Window Energy Performance Calculator: Methodology and Data Sources

How the calculator works

The calculator estimates how much heating and cooling energy a window configuration saves each year compared with a reference window, at your ZIP code. It combines typical hourly weather for the nearest weather station with the U-factor and SHGC of the window you configure. The results are estimates for comparing options.

Weather data

• Source: NREL's TMY3 (Typical Meteorological Year) data sets. A TMY is one year of hourly solar radiation and weather values, built from 12 typical months. NREL derived TMY3 from a base period of 1976–2005 where 30 years of data exist, and 1991–2005 elsewhere.
• Stations: the calculator uses 1,010 TMY3 weather stations.
• Your location: your ZIP code is converted to coordinates, and the calculator uses the closest station.

What is pre-computed for each station

• Heating degree-hours: for every hour colder than 18 °C (64.4 °F) outdoors, the difference between 20 °C (68 °F) indoors and the outdoor temperature, summed over the year.
• Cooling degree-hours: for every hour warmer than 24 °C (75.2 °F), the amount by which the outdoor temperature exceeds 24 °C, summed over the year.
• Solar radiation on a wall facing each of eight directions (N, NE, E, SE, S, SW, W, NW), summed separately over heating hours and cooling hours.

How savings are calculated

The same steps run for your window and for the reference window:

• Heating loss = Uw × window area × heating degree-hours.
• Solar gain = glass area × SHGC × shading factor × solar radiation on that wall.
• Heating need = heating loss minus the useful part of the winter solar gain. The useful part comes from the gain utilization factor of ISO 13790:2008 (since replaced by ISO 52016-1:2017). Solar gain that arrives when the house needs no heat does not count.
• Cooling load = Uw × window area × cooling degree-hours, plus the full summer solar gain.

Annual savings = reference window load − your window's load, for heating and cooling separately, multiplied by the number of windows.

Uw is calculated from frame (Uf), glass (Ug) and glass-edge (Ψ) values in the GlassBase product database with the area-weighted formula of ISO 10077-1: Uw = (Uf × A_frame + Ug × A_glass + Ψ × glass perimeter) / (A_frame + A_glass). SHGC and visible transmittance are area-weighted from glass and frame values. U-factor is shown in BTU/(h·ft²·°F). One BTU/(h·ft²·°F) equals 5.678 W/(m²·K).

Reference window

The calculator compares your window with a reference window of the same size, orientation and shading. You pick the reference under "Comparison"; its values come from the GlassBase reference-window table:

• Single-pane window: Uw 5.75 W/(m²·K) (about 1.01 BTU/(h·ft²·°F)), SHGC 0.87 (the default)
• Double-pane window 1": Uw 2.63 W/(m²·K) (about 0.46 BTU/(h·ft²·°F)), SHGC 0.78

Inputs

ZIP code, window material, brand and profile system, glazing, spacer, window size and number of windows, window facing, exterior shading and the comparison window. If no facing is chosen, the calculator uses south. In the savings panel you can also set the heating type, energy prices and cost per window.

Outputs

Calculated U-factor, SHGC and visible transmittance; annual heating and cooling energy saved; yearly and cumulative dollar savings; and the payback period based on the cost per window you enter.

Assumptions

Limits

• Only heat conducted through the window and solar gain are counted. Air leakage is not part of the savings figure.
• The house itself is not modeled: no internal gains, thermal mass, ventilation or thermostat schedules.
• All windows are assumed to be the same size and to face the same direction.
• The nearest station's climate can differ from your site's. A TMY is a typical year, not a forecast.
• Values are calculated with ISO methods. They are not NFRC 100/200 ratings, ENERGY STAR certification or proof of code compliance. For certified values, check the product's NFRC label or the NFRC Certified Products Directory.

See also: Window energy ratings in the US and the window and glazing glossary.

Sources

• NREL, Users Manual for TMY3 Data Sets (NREL/TP-581-43156, revised May 2008)
• TMY3 files as distributed by climate.onebuilding.org
• ISO, ISO 13790:2008, Energy performance of buildings — Calculation of energy use for space heating and cooling; replaced by ISO 52016-1:2017
• ISO, ISO 10077-1:2017, Thermal performance of windows, doors and shutters — Calculation of thermal transmittance — Part 1: General
• NIST, Guide to the SI, Appendix B.9 (coefficient of heat transfer conversion)
• NFRC, Certified Products Directory

• U.S. EIA, Electric Power Monthly, Table 5.6.B, Average Price of Electricity to Ultimate Customers by End-Use Sector, by State (residential sector)
• U.S. EIA, Natural Gas Prices, price delivered to residential consumers, and Heat Content of Natural Gas Delivered to Consumers
• U.S. EIA, Weekly Heating Oil and Propane Prices, residential No. 2 heating oil