Back payment air heat pump disaster, contracting party, electricity supplier

Hello everyone,
We have now been living in our beautiful house for a year. But already in January of this year, our consumption of the air heat pump seemed very high to us. Our general contractor came, looked, and said everything was okay, that's just how it is. Now the drama: we are facing an additional payment (electricity) of over €2000 for almost 16,000 kWh in ONE year!!! Today the heating installer came and said "Oops, software error, the heating rod was running the whole time and took over the work of the air heat pump, the heating guy from Va**lant has to take care of that." That's when we learned that the water installer connected the heating, but no Va**lant guy ever checked it! So, who is going to pay us the additional payment now? We are absolutely not responsible for this! I am so angry

Since you only have one contracting party, namely the GU, there are only two possibilities. Either you or the GU. How he then handles it internally is his business.

You seem to have reported a defect, as I understand you, which has actually turned out to be one afterwards.

So I would try it with the GU if I were you. You can see how he reacts.

Suggestion: Somewhere in the heating system's datasheet there is a [Jahresarbeitszahl (Jahresarbeitszahl)], the value should be around 4. This is used to calculate the theoretical electricity consumption if the heating system had worked properly (i.e., 16,000 kWh divided by the [Jahresarbeitszahl]). With the help of the electricity bill, the damage can now be determined (a value in euros; not the additional payment amount). You claim this from your contractual partner who supplied you with the heating system. With a deadline. After the deadline expires, you seek legal advice.

Do you have a separate electricity meter for the heat pump? That would also not be insignificant.

Yes, we have, so we have two electricity meters. So my next step is to contact the GU?

You will probably have to pay first, as you are the contracting party of the electricity supplier. Step two is the question of who could reimburse you.