An electricity supplier must annually disclose their fuel-mix to their customers according to the European Directive 2009/72/EC.  The fuel-mix is the mix of different electricity production sources that make the total electricity a supplier sells to consumers.  These different production sources combine their emission information (g of CO2/kWh) total, as well as a mg of radioactive waste per kWh total.  A consumer can obtain a different fuel-mix than the one they are delivered by correctly using an electricity tracking certificate like the Guarantee of Origin.

An electricity supplier delivering different products, with different fuel-mixes, must us GOs to prove these different fuel-mixes.  This is regulated in some countries.