, energy price, and emission factor.
- Use the optional comparison value to estimate savings.
- Press the calculate button, then download the CSV or PDF report.
Example Data Table
| Assembly |
Area |
U Factor |
Temperature Difference |
Season Hours |
Thermal Energy |
| Double pane window |
100 m² |
1.80 W/m²·K |
19 K |
2880 h |
9849.6 kWh |
| Improved window |
100 m² |
1.20 W/m²·K |
19 K |
2880 h |
6566.4 kWh |
| Roof assembly |
150 m² |
0.25 W/m²·K |
22 K |
2400 h |
1980.0 kWh |
Understanding U Factor Energy Use
U factor measures how easily heat passes through a building part. A lower value means better resistance. A higher value means more heat moves through glass, walls, roofs, or doors. This calculator turns that rating into energy use. It uses area, temperature difference, time, and equipment performance.
Why U Factor Matters
Heat transfer can be small each hour. It becomes large over a season. A single window may lose little heat during one cold night. Many windows can create a large heating load over months. The same idea applies during cooling season. Warm outdoor air pushes heat through the envelope. The cooling system must remove that heat.
Main Inputs
Start with the surface area. Use square metres or square feet. Enter the U value or an R value. The tool can convert common units. Next enter indoor and outdoor temperatures. The difference drives heat flow. Add operating hours and season days. Use a COP or efficiency factor for the system. Add local energy price and emission factor for practical outputs.
Result Meaning
The peak heat flow shows instant load. It helps size envelope loads. Thermal energy shows heat transferred during the chosen period. Site energy adjusts that load by equipment performance. Cost uses your entered utility rate. Emissions use your local grid or fuel factor. A comparison U value can show possible savings.
Design Use
Use the result to compare window upgrades, insulation levels, and roof assemblies. Keep assumptions realistic. Average outdoor temperature is useful for simple estimates. Degree hour data gives a better seasonal result. Shading, air leaks, solar gain, and moisture can change real performance. Still, this method gives a clear physics baseline.
Better Decisions
A U factor calculation supports early design choices. It can show where improvements matter most. Large areas with poor U values often deserve attention first. Better assemblies lower peak load and seasonal cost. They may also improve comfort near cold or hot surfaces. Use this estimate with local codes, product ratings, and detailed energy modeling when accuracy is critical.
Review several scenarios before buying products. Small input changes can move the final cost. Record each run and compare the CSV file. Keep notes for future design revisions later.
FAQs
What is U factor?
U factor is the rate of heat transfer through a surface. It is often used for windows, walls, roofs, doors, and skylights. Lower values usually mean better thermal performance.
How is U factor different from R value?
R value measures resistance to heat flow. U factor measures heat flow. They are reciprocals when units match. High R value means low U factor.
Can I use this for windows?
Yes. Enter the total window area and the rated window U factor. Use seasonal temperature difference and operating hours to estimate heat gain or loss.
Can I use this for walls or roofs?
Yes. Use the assembly area and its effective U factor. Include framing effects if your U factor already includes them. Otherwise use the adjustment field carefully.
What does COP mean?
COP means coefficient of performance. It compares delivered heating or cooling to purchased energy. A higher COP reduces estimated site energy use.
Why is my result zero?
Heating mode gives zero when outdoor temperature is higher than indoor temperature. Cooling mode gives zero when outdoor temperature is lower. Use absolute mode for unsigned heat transfer.
What is the adjustment percent?
It is a multiplier for practical changes. Use 100 for normal calculation. Use higher values for added losses. Use lower values for reduced operating exposure.
Is this a complete building energy model?
No. It is a steady-state envelope estimate. It does not fully model solar gain, thermal mass, air leakage, occupancy, controls, or hourly weather changes.