Measured, not estimated
What this website costs the planet
Every website has a carbon cost: moving bytes takes electricity, in the data centre, across the network and in your device. The honest way to talk about it is to measure this page, right now, in your browser — so that is what the figure below is.
Already lighter than most of the web
This matters more than any setting we could offer you. The median web page in 2025 weighed 2,412 KB on desktop and 2,164 KB on mobile. Our home page transfers around 550 KB — roughly a quarter of that — before you change a single setting. The choices that get it there apply to everyone, all the time, with nothing to opt into:
- Photographs are served as AVIF or WebP at a quality chosen to be visually indistinguishable rather than maximal, and cached for thirty days once encoded.
- Pages are rendered ahead of time and served from cache, so a visit rarely costs any database work at all.
- Fonts are self-hosted, and only the two faces that draw the largest text are preloaded.
- The interactive map loads only when you ask for it, and only after you have accepted the cookies it needs — so most visits never download it.
- Below-the-fold sections are skipped by the browser’s layout engine until they are near the viewport.
- The site is hosted on renewable-powered infrastructure (see below), which lowers the data-centre share of every figure on this page.
How that figure is worked out
- Bytes. The browser’s Resource Timing API reports what this page actually transferred, including anything already in your cache (which counts as no new bytes, because it is).
- Grams. co2.js from the Green Web Foundation converts those bytes using the Sustainable Web Design model, version 4.
- Green hosting. The site runs on Hetzner, which the Green Web Foundation lists as a green host with renewable-energy and ISO 14001 evidence on file — you can check that yourself. Under this model that lowers the figure by roughly 18%, because only the data-centre part of the chain is affected — not the network or your device.
- What we can’t see. Browsers hide the size of some third-party responses. Where any of those exist on a page, the figure is labelled a minimum and the hosts are named, rather than quietly counted as zero.
Low-carbon mode
The leaf in the header can serve you a deliberately lighter version of the site. It is off by default — the page you are reading is already lean, and we would rather not degrade anyone’s experience without being asked. Three settings:
Off
YoursThe default. Nothing is changed.
You get the site exactly as designed — full-quality photography, entrance animation, the lot. It is already lighter than most of the web, so this is a perfectly reasonable place to stay.
Auto
Lighter only when it counts.
Stays off until there is a reason: your device or browser asks for reduced data, or the UK electricity grid is running dirty enough that the same page costs more carbon to serve. The grid is re-checked every half hour, and the site returns to normal by itself.
On
Always lighter.
Images are re-encoded at roughly a quarter of their usual weight, motion is switched off, and decorative imagery is never requested. The site looks plainer and photographs are visibly softer — that is the trade.
The UK grid is currently generating at 191 gCO₂e/kWh (high) — leaning on gas right now.
Your setting is not Auto, so this reading is information only — it is not changing anything for you.
Grid data from the National Grid ESO carbon-intensity API, cached for half an hour — the interval at which it updates.
Dark mode
The sun and moon in the header switch the whole site between light and dark. Dark mode is offered mainly for comfort, and it is worth being straight about its energy effect, because it is routinely overstated.
- It only saves power on OLED screens — most modern phones, few laptops and desktops. On an LCD the backlight is on regardless of what is drawn, so dark pixels save essentially nothing.
- At normal brightness the saving is small. Purdue University researchers measured switching from light to dark at 30–50% brightness — where most people actually sit — as 3–9% of total phone power. At full brightness it rises to around 42%.
- Turning your brightness down does far more. The same study found dropping from 100% to 50% brightness cuts OLED power draw by roughly ten times as much as the light-to-dark switch does — whatever theme you are using.
- It changes nothing on our side. The theme is a CSS choice applied in your browser: identical bytes, identical server work. It is not part of the figure at the top of this page, and we do not count it as a saving.
The site opens in light mode and remembers your choice in a cookie; it deliberately does not follow your operating system, so switching is always something you did rather than something that happened to you.
