Two numbers tell you most of what you need to know about how efficient an air conditioner is.
SEER: efficiency when cooling
SEER stands for seasonal energy efficiency ratio. It tells you how much cooling you get for each unit of electricity, averaged across a typical cooling season. A SEER of 7 means about 7 kWh of cooling for every 1 kWh of electricity.
SCOP: efficiency when heating
SCOP stands for seasonal coefficient of performance. It’s the same idea for heating: how much heat you get for each unit of electricity across a heating season. A SCOP of 4 means about 4 kWh of heat for every 1 kWh of electricity.
Why “seasonal” matters
Older ratings measured efficiency at a single test temperature. Seasonal figures are averaged across the range of temperatures a unit actually meets, so they’re a better guide to what you’ll pay.
What good looks like
Higher is better for both. On the UK energy label, the best-rated units (A+++) have a SEER of 8.5 or more and a SCOP of 5.1 or more. Labels can show SCOP for several climate zones; for most of the UK, the “average” figure is the one to compare.
Which matters more for you?
If you mainly want heating, which is the case for most UK homes, compare SCOP first. If you mainly want summer cooling, compare SEER.