The rainbow-style energy label like the one to the left has been endorsed by the British Fenestration Rating Council (BFRC). There are various other Competent Persons Bodies operating within the window industry who can also endorse these labels. The other main ones are CERTASS and BSI. This rating is based on the overall performance of the window including the frame and glazing.
An 'A' rated window means that there is theoretically no energy lost through the window and some installers now offer an 'A' rating as standard. It is also worth noting that there is now an 'A+' energy rating available from the BFRC and an 'A*' rating from CERTASS.