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Window Performance Comparison Calculator

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Energy-efficient replacement windows in a Texas home

Compare two whole-window NFRC ratings under the same conditions. Enter the total window area, temperature difference, incident solar intensity, and the old/new U-factor and SHGC. The tool estimates instantaneous conductive heat flow and solar heat gain in Btu per hour. It does not predict annual energy bills.

Add the width × height of the windows being compared.
Use the absolute difference for the snapshot you want to compare.
Use the whole-product NFRC value when available.
Lower values indicate less heat flow under the rating conditions.
This is a user-controlled scenario input, not a local weather forecast.
SHGC is a dimensionless number from 0 to 1.
Lower values admit less solar heat in this simplified comparison.
Important limitation: This is a simplified comparison of window ratings at one user-selected moment. It does not model orientation, sun angle, shading, air leakage, humidity, thermal mass, installation quality, HVAC efficiency, occupant behavior, weather over time, utility rates, or rebound effects. Do not treat the output as an annual energy, cost, equipment-sizing, code-compliance, or savings guarantee.

Calculation method

Component Formula Units
Conductive heat flow U-factor × window area × temperature difference Btu/h
Solar heat gain SHGC × window area × incident solar intensity Btu/h
Combined snapshot Conductive heat flow + solar heat gain Btu/h

The U.S. Department of Energy describes U-factor as the rate of heat flow through a fenestration product and explains that heat transfer depends on U-factor and the indoor/outdoor temperature difference. Berkeley Lab describes solar heat gain as the product of SHGC and incident radiation. NFRC ratings should be taken from the whole-product label or certified record.

How to use the result

  1. Run the same area and environmental inputs for both products.
  2. Use whole-product NFRC values rather than center-of-glass values.
  3. Try multiple temperature and solar scenarios instead of relying on one default example.
  4. Compare installation scope, air leakage, orientation, shading, warranty, and applicable code separately.
  5. Use an approved building-energy model or qualified professional for annual savings and equipment decisions.

Sources and related guidance

Method reviewed: July 21, 2026.