How Much Does It Cost to Run a Heat Pump?

A heat pump's power draw depends on its size and on the weather. Mitsubishi Electric's figures for its Ecodan air-to-water range show a 6 kW model drawing about 1.9 kW, and an 8.5 kW model about 3.3 kW, when each delivers its full output at −7°C (19°F). In milder weather they slow down and draw much less.

We use 2,000 W as a typical running draw for a home-sized unit, and assume it averages half that over a cold winter day. That's about 1 kWh an hour, or 24 kWh a day: about $4.36 a day in the US, £6.32 in the UK and €8.68 in Germany.

A whole month of days like that comes to about 730 kWh: $132.50 in the US, £192.03 in the UK and kr 2,410 in Sweden. Spring and autumn months cost far less, and in summer an air-to-water heat pump may only heat hot water.

A year therefore costs much less than twelve cold months. Mitsubishi's lifetime energy figure for the 8.5 kW Ecodan, worked out under the EU test scenario (EN 14825), comes to about 5,700 kWh a year. The EU energy label for one small wall-mounted air-to-air unit gives 861 kWh a year for heating in an average European climate.

A heat pump doesn't make heat from electricity. It uses electricity to move heat from the outside air into your home, so it delivers more heat than the electricity it uses.

The coefficient of performance (COP) is the ratio of heat out to electricity in at one moment. A COP of 3 means 3 kWh of heat for every 1 kWh of electricity. Natural Resources Canada gives typical COPs of 2.0 to 5.4 at 8°C (46°F) and 1.1 to 3.7 at −8°C (18°F).

SCOP (seasonal COP) averages this over a heating season, and it's the figure on EU and UK product labels. The same wall-mounted unit is rated at a SCOP of 6.6 in Europe's warmer climate, 5.2 in the average climate and 4.0 in the colder climate, because it has to work harder when the outside air is colder.

Real homes usually do worse than the label. The UK's Electrification of Heat trial measured a median seasonal performance factor (SPF) of 2.80 for air-source heat pumps over a year, and 2.44 on the coldest days.

Water temperature matters too. Mitsubishi rates the 8.5 kW Ecodan at a SCOP of 4.84 when it heats radiator or underfloor water to 35°C, and 3.48 at 55°C. Larger radiators, underfloor heating and weather compensation let a heat pump run cooler and cheaper.

Every electric resistance heater, from a fan heater to a baseboard or oil-filled radiator, turns 1 kWh of electricity into 1 kWh of heat. Natural Resources Canada describes electric resistance heating as typically 100% efficient.

A heat pump with an SPF of 2.8 gives the same heat for about 36% of the electricity. ENERGY STAR says ductless mini-split heat pumps use up to 60% less energy than standard electric radiators.

For example, Panasonic's Japanese wall unit for a 5–6 tatami room (about 8–10 m²) delivers 2.2 kW of heat for about 470 W at its rated output. An electric heater would need 2,200 W for the same heat.

Compared with gas, the IEA says heat pumps on the market are three to five times more energy efficient than gas boilers. Whether that makes them cheaper to run depends on the price of a kWh of electricity compared with a kWh of gas where you live.

As a rule of thumb, with an SPF of 3 and a 90%-efficient boiler, a heat pump costs less to run whenever a kWh of electricity costs less than about 3.3 times a kWh of gas.

Air-to-air heat pumps blow warm air into a room. They include ductless mini-splits and the wall-mounted air conditioners used for heating across Japan, Korea and much of Asia. In Australia and New Zealand they're called reverse-cycle air conditioners.

Air-to-water heat pumps heat water for radiators, underfloor heating and often a hot water cylinder. They are a common replacement for gas and oil boilers in the UK, Germany, France and the rest of northern Europe.

Ducted (central) heat pumps, common in the US and Canada, heat the whole house through air ducts. Many have electric resistance backup that switches on when the heat pump can't keep up, and that backup uses two to three times as much electricity for the same heat.

Heat pumps are widespread in the Nordics: the IEA reports that 60% of buildings in Norway have one, and over 40% in Sweden and Finland. Natural Resources Canada says newer air-source models keep working down to about −15°C to −25°C (5°F to −13°F).

Heating season runs roughly from October to April in the Northern Hemisphere and from May to September in Australia, New Zealand and southern South America.

Heat pumps change speed, so they draw their full input only when working hard; most of the time they draw less. Air-to-water figures are for 35°C water, from Mitsubishi Electric's Ecodan data. Your model's rated input is in its spec sheet or product information sheet; use it, and the share of time it runs, in the calculator.

• Keep the flow temperature as low as keeps you comfortable, and use weather compensation if your system has it.
• Avoid deep night-time setbacks. Natural Resources Canada advises setting back no more than about 2°C, because a cold house can bring on backup heat while the heat pump catches up.
• Keep the outdoor unit clear of snow, leaves and clutter, and clean the filters of air-to-air units regularly.
• Insulate and draught-proof: a smaller heat loss lets the heat pump run slower and more efficiently.
• Ask your supplier about heat pump or time-of-use tariffs, and heat the hot water cylinder in the cheapest hours.
• Check that any electric backup heater is set to come on only when it's really needed.

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