Single- or dual-tariff meter in new construction

Normally not. Except [Wärmepumpe am Haushaltsstrom ist verboten], then the second meter is mandatory anyway.

Except for heat pumps on household electricity, it is prohibited, then the second meter is mandatory anyway.

Then you simply choose another electricity provider. Or ignore this clause in the terms and conditions.

I don’t understand that.

Because if the air-to-water heat pump is on a different meter than the photovoltaic system, it never makes sense to install a second meter when combining air-to-water heat pump and photovoltaic.

Or am I making a mistake in my thinking?

Sort of, you are basically right, unless you consume so much household electricity that it pays off with photovoltaic. The heating can be neglected anyway, but the hot water is interesting.

Now I really don't know what to do anymore.

In our apartment, we currently have an electricity consumption of 4500 kwh. Our house will be twice as big, and as described, we will heat with an air-to-water heat pump and have a 9.9 KWp system on the roof.

Which of my craftsmen should I ask what the best solution is for me?

You have to calculate whether a second meter is worthwhile! Only you can do this with the conditions of your electricity provider, provided they offer discounted heat pump electricity. If it’s not worthwhile, or just barely, then choose a single-rate meter. Yes, there are electricity providers who exclude the use of heat pumps in their terms and conditions, but there are also thousands who don’t care. And ultimately, it’s just a clause in the terms and conditions – where there is no plaintiff, there is no judge.

By the way, the issue with two separate meters and self-consumption of photovoltaic electricity can also be managed, your electrician can help you with that. But as I said, you probably don’t want a second meter anyway, because it’s not worthwhile.

However, it is always advisable to have an intermediate meter for the heat pump to be able to read its consumption. But that has nothing to do with your electricity provider anymore; it’s a little device, uncalibrated, costs 30€. You will get one or two of these anyway (photovoltaics).

In principle, I agree with the previous speakers:
You have to calculate this once, with pen and paper and a calculator.

First clarify the prerequisites:
- you need a meter for household electricity
- if you have a photovoltaic system, it may need its own meter (e.g. at the latest when you want to feed electricity into the grid (at least that was the case with my parents-in-law))
- for the air-to-water heat pump, you may need another meter if you want to use a heat pump tariff from an electricity provider

Since we are facing the same consideration, here is my calculation:

- no photovoltaic system
- household electricity one meter
- air-to-water heat pump possibly one meter

= one or two meters

According to the local utility company, I have the following information:
The WärmeÖkoStrom tariff for systems with combined or separate measurement, each with a contract period until [...].

In combined measurement, the heating electricity is measured together with the household electricity. In separate measurement, the electricity consumption for heating is recorded independently from household electricity via a separate dual-tariff meter.

Additionally:
All legally valid charges and taxes for 2017 are included in the tariffs (valid for consumption points under 100,000 kWh/a). Changes as well as possibly further legal charges and taxes will be passed on accordingly in the respective supply year.

This means:
- either we have ONE dual-tariff meter (dual = HT and NT), where we then have different prices for HT and NT (because it can be assumed that most household electricity is consumed during HT and at night in NT basically only the heating consumes electricity)

- or two separate meters where the air-to-water heat pump can run all day at the cheaper rate (meaning its own dual-tariff meter for the air-to-water heat pump)
- household electricity must then be billed at a separate tariff

Here are the prices from our utility as an example:

combined measurement heat pump and household electricity together (dual-tariff meter):
Price per kWh HT: 16.234 (net)
Price per kWh NT: 21.634 (net)
Basic charge per year 90€ net

separate measurement only heat pump (dual-tariff meter):
Price per kWh HT: 16.234 (net)
Price per kWh NT: 16.234 (net)
Basic charge per year 60€ net

separate measurement only household electricity (single-tariff meter):
Price per kWh 21.234 (net)
Basic charge per year 84€ net

In summary, for separate measurement there are basic charges of 84€ + 60€ = 144€ (net) versus basic charges of 90€ for combined measurement.
However, the air-to-water heat pump can also consume cheaper electricity during the day in separate measurement (about 0.05€/kWh cheaper).

The question now is, how much does the air-to-water heat pump consume? There are already many threads here in the forum for that, so please search yourself.
Otherwise, let's say you need 18,000 kWh of heating!
Let’s say the air-to-water heat pump has an annual performance factor of 3, which theoretically means the air-to-water heat pump can produce three kWh of heat from one kWh of electricity.
That means the air-to-water heat pump consumes 18,000 / 3 = 6,000 kWh of electricity per year.

Now one would have to know how much electricity the air-to-water heat pump consumes only at night.
Let's say to approximate it, the air-to-water heat pump consumes 50% of electricity at night (= NT) and the other 50% during the day (= HT).

6,000 kWh / 2 = 3,000 kWh
3,000 kWh * 0.05€ = 150€
-> so separate meter measurement would save 150€ electricity (net) per year, but it costs about 50€ more in the basic price.
In total, there remains a "profit" of 100€ net per year.

If only 33% instead of 50% of electricity is consumed by the air-to-water heat pump at night, we still have a "profit" of 50€ net.

Costs and installation of an electricity meter vary after a short Google search between 50 to 100€ each (pay attention to the topic of smart meters, I am not very familiar with this).
In my opinion, these one-time costs can be neglected since they would be recouped after one to two years.

According to my calculation, separate measurement is worthwhile to a small extent.
My calculation is based on the numbers of our utility company, it may be different for you.
It may also be that the air-to-water heat pump consumes much more electricity than I calculated or that my 50% rule is completely off.

For the calculation to work, the following must fit:
The savings in electricity price (in my example 0.05€) must cover the additional costs (increased or additional basic charge as well as the one-time costs due to the installation of the appropriate electricity meter).

If we very roughly say that a special tariff for the air-to-water heat pump causes 100€ extra per year, but you can save 0.05€ per kWh (within the reduced tariff, possibly only at night).
Then you have to consume at least 100€ / 0.05€ = 2,000 kWh of electricity (within the reduced tariff, possibly only at night) for it to be worthwhile.

In the end, I even consider what I have written here to be realistic. However, I will still specifically look for experience values here in the forum and especially with acquaintances to confirm my consumption values.