A 10 kW heat pump delivers 10 kW of heat; it does not use them. The ratio between the heat it gives and the electricity it draws is the COP: a Kita SP 10 at +7 °C outdoors, with water at 35 °C, gives 10 kW of heat and draws 1.84 (COP 5.44). At -7 °C it gives 7.4 kW and draws 2.1 (COP 3.52). It is the opposite of a boiler, where 24 kW on the plate are 24 kW of gas burnt. The first thing to get out of your head is the equation size equals consumption.

A day’s consumption is the heat the house asked for that day divided by that day’s COP. The physical maximum is quick to work out: an SP 10 running 24 hours at full output at -7 °C would use 2.1 kW for 24 hours, that is 50 kWh. It almost never happens: the machine is chosen to reach full output only on the coldest day of the year and for the rest of the winter it works at half capacity or less. For normal days there are the figures of the European HP Keymark certification, which tests the machine in an independent laboratory at part load too, as it really runs in winter: at +2 °C outdoors an SP 10 gives 4.8 kW of heat drawing 0.82 (COP 5.84); at +7 °C it gives 4.3 kW drawing 0.57 (COP 7.64); at +12 °C 4.2 kW with 0.40 kW (COP 10.37). A whole day at +2 °C is about 20 kWh of electricity, one at +7 °C about 14. Owners read it straight off the display: our machines count the kWh produced and the kWh used.

The most reliable way to estimate annual consumption is not square metres but the bill you already have. The cubic metres of gas of the last year say how much heat your house really asks for, the way you live in it.

1. take the cubic metres of gas of a whole year, heating and hot water together;
2. one cubic metre is worth about 10 kWh of energy; an old boiler brings about 85% of it into the house, a condensing one about 95%. With 1,200 cubic metres the house received about 10,000-11,000 kWh of heat;
3. divide by the SCOP at your system’s temperature: with radiators our air-to-water units sit between SCOP 3.8 and 4.3 (SP 10: 4.20), with underfloor heating between SCOP 5.1 and 5.75;
4. the result is a year’s electricity: for that house about 2,400-2,600 kWh with radiators, about 1,800-1,900 with underfloor heating. For every 1,000 kWh of heat an SP 10 needs 238 kWh of electricity at 55 °C and 174 at 35 °C.

The same certification also gives the annual consumption in the average European climate, for a house whose design load equals the machine’s rated output: an SP 10 uses 2,999 kWh a year with water at 35 °C and 3,881 with water at 55 °C; an MP 18 5,759 and 7,081; an MP 20 7,306 and 8,861. These are the figures to put next to the sum made from the bill.

• Flow temperature as low as possible: the HCC controller works on the rooms and extends the running hours instead of raising the temperature; every degree less of flow is more efficiency. How to test your own system is in the tip on radiators.
• Do not switch off and on: a house that cools down has to be reheated from scratch, at full output and with a worse COP. Better a constant temperature and a machine that modulates.
• Use the sun: with photovoltaics the daytime electricity comes from the roof. The energy optimiser shifts heating into the sunny hours and stores heat in the walls.
• Domestic hot water at 45-50 °C when that is enough, higher only for the anti-legionella cycle.
• The right size: chosen on the demand of the coldest day, not on the boiler. Why is in the tip on sizing and in the one on useful capacity.
• A machine that modulates right down and defrosts only on demand: that is the job of the inverter and the on-board electronics, not of the user.

The same machine cools, and in cooling the ratio between cold produced and electricity drawn is called EER. An SP 10 with underfloor or fan coils at 18 °C water has an EER of 5.43; with water at 7 °C 3.62. The same rule applies: the closer the water is to room temperature, the less it uses.