Practical guide · stated assumptions and checkable examples
Window replacement: use whole-window U-values and separate the cost case
Published · Sources checked
Source and worked-example review; not professional advice or approval. Examples are synthetic unless explicitly identified otherwise. Follow the cited authority for current eligibility and legal requirements.
A window quote can change appearance, maintenance needs, comfort and energy performance at the same time. An energy worksheet should isolate the part it can calculate instead of assigning all of the project’s value to an unsupported bill-saving claim.
Read comparable performance ratings
NFRC distinguishes U-factor, solar heat gain coefficient and visible transmittance. A lower U-factor means less heat transfer under the rating conditions; it does not by itself describe solar gain or daylight. Compare whole-window ratings in the same units rather than using centre-of-glass performance for one product and an assembly rating for another.
References: National Fenestration Rating Council — Window performance ratings
Model one mechanism at a time
For a heating-only conduction scenario, use window area × change in U-value × heating degree-days × 24 / 1,000. With area 15 m², a reduction of 1.5 W/(m²·K) and 2,500 K·days, the result is 1,350 kWh of thermal heat reduction. At an assumed 90% heating efficiency, modeled fuel input reduction is 1,500 kWh. At an invented fuel price of 0.10/kWh, that component is 150 per year.
Calculation: Thermal heat reduction (kWh) = area (m²) × ΔU (W/m²K) × HDD (K·days) × 24 / 1000
Do not convert a temperature difference as an absolute temperature
Degree-days and temperature differences do not use an offset. Celsius and Fahrenheit differences have a ratio of 1.8, while absolute temperatures also require the freezing-point offset. Mixing these conversions in a degree-day estimate creates a numerical error. Also verify whether the U-value is expressed in SI units or Btu/(h·ft²·°F).
References: NIST — Guide for the Use of the International System of Units
Explain what that 150 does not include
DOE’s window guidance discusses frames, glazing, gas fills and operation. Actual performance also depends on the installed assembly. The simplified example does not model solar gain, shading, air leakage, cooling loads, heating schedules or variations in weather. The 150 is an output of selected inputs, not an independently measured saving or a quote from an installer.
References: US Department of Energy — Window types and technologies
Put the project cost beside the right benefit
List product cost, removal, making good, access and installation separately. For a synthetic 9,000 project, dividing by 150 gives a 60-year simple energy-only payback under unchanged assumptions. That result neither measures comfort nor decides whether replacement is necessary. It shows why the energy component and the wider replacement decision should remain distinct.
Questions about this guide
Can I use the glazing U-value from a brochure?
Use a value comparable to the modeled area and assembly. A glazing-only value is not automatically a whole-window rating.
Does this worksheet guarantee a heating-bill reduction?
No. It calculates a stated conduction scenario. Tariffs, installation, solar gain, leakage and household use can change the realized result.