How can a heat pump be more than 100% efficient?

It cannot create energy — it moves it. A heat pump does not generate heat from electricity the way a heater does; it uses electricity to transfer heat that already exists from outside to inside. That is why the output exceeds the electrical input without violating anything.

The mechanism is a refrigeration cycle run in reverse, and it is the same technology as a fridge:

A refrigerant with a very low boiling point absorbs heat from the outside air, ground or water — even at low temperatures, because it boils well below zero.

A compressor compresses the now-gaseous refrigerant, which raises its temperature substantially. This is the step that uses the electricity.

The hot gas passes through a condenser indoors, releasing heat into your home and condensing back to liquid.

An expansion valve drops the pressure, cooling it again, and the cycle repeats.

The measure is coefficient of performance (COP) — heat delivered divided by electricity consumed. A COP of 3.5 means 3.5 kWh of heat for 1 kWh of electricity. The extra came from the environment, not from nowhere.

Why "efficiency" is the wrong word. Efficiency implies converting one form of energy to another with losses. A heat pump is a transport device, so COP is the appropriate metric.

COP falls as it gets colder, because there is less ambient heat to extract and the temperature difference the compressor must bridge is larger. Modern units work well below freezing but perform better in mild weather. SCOP (seasonal COP) gives a more realistic annual figure.

Practical implications: heat pumps deliver water at a lower temperature than gas boilers, so they suit larger radiators or underfloor heating and a well-insulated building. Running them steadily rather than in short bursts generally works better.

Even at a COP of 3, they beat a gas boiler on emissions on most modern grids.