To estimate electricity use, divide the required useful heat by the heat pump's effective coefficient of performance (COP).
Example
Suppose a home needs 900 kWh of useful heat per month. At an effective COP of 3, the heat pump consumes around 300 kWh. At €0.21/kWh, this represents approximately €63 in variable electricity costs.
Why the COP varies
Actual performance is influenced by outdoor temperature, defrost cycles, heat distribution, setpoint, maintenance and installation. The advertised peak COP may not reflect seasonal use.