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The running cost of a heat pump depends on a ratio: how much heat it delivers for each kWh of electricity. There are three ways of expressing it, and they are not interchangeable.

Three numbers

• COP: the ratio at a test point, at a specific outdoor temperature and water temperature. It describes a moment, not a year.
• SCOP: a seasonal value calculated under standard test conditions, for the flow temperature stated next to it. It is the data sheet figure and is used to compare different models under the same conditions.
• JAZ (Jahresarbeitszahl): the heat produced divided by the electricity used over a year of operation, in a real house. It is only known afterwards, from the meters.

The calculator asks for the JAZ because the cost of a year depends on real operation. If you only have the SCOP from the data sheet, you can use it as a first value and check in the table what changes with lower values.

What has been measured

Sources: Fraunhofer ISE [7], OpenEnergyMonitor on the Electrification of Heat trial [11]. Fraunhofer ISE found no relationship between the year the houses were built and efficiency, and in its air-to-water systems the electric backup heaters accounted for 1.3% of the electricity [7].

How much the JAZ affects the cost

For a house that burns 20,000 kWh of gas with a 90% boiler, with gas at 11.93 ct/kWh and electricity at 37 ct/kWh, the 2026 BDEW averages [9] [8], without standing charges:

At these prices the two costs break even at a JAZ of about 2.8. Between the lowest and the highest values measured by Fraunhofer ISE in air-to-water systems, the difference is about €1,200 a year. Open this case in the calculator.