Free tool

Solar Gain Calculator

Estimate the peak solar heat gain (in watts) a window will get on a given day, and what time it peaks, based on where it is, which way it faces, how big it is, and what glazing it has.

Location

Using South East (51.5074, -0.1278)

Date

We scan the day in 15-minute steps to find the peak gain and when it happens.

Window orientation

Bearing (degrees, 0 = N, 180 = S)

Window size

Width (m) Height (m)

Sizes include the frame — measure the overall opening.

Glazing Glazing type

Advanced

Frame allowance (% of area) Window tilt (degrees from horizontal — 90 = vertical wall window) Ground surface below the window Interior shading (blinds/curtains) reduction: 0%

Known simplifications

Assumes a clear sky — real-world gain will be lower on cloudy days.
The SHGC angle correction is an approximation, not a manufacturer-tested curve.
This is instantaneous heat gain through the glass, not a full room heat balance — it doesn't account for thermal mass, re-radiation, or how the room retains the heat.
Window sizes are treated as overall (frame-included) dimensions, with a simple area-based allowance (default 12%, adjustable in Advanced) subtracted for the frame — not a true frame profile.
No overhang or reveal shading yet — every window is treated as fully unshaded.
Peak gain is found by sampling the day every 15 minutes, not continuously — the true peak time could be a few minutes either side.