Window Heat Gain Calculator

How the calculator works

It follows the sun across the sky at your place every 10 minutes of the day you pick, with the NOAA solar position equations. For each moment it works out three things.

• How strong the sun is. On a cloudless day the direct sun on a surface square to it is about 1,353 W/m² × 0.7^(AM0.678), where AM is the air mass, the length of the sun's path through the air. That is the formula PVEducation gives, with the air mass corrected for the Earth's curvature. The same source puts the scattered light from the rest of a clear sky at about 10% of the direct sun; a wall sees half the sky, so a window gets half of that.
• How squarely it hits the glass. The direct sun on a vertical window is cut by the cosine of the angle between the sun and the window's face. At steep angles glass also reflects more of it away. A window's SHGC is measured with the sun square on, so the calculator takes off the extra reflection for each pane using the Fresnel equations, as in the physical glass model in pvlib (De Soto and others, 2006).
• How much gets inside. Heat through the window = area × SHGC × (direct + diffuse sun on the glass). The shading then keeps out a share of it, or for an overhang, takes the direct sun off the part of the window in its shadow.

The card's own check, which the calculator is tested against: a 1 m² (10.8 ft²) south window at 40°N, SHGC 0.7, at solar noon on 21 June. The sun stands 73.4° high, due south, so it strikes the glass 73.4° off square. The direct sun is 937 W/m², and cos 73.4° = 0.285 leaves 267 W/m² on the glass. One pane passes 80.5% of that at this angle, compared with square on, giving 215 W. The sky adds 47 W, and × 0.7 gives 183 W into the room. The same window facing west takes nothing at noon and much more later: in Denver at 4 pm, 2 m² of double clear glass takes 622 W.

Which way the window faces matters most

In summer the sun rises north of east and sets north of west, and at midday it is high. A south window then gets the sun at a glancing angle, while east and west windows face it low and square. The calculator's figures for 1 m² (10.8 ft²) of window on 21 July, a clear day, no shading:

Facing	Phoenix, double clear (SHGC 0.53)	London, double low-e (SHGC 0.44)
North	0.43 kWh	0.41 kWh
East	1.77 kWh	1.64 kWh
South	0.50 kWh	1.24 kWh
West	1.77 kWh	1.64 kWh

London's south window takes more than Phoenix's because London is 18° further north, so the summer sun is lower there and hits the glass more squarely. East and west match on a clear day. In practice the west side is the one that overheats, because its sun arrives in the afternoon, when the air outside and the house are already at their hottest. How the sun moves across your garden explains the path behind these numbers.

What each kind of shading keeps out

The calculator starts each shade at the figure below, and you can type your own product's figure over it. Outside shading stops the sun before it reaches the glass, while inside shading meets it once it is already through. Efficient Window Coverings, built by Lawrence Berkeley National Laboratory's windows group, says that is why awnings "can be far more effective than interior blinds and shades".

Shading	Share of the sun's heat kept out	Source
Exterior awning	up to 65% on south windows, 77% on west	US DOE Energy Saver
Exterior solar screen	80 to 90% (SunTex 80/90; 80% used)	Phifer
Interior cellular shade, closed	up to 60%	US DOE Energy Saver
Interior blinds, reflective, closed	around 45%	US DOE Energy Saver (2018)
Interior roller shade	20 to 60% by color and weave (40% used)	Efficient Window Coverings
Medium-colored drapes, white plastic backing	33%	US DOE Energy Saver
Window film	from the film's SHGC on its label	Efficient Window Coverings

Read the "up to" figures as best cases. DOE gives awning figures only for south and west windows, so the calculator uses 65% for a window within 45° of south and 77% for any other direction; below the equator, north takes the place of south. The roller-shade figure is just the middle of a range, not a measurement, so if you know your fabric's own figure, use it. Films differ too much for one typical number. Their effect is on the label: if your window's SHGC is 0.53 and the film's label gives 0.30 on the same glass, the film keeps out 1 − 0.30 ÷ 0.53 = 43%. Efficient Window Coverings also notes that indoor shades make less difference behind double low-e glass, which already keeps out much of the heat.

