How does it work?

We have updated the background data in V2 of the Website Carbon Calculator. The new data shows carbon impacts being roughly one third of those reported by the V1 carbon calculator, partly thanks to more robust data and partly due to the efficiency of web technology increasing rapidly over the past few years. V1 used data from 2012 whereas V2 uses data from 2017/18.

If you have previously tested a website with the v1 calculator and wish to adjust it, you can do so by dividing your V1 result (the grams of CO2 produced) by three. 

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

  1. Data transfer over the wire
  2. Energy intensity of web data
  3. Energy source used by the data centre
  4. Carbon intensity of electricity
  5. Website traffic

Data transfer over the wire

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 intensity of web data

Energy is used at the data centre, telecoms networks and by the end user’s computer or mobile device.  Of course, this varies for every website and every visitor and so we use an average figure. The figures used are for 2017 from the report On Global Electricity Usage of Communication Technology: Trends to 2030 by Anders Andrae and Tomas Edler, adjusted to remove manufacturing energy as this is not relevant to this calculator. We then divide the total amount of energy used by the total annual data transfer over the web as reported in the Nature article, How to stop data centres gobbling up the world’s electricity. This gives us a figure of 1.8kWh/GB.

Energy source used by the data centre

To gauge the energy source, we assume that all websites use standard grid electricity for the telecoms 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. We estimate the proportion of energy used in the data centres to be approximately 10% by comparing the global data centre energy usage stated in the Nature article to total ICT energy usage (excluding manufacturing) in the Andrae and Edler report. Of course, the GWF database is not 100% perfect and also includes data centres that purchase standard grid electricity but offset their emissions.  For the purposes of this calculator, we treat them all the same.

Carbon intensity of electricity

The carbon intensity of grid electricity is based on the international average as reported by the International Energy Agency (475 grams CO2e per kWh) and renewable energy is based on data for wind energy from Ecotricity (33.4 grams CO2e per kWh).

Website traffic

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 annual 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.

Our code is available on GitLab for those who would like to access it.

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