How the rating is actually calculated
Domestic EPCs are produced using RdSAP, a government-defined methodology, which means assessors are not exercising judgement about how efficient a property feels — they are recording defined inputs that drive a fixed calculation. The main inputs are floor area and built form, wall construction and insulation, loft and floor insulation, glazing type and age, the main heating system and its fuel, heating controls, hot water provision, and any low-carbon technologies such as solar PV. Two things follow. First, the rating reflects the fabric and the systems rather than how the occupants behave. Second, and critically, the assessor can only record what can be evidenced — which is where most avoidable low ratings come from.