The Energy Performance Certificate will give rating of the property between
A-G,   A being very efficient and G is being very poor.  It’s similar to the ratings found on fridge freezer.

EPC’s are produced using the RdSAP methodology which makes various assumptions about energy usage thereby allowing the energy efficiency of one building to be easily compared with another building of the same type. Prospective buyers, tenants, owners, occupiers and purchasers can then consider the energy efficiency, carbon emissions and potential running costs of a building and factor this into their investment.
In addition to the rating report there is also a recommendation report on how you can actually improve the rating.  There are high cost measures and low cost measures which will show you how much you can save if these changes are made.
An inspection takes on average one hour to complete.  Access is required throughout the property including the loft.  After the inspection the assessor will lodge the report on the national database to make it avalible for the client.