Window film blocks a substantial amount of the sun's heat — up to 79% with high-performance films — but the exact figure depends on the film grade and your glass. Understanding the numbers helps you choose the right film rather than assuming more is always better.
The key figure: TSER
Total Solar Energy Rejected (TSER) is the percentage of the sun's total energy the film stops. Films range from around 40% TSER for light, near-clear grades to about 79% for premium reflective and ceramic films. A higher TSER means a cooler room.
Where the heat goes
Solar energy is mostly infrared (the heat you feel) plus visible light and UV. Solar films reflect and absorb a large share of the infrared and UV while transmitting visible light — so they cut heat without dimming the room. That's why film beats blinds, which only block heat after it's already inside.
What it feels like in a real room
Numbers aside, most customers describe the difference as going from "unusable on a sunny day" to "comfortable" — a conservatory that no longer bakes, a home office that stays workable, a living room that loses the greenhouse feel. It reduces heat dramatically; it isn't air-conditioning, so the very hottest days are far better rather than cold.
Choosing the right amount
The most heat blocked isn't always the right answer — a heavy reflective film on a north window is overkill. We match TSER to orientation and the look you want. Our Heat Reduction Calculator gives an indicative figure for your glass.
The bottom line
Expect up to 79% of solar heat rejected with the best films, matched to your windows on the survey. Try the Heat Reduction Calculator, then book a survey for the exact spec.
Thinking about window film? We offer a site survey anywhere in Scotland, with most quotes returned within 24 hours.