Good question! This answer is hard to understand without an understanding a little bit about how the electricity market works in the UK.
You, as an electricity consumer, buy your electricity from an electricity supplier. These suppliers get their electricity from generators (e.g. a coal power station or a wind farm) that they own and also from other generators that they don’t own. These generators could be spread across the country. For example, you may be based in Birmingham, but the electricity you buy from a supplier could be generated in Scotland. A green tariff means that your supplier is obligated to ‘match’ the electricity they supply to you by buying renewable energy on your behalf.
When electricity is generated from renewable and non-renewable sources it goes into the electricity network. Once inside the grid, it is impossible to differentiate one unit of electricity from another. Therefore it is impossible to state that the physical electricity that powers your home as being from the renewable generation that your supplier bought on your behalf. The electrons generated from that power plant in Scotland are most likely not those that end up charging your smart phone in Birmingham.
The physical reality is that the electricity that supplies your home is most likely from the nearest generators to your home – it is this that Carbon Tracer focuses on.
For more information, please visit the Energy Saving Trust’s website.