Digital emissions

How we measure the site’s carbon emissions

For our site’s homepage, we estimate emissions at 0.07g of CO2-equivalent per page view as of April 2024.

We review the site’s emissions with the Sustainable Web Design (SWD) methodology. It’s an established way to calculate the carbon footprint of content websites, and individual web pages, based on the weight of the pages and how much traffic they receive. This is the same methodology as established tools like the Website Carbon Calculator.

Here are the underlying factors we use to calculate emissions:

  • We measure the page weight at 334 kb. This is measured with Google Lighthouse (via DebugBear) set up for Mobile emulation.
  • We use a grid carbon factor of 338 gCO2e/kWh. This is a weighted average based on the site’s demographics, using country-level carbon factor data from Ember.

We use this carbon factor to reflect our understanding of our site’s audience, in place of the global grid carbon factor of 442 g/kWh. With the global grid carbon factor our emissions would be 0.1g of CO2/view.

Why we use this methodology

We use a wide range of digital emissions methodologies depending on the needs of the project. For, we chose:

  • SWD to make figures more comparable with common emissions calculators.
  • Google Lighthouse, as its assessment of page weight on first page load is well established.
  • Simulated Mobile testing, rather than desktop as the majority of web traffic worldwide is on mobile devices.
  • The homepage, as it’s the most-viewed page of the site.
  • A static “one-off” measurement, to avoid adding a badge that would necessarily use up more resources to load.

Monitoring emissions over time

We like the instant recognition and ease of use of ready-made badges like those provided by Website Carbon, however we had concerns about the footprint of the badge itself.

Instead, we use DebugBear to check representative pages of the site for performance and other content health considerations on a weekly basis. There are alerts in place to make sure page weight (and emissions) stay within our target.