Quick answer: Clean glass transmits around 90% of visible light; add a season of film and grime and glazing-industry measurements suggest losses of 5–10% for typical soiling and up to 20% for heavily soiled panes — like fitting a permanent grey filter to every window. In a UK winter, when the sky delivers a tenth of its summer light on a good day, that loss lands on your mood, your houseplants, your daylight hours and even your heating (dirty glass also cuts free solar warmth). Window cleaning is the only home-lighting upgrade that's free.
Nobody notices their windows getting dimmer, because it happens at the speed of dust — a fraction of a percent at a time, evenly, everywhere. Then one January afternoon the house feels like a cave by 3pm and everyone blames the season. Some of it is the season. Some of it is washing off with vinegar water.
The transmission numbers
Fresh modern double glazing passes roughly 70–90% of visible light depending on coatings. Soiling stacks its own loss on top: measurements in the glazing and solar industries put typical urban soiling at 5–10% light loss, with heavy grime — traffic film, algae bloom, coastal salt — reaching 15–20%. (The solar industry knows these numbers coldest, because for panels the loss is billable — the same physics, with an invoice.)
A fifth less light doesn't read as "dirty windows". It reads as "this room is a bit gloomy", "the plants keep dying", "we need more lamps".
Winter multiplies the cost
A UK December day delivers a small fraction of June's light, through a low sun that hits your glass at a shallow, haze-maximising angle — low-angle light scatters off grime far worse than overhead summer sun, which is why windows look dirtiest (and rooms dimmest) exactly when light is scarcest. Anyone whose winter mood tracks the daylight — which is most of us, clinically or not — is paying the soiling tax at the worst possible exchange rate. The cheapest response to a dark house isn't a lightbox; it's cleaning the glass in winter, which is exactly when most people stop.
Three more bills the grime quietly pads
- Heating: south-facing glass is a free radiator — passive solar gain matters in UK homes, and film on the glass skims it. Clean south glazing is worth real watts on a sunny cold day.
- Plants: windowsill herbs and houseplants live on window light; a 10–20% cut is the difference between thriving and leggy. (Greenhouse growers treat glass cleaning as crop management — their guide.)
- Electric light: lamps on at 3:30 instead of 4:15, every winter day, in every room that's dimmer than it should be. Small numbers, multiplied by a season.
The free upgrade (and keeping it)
Clean every pane properly once — both sides, the fast method — and stand in the room after. The difference is not subtle; people repaint rooms for less change than this. The problem is persistence: light loss rebuilds at the speed of your postcode, and the panes that matter most (big, high, south-facing) are the ones least likely to get re-cleaned by hand.
That's the case for automating it: a CleanBot R1 holds every pane — including the tall and unreachable ones — at week-one clarity all winter, for the effort of placing it and anchoring a rope. Maximum daylight becomes the house's default state rather than a twice-a-year event.
FAQ
Which windows matter most for light?
South-facing and largest first, then the windows of rooms you use in the afternoon. One big clean pane out-delivers three small ones.
Inside or outside face — which steals more light?
Outside, usually, by far — weather does more than fingerprints. But kitchen-adjacent inside faces carry a grease film that scatters light badly for its thickness.
Can I measure the difference?
Any phone light-meter app: same spot, same hour, before and after cleaning. Seeing "+15%" in numbers is weirdly motivating.
Your windows are the biggest lamps you own. The CleanBot R1 keeps them switched all the way on — every grey British week of the year.