An overhang, such as an eave, a porch roof or a fixed awning, is the one shade worked out from the sun's position, not from a fixed share. At each moment the calculator finds where the overhang's shadow line falls on the glass and takes the direct sun off everything below it. It counts the overhang as running well past both sides of the window, and it leaves the light from the sky as it is, so it slightly understates what the overhang does. That is why the same overhang works so differently by facing. On the Phoenix window below, a 2 ft (0.61 m) overhang set 1 ft (0.30 m) above the glass keeps out 45% of the day's heat facing south but only 16% facing west, where the afternoon sun gets in under it. Shading windows to cut cooling costs goes through the choices, the types of awnings and what they cost the prices, and retractable or fixed the choice between an awning you can wind in for winter and one you cannot.

The glass

The solar heat gain coefficient (SHGC) is the share of the sun's heat striking a window that ends up inside. In the US it is the top-right number on the window's NFRC label; in the UK the same idea is the g-value. If you know your window's figure, choose "My window's label" and type it. If not, the defaults are whole-window figures, frame included:

Glass	SHGC used	From
Single, clear	0.60	g 0.85 × frame factor 0.7
Double, clear	0.53	g 0.76 × 0.7
Double, low-e (soft coat)	0.44	g 0.63 × 0.7
Double, low-e, low solar gain	0.25	"0.25 or less"
Triple, low-e	0.40	g 0.57 × 0.7

The g-values and the 0.7 frame factor for wood and PVC frames come from Tables 6b and 6c of SAP 10.3, the UK government's method for rating home energy use, published by BRE in June 2026. The low-solar-gain figure is the top of the band the Efficient Windows Collaborative gives for that glass. The Collaborative calls low-solar-gain glass "ideal for buildings located in cooling-dominated climates" and high-solar-gain low-e the choice where heating matters most.

From heat to a cooling bill

An air conditioner moves several units of heat for each unit of electricity it uses. That ratio is its COP. US equipment is rated in SEER2, Btu of cooling per watt-hour, and since 1 Btu per hour is 0.2930711 W (NIST), COP = SEER2 × 0.293. The default, 14.3 SEER2 (COP 4.19), is the federal minimum for a new split-system air conditioner in the Southeast and Southwest; in the North it is 13.4 (10 CFR 430.32). Heat kept out ÷ COP is the electricity saved.

The default prices are the US average residential price, 18.31¢ per kWh in July 2026 (EIA, Electric Power Monthly, Table 5.6.A), and the UK price cap unit rate, 26.32p per kWh by Direct Debit from 1 October to 31 December 2026 (Ofgem). Type your own from your bill. The number of days is yours to set: 90 stands for a summer of clear, hot days with the cooling on, not a researched figure, and cloudy days let in less.

Worked through for a west window in Phoenix, 4 × 5 ft (1.22 × 1.52 m), double clear glass, on 21 July: 3.28 kWh of heat comes in over the day (11,200 Btu), peaking at 686 W (2,340 Btu/h) in the late afternoon. An awning keeping out 77% stops 2.53 kWh of it. At COP 4.19 that is 0.60 kWh of electricity a day, and over 90 such days at 18.31¢, about $9.90. In London, a 1.2 × 1.5 m (3 ft 11 in × 4 ft 11 in) south-west window of double low-e glass takes 2.93 kWh that day, and an indoor roller shade at 40% keeps out 1.17 kWh. With air conditioning that would be £6.60 over 90 days at the price cap. Without it, the saving is in comfort: 1.17 kWh is a 1 kW heater left running for over an hour.

The figures are small for one window, which is why the calculator has a box for several windows. The bigger point is often comfort: the peak, not the day's total, is what makes a west room hot at 5 pm. Shade for hot climates looks at whole-house shading, and the louvered pergola guide at shading a patio door or a wall of glass.

What it leaves out

This is an estimate for a clear day, not a survey of your house. Haze, cloud, trees and neighboring buildings all cut the sun. Light reflected up off a pale patio or a lawn is left out, which makes the figures low for a window over bright paving. The heat counts as cooling load the moment it comes in, though in a real room some of it is stored in the walls and floor and released later. Shading that keeps out summer sun also keeps out winter sun you might want, and DOE notes that awnings "can increase energy used for heating", which is the case for one you can take down or wind in. For an exact figure, an energy assessor (a home energy rater in the US, a domestic energy assessor in the UK) models the whole house.

Sources: NOAA (sun position); PVEducation (clear-sky sun); pvlib (glass at an angle); US DOE Energy Saver, archived copies of 1 January 2026 and 17 January 2018, since the pages are no longer on energy.gov; Efficient Window Coverings; Phifer; SAP 10.3; Efficient Windows Collaborative; 10 CFR 430.32; NIST SP 811; EIA; Ofgem. Checked 19 October 2026.