The EPC Rating is split into two A-G charts, where A is extremely efficient, and G is not efficient at all. These figures are based on two main factors: the amount of Carbon Emissions expected to be produced, and how much you will be expected to pay in fuel bills.
The most common reason for a different figure is if you have made alterations to the building fabric or heating systems since we issued the PEA to you. Any change which ultimately affects the amount of fuel needed to heat and power your home will have a knock-on affect on the EPC.
Another key reason is a change in methodology which occured in April 2011. Reports which were issued before this date were based on older fuel prices and emission factors, so your rating would typically be slightly better than it is now.
If you have scored a very good result on your air test, this can usually show an improvement on the EPC, particularly if the design calculation was based on a much higher figure.