@Oinq I don’t know the fuel price comparison between gas and electricity where you are. But where I am in the UK electricity is about 3x the price of gas.
However, a well installed heat pump can compensate price-wise for this differential and end up having lower bills. My A2W heat pump has a sCoP of around 4. So my running costs are lower compared with if I had a gas boiler.
I know little of your situation, but at first glance it would seem worth investigating the payback time in making the 10k euro investment switching to heat pump and offsetting more than 2k euro gas bill in the winter months.
the 2k bill comes up because we pay their heating. I’m sure of it.
If you go ahead with your plan and you’re correct, you’ll be pleased to know that history is repeating itself. This won’t be the first time someone has come to OEM with doubts about the accuracy of a bill and has proved the bill is wrong. (But don’t ask me to find the evidence - I think it will be in the old forum that’s now archived.)