How does it work?

How do we calculate carbon emissions?

Calculating the carbon emissions of website is somewhat of a challenge, but using two key pieces of data we can make a pretty good estimate:

  1. Data transfer over the wire
  2. Energy source used by the data centre

When a website is loaded, the energy used is roughly proportional to the amount of data transferred.  We measure the data transferred over the wire when a web page is loaded, and multiply that by the energy usage data that we have.  We also make an adjustment for repeat visitors who may have website assets cached on their devices.

Energy is used at the data centre (48%), telecomms networks transmitting the data (14%) and by the end user’s computer or mobile device (38%).  Of course, this varies for every website and every visitor and so we use an average figure from a study by the ACEEE, ‘The Megawatts behind your megabytes: Going from data centre to desktop‘.

To gauge the energy source, we assume that all websites use standard grid electricity for the telecomms network and end user, since we have no way to determine otherwise.  For the data centre energy use, we check the Green Web Foundation (GWF) database to see if the data centre is using green energy.  If so, then we reduce the carbon emissions attributed to that portion of the energy accordingly.  Of course, the GWF database is not 100% perfect and also includes data centres that use standard grid electricity but offset their emissions.  For the purposes of this calculator, we treat them all the same.

The carbon intensity of grid electricity is based on the UK grid and renewable energy is based on data for wind energy from Ecotricity.

When we put all of this information together, we get a pretty good idea of the emissions associated with an average user visiting any given website.  Multiply carbon per page view up by the typical number of page views and we can estimate the total annual CO2 emissions.

Which pages do we test?

The public version of the tool is designed to be simple and give a rough idea of website efficiency and so we only test the homepage of a site.  We hope to later release a more advanced version with the option to test specific internal pages.

Any website can be tested but only sites that comply with the following guidelines are included in our ranking tables:

  • Can be accessed by the public through a standard web browser
  • Do not require login
  • Allow search engines
  • Contain unique content aimed at human visitors – this excludes holding pages, error pages, server notification pages, demo pages or pages that are generally useless (this is highly subjective)
  • Are free from illegal or explicit content

Websites that do not meet the above criteria may be deleted from our database and/or excluded from ranking tables.