Single- or dual-tariff meter in new construction

If you have a photovoltaic system, it may require 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))

No, that does not have to be the case. You use bidirectional meters (please do not confuse with dual-tariff meters (HT/NT)). Cheaper to maintain because there is only one and cheaper to purchase because they require less space in the panel. These are supplemented with additional intermediate meters to measure self-consumption, heat pump consumption, etc. These things only cost 20-30€, are not calibrated, not from the power supplier, and are about the size of a fuse.

- for the air-to-water heat pump, you may need a separate meter again if you want to take advantage of a heat pump tariff from an electricity provider

Yes, correct. For extra heat pump electricity, you need a separate meter because household electricity should not be measured through it. If you have a photovoltaic system, however, this can be tricky or leads to extra effort because the heat pump then may not benefit from the electricity produced by the photovoltaic system (which can be a crucial aspect of the profitability of the photovoltaic system).

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

Stop, dual-tariff meter. That is old thinking from the days of night storage heaters and often nonsense for heat pumps. Air-to-water heat pumps ideally run as little as possible at night, preferably not at all! Is this really the product for a heat pump you are looking at? Locally, both exist: dual-tariff meters for night storage heaters and single-tariff meters for heat pumps. Dual-tariff meters are usually more expensive than single-tariff meters and greatly restrict the choice of electricity suppliers (!!). Just try a comparison portal of your choice and specify that you have a dual-tariff meter (if the portal even allows this). This usually binds you to the local electricity supplier, which in my experience, besides reducing electricity demand, is the most important aspect of saving electricity costs – moving away from the local supplier!

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

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

Or everything through one meter, completely normal, with free supplier choice. Maximally flexible in supplier choice, no basic fee for a second meter and/or increased fee for dual-tariff meters, no need for space in the panel.

Otherwise, let’s say you need 18,000 kWh of heat!.

Rather half that, if even. Unless you have 400 sqm of heated space or 20 people shower daily.

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

This assumption is the biggest flaw in your calculation as it is based on incorrect premises. As already mentioned above, the air-to-water heat pump consumes as little electricity as possible at night or only marginally because it is most uneconomical at night due to the low outside temperature! It is therefore set to run during the day. Night operation is even more nonsensical if a photovoltaic system is present. The air-to-water heat pump should ideally be set (or dynamically controlled) to heat when the photovoltaic system is producing a lot of electricity in order to use this in self-consumption (or in other words: store it in the house in the form of heat).

Costs and installation of an electricity meter are between 50 and 100€ each according to a short Google search (pay attention to the topic of smart meters, I'm not very familiar with this here).

Your electrician will want more for that. The switch cabinet also needs appropriate space; these things are not exactly cheap.

I took the number from a calculation example for heat pumps from the internet; if it is less, the calculation can simply be adjusted accordingly. Your counterexample with 20 people showering is somewhat exaggerated.

The heating demand of current newly built single-family houses is between 7000-9000 kWh/a. The included hot water portion is between 1500-2000 kWh/a for 3-4 persons. Although my example with 20 showering people was a rough estimate, it actually fits quite well and is not exaggerated.

And if what you say is true, that heat pumps should rather not run at night, then a joint measurement as described above would be totally nonsensical.

The efficiency of the air-to-water heat pump depends significantly on the outside temperature, and at night it is usually colder than during the day, isn't it.

On the other hand, a separate heat pump tariff, which applies around the clock, would still be an advantage from my point of view.

Yes, that is possible. Everyone has to calculate this for themselves. Here it would be about 10-20€ p.a. savings, not much more in your example, but with corresponding additional effort in the installation of the photovoltaic system, more space in the installation cabinet, etc. – in my opinion, not worth it. Also, read the fine print carefully. Some heat pump tariffs also require that household electricity must be registered with the same supplier. At the latest then, the heat pump tariff is complete nonsense.

4 persons = 3,400 kWh for hot water plus standby losses according to Dr. Rüdiger Paschotta (RP Energie Lexikon). Open-ended upwards

It should perhaps also be noted that at least with a dual-rate meter, the energy supplier often wants to have a disconnect installed, with which the heat pump can be unilaterally switched off by the energy provider during peak loads. This is the case with my sister, so they regularly have hardly any hot water at noon and in the afternoon because it’s already used up in the morning and is switched off around noon.

Whether this also happens with separate metering depends possibly on the energy supplier; with the municipal utilities I mentioned, there is also a corresponding clause in the separate tariff. A heating engineer told me that this is rather not the rule with separate metering.

is such a small little box, uncalibrated, costs 30€. You will get one or the other anyway (Photovoltaic)

What is the name of the little box and where is it connected?

What is the name of this box and where is it connected?

Google for example AC meter and DIN rail