BREEAM In-Use is an assessment scheme for rating the environmental performance of existing buildings. It covers energy, water, waste, and other categories. Buildings score higher on energy when they can prove their electricity comes from renewable sources, and the way you prove that is with Energy Attribute Certificates (EACs). In the UK these are REGOs, in Europe they're Guarantees of Origin, and in other markets they're called I-RECs. Buy the right certificates, retire them against your consumption, and your building's BREEAM In-Use energy score goes up.