Boiler efficiency is measured and expressed in multiple ways, which creates confusion. Here's what each figure actually means.
SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK)
SEDBUK measures how efficiently a boiler converts gas into heat over a typical year — accounting for seasonal variation in load. It's expressed as a percentage. All boilers now have a SEDBUK database entry. The old banded system (A–G) used SEDBUK as its basis.
ErP (Energy-related Products) efficiency
ErP replaced SEDBUK as the primary EU/UK efficiency metric. Expressed as a percentage, it's very close to SEDBUK. Most modern condensing boilers achieve 92–96% ErP.
Net vs gross efficiency
You'll sometimes see figures like "93% net" or "107% gross" — these are the same boiler. Net efficiency measures against the net calorific value of gas; gross efficiency against the gross calorific value. Gross figures are always higher by about 11 percentage points. The UK increasingly uses gross calorific values, so a boiler quoted at "93% net" would be approximately "82% gross" — not actually less efficient.
What the number means in practice
A 94% efficiency boiler converts 94p of every £1 of gas into heat. The remaining 6p is lost through the flue. Compare this to a 1990s G-rated boiler at 65% — which loses 35p in every £1.