Sorry for the late answer! Thank you @Timbones and @glyn.hudson. We don’t have a home battery, because financially it doesn’t seem that interesting (at least in Belgium). We also don’t have the “normal” central heating: our passive house does reasonably well without. We use a simple electric heater during some colder days/hours (consumed about 125 kWh last winter in our living room). This means that our heatpump is for domestic hot water only (and ventilation).
So I guess it is possible that a cloud passes by and that electricity is imported from the grid during it heatpump cycle.
Our heatpump consumes about 600 Watt during its cycle. Would it be possible to produce a heatpump that can have different ‘levels’. Eg. 300 Watt cycle if there is not much sun (of course it takes double as long) and 600 Watt cycle if there is more sun. If this could be possible: are they on the market already?