Construction power meter reading not recorded. Is the incorrect billing correct?

What does the electrician say?

Well...

the electricity meter comes from the electrician, but the construction power box in which it is installed comes from the roofer. The meter remained in the roofer’s box and he apparently continued to use it assuming that the electrician had registered it correctly.

Now the roofer shifts the responsibility to the electrician by saying that the correct registration is his responsibility and that he relied on that, and the electrician points out that he did not receive the meter and therefore could not have deregistered it. Furthermore, the roofer should have known that a meter in the cabinet cannot just be used casually.

Strange. Wherever the construction power box was used, there must have been a registration. That means a responsible electrician – meter number – meter reading must have been reported. It should be possible to find out from the roofer where the box was used. Then ask whether the roofer/users were actually aware that they were committing an offense by simply connecting a meter without properly registering the whole thing. You agree: you don’t report anyone, but want to have the 2000 kWh paid! If they don’t cooperate: report to the supplier that electricity was stolen without registration.

Finding out who is "to blame" and who knew what is a reflex. It is about a few hundred euros. Another builder has benefited. I would talk to this one. The electricity theft report variant seems to me a good hint in case no agreement is reached.

Strange. Wherever the construction power box was used, a registration must have been made. That means a responsible electrician - meter number - meter reading must have been reported.

Power is on-site...

Finding out who is "to blame" and who knew what is a reflex. It's about a few hundred euros.

For us, construction power is 8/1 to household power, so not quite trivial to handle...

But it is like this: if you miss something to re-register, then you probably have to bite the bullet.

With us, construction electricity is 8/1 compared to household electricity, so it’s not something to be taken lightly...

But it’s like this: if you miss switching it, then you probably have to bite the bullet.

It shouldn’t sound trivial. With our construction electricity tariff it would be 800€. All parties are making excuses. That leads nowhere. The search for the guilty party is therefore counterproductive for an attempt at reconciliation, as positions harden instead of resolve. The OP’s negotiating position is weak; a different strategy is needed to avoid being stuck with the costs.