Away from fossil fuels - House from 1999

I would really be interested to know how the electricity bill has changed there. On paper, actually very suboptimal for a heat pump (no insulation, old windows, you don’t mention anything about underfloor heating).

No, they are normal radiators. I think she mentioned something about a €120 heat pump electricity advance payment, if I remember correctly. So only for the heating, without the household electricity. This is a 4-person semi-detached house over 3 floors. So the mother still lives down in the granny flat, which is partly in the ground (the house is on a slope), I think that costs a lot of energy, it is supposed to be insulated next year. It’s really hard to compare with before, because there was an oil heating system before and oil is not exactly known for being cheap nowadays. They also have photovoltaic panels on the roof, but I think that will play a minor role with the heat pump electricity; in the really cold months the performance of the photovoltaic system leaves much to be desired.

... mom still lives there, she is partly in the ground ...

Then it doesn't cost much anymore. ;-) And insulating afterwards is of course inconvenient. Either they have now greatly undersized the heat pump and help with fan heaters, or they carry an oversized heat pump around for 15 years...

Then it doesn't cost much anymore. ;-)

And retrofitting insulation is obviously inconvenient. Either they have now severely undersized the heat pump and are supplementing with fan heaters, or they've been lugging around an oversized heat pump for 15 years...

Probably the latter, although I'm not sure. Maybe also a compromise until the insulation is done.
An energy consultant was involved, who must have thought of something. This year the house was also a bit cooler than usual (but not extremely - it could also be because we always keep it a cozy 24°C at home), maybe they took that one winter into account until the insulation is done next year. That probably couldn't be solved quickly since you really have to dig quite deep around the house.

What kind of supply temperatures are actually needed for something like this? Do you then need a special high-temperature heat pump? (I once read that such things exist, or for example with brine heat pumps sometimes two heating circuits.) Or is it just a normal one that then simply runs a bit less efficiently? At my parents' house, they are currently going back and forth because the existing oil heating system is getting old. So far, they have rather ruled out a heat pump because there are no large surface heaters (only regular radiators), but they haven't fully consulted yet.

The best way to test this is with the old heating system. There are plenty of instructions for this. Everything up to 50, maximum 55° flow temperature at [NAT!] is still acceptable. Often, replacing individual radiators is sufficient.

I would really be interested to know how the electricity bill has changed there. On paper, actually very suboptimal for a heat pump (no insulation, old windows, you don't mention underfloor heating).

You can roughly deduce it. 1000l/a of oil equals 10,000kWh. Heat pump with radiator heating has a COP between 2.5 and 3 or even worse, so 3,300 to 4,000 kWh/a. At 30 ct/kWh about 1000€/a. The COP depends on the flow temperature and from a COP of 3 it is worth it.