The energy efficiency rating is a measure of the overall efficiency of a home. The higher the rating the more energy efficient the home is and the lower the fuel bills will be. This score is found on an Energy Performance Certificates (EPCs), a mandatory document, in England and Wells, which states the energy efficiency of the building when rent or sold.
This is a result of European Union Directive 2002/91/EC relating to the energy performance of buildings. The principle of this directive is to make energy use in buildings transparent by the issuing of a certificate showing the energy rating of a property, along with recommendations on how to improve efficiency.
As Mr D. Tail demonstrates, a Passive house has a high rating because it has an excellent level of insulation and airtightness.
On the contrary an old Victorian house, non-insulated with uncontrolled draughts that let air come in and out, show a low rating.
However we should not lose hope if we own an old house. Today the construction market offer plenty of products that help us to improve the building performance of a house to an acceptable level, even a Victorian house.