COP and EER: what efficiency means

Efficiency figures turn up on specification sheets, energy labels and quotes, often as a string of similar-looking abbreviations. Together with your electricity price, they decide what each unit of heating or cooling costs you, so they're worth understanding before you compare models.

Why the figure can be more than 1

A plug-in heater turns electricity straight into heat: 1 kWh in, 1 kWh of heat out. Its efficiency, expressed the same way, is 1.

An air conditioner works differently. It uses electricity to run a compressor and fans that move heat from one place to another through a refrigerant circuit. Moving heat takes less energy than making it, so more heat comes out than electricity goes in. How air conditioning heats a room explains the process.

No energy is created. When heating, the extra comes from the outdoor air, which still holds usable heat on a cold day. When cooling, the unit moves heat out of the room and releases it outside.

COP, EER, SCOP and SEER

COP stands for coefficient of performance and EER for energy efficiency ratio. Both are measured at a single standard test point, usually 7°C outside and 20°C inside for heating, and 35°C outside and 27°C inside for cooling. A UK winter spends a lot of time colder than that heating test point.

The seasonal figures, SCOP and SEER, are calculated by a standard method across a range of outdoor temperatures and levels of output, for a reference climate. That makes them a better guide to a year's use than a single test point, and they're the figures shown on the energy label. Energy labels: SEER and SCOP explained covers the label itself.

What changes a unit's efficiency

A unit doesn't have one fixed efficiency. It changes from hour to hour with the conditions.

• Outdoor temperature. The further the unit has to lift heat, the lower its efficiency. In mild weather small air-to-air systems commonly reach a COP of 3–5, and the figure falls as it gets colder. When cooling, efficiency falls as it gets hotter outside.
• The temperature you set. A warmer setting when heating, or a cooler one when cooling, widens the gap the unit has to work across.
• Defrosting. In cold, damp weather the outdoor unit periodically melts ice off its coil, which uses energy and pauses heating for a short while. Why the outdoor unit defrosts explains why this happens.
• How hard it's working. Inverter units usually run most efficiently when they can tick over steadily, rather than working flat out or repeatedly stopping and starting.
• Installation and upkeep. An incorrect refrigerant charge, dirty filters or a blocked outdoor unit all lower real-world efficiency.

Turning efficiency into cost

For every kWh of heat, the unit uses 1 ÷ COP kWh of electricity. So the cost of a kWh of heat is your unit rate divided by the COP, and the same works for cooling with EER.

The rate in the table is only an example, so check your own tariff and divide it by the efficiency you're interested in.

When comparing two models, use the seasonal figures from their energy labels rather than a COP at one test point. The gap between two models' running costs grows with the hours you use them, so efficiency counts for more on a unit that runs most days than on one used for a few hot nights.

To weigh electricity against a boiler, the same arithmetic applies with the boiler's efficiency and the gas price, as set out in heating with air conditioning compared with gas.