Photovoltaic system battery storage subsidy feed-in tariff?

I will soon measure myself since I have no access to the friend's measurement. Some simply assume that the storage is only used at night. But that is not the case. Whenever the sun provides less energy than is currently "requested," the hybrid inverter will draw energy from the storage and not from the grid. This causes charging and discharging periods of the battery to occur again and again during the day.

In the statements "up to 200 charging cycles per year," one means complete charging and discharging. This results in the amount of electricity the battery could actually deliver. For example, 200 x 9 kWh p.a. = 1800 kWh p.a. 10 years of durability = 18,000 kWh delivered. Divided by purchase cost = storage cost per kWh. Without taking generation costs into account. The 10 years are, of course, only assumed.

Yes, these are the kind of calculation examples. There are simply many assumptions. Besides the assumptions mentioned above, one can also speculate about the development of electricity prices. If you calculate 18,000 kWh at 30 cents, you have produced electricity worth €5,400. At 35 cents, it is already €6,300. If you then calculate with only 150 charging cycles, you suddenly end up below €5,000. Then you simply increase the lifespan... Then you calculate with the decline of the batteries... Then you tweak the electricity price again... These are all very imprecise forecasts.

These are rule-of-thumb formulas based on experience and used accordingly.

Personally, I consider 200 full cycles already too much. I have a 6.8 kWp system on my roof myself, and now in summer it delivers 40-45 kWh per day; the household needs 10-14 kWh per day including hot water preparation. A household-sized storage is full in no time and doesn’t get emptied at night anymore ... unless it is really small. In winter, photovoltaics bring nothing or everything is consumed directly anyway. Storage is empty. It even needs power from the grid, maintenance charging. Which season remains as a meaningful use scenario for the storage? What about storage losses (10-20%)? Lost feed-in compensation? VAT on the self-consumed (and consumed by storage losses) electricity?

Yes, many factors. Overall, it makes at most idealistic sense. Or you get the thing practically for free due to favorable conditions.

If you calculate 18,000 kWh at 30 cents, you have produced electricity worth €5,400.

The storage of course produces nothing. And the electricity stored in it is not free either, quite the opposite.
Every kWh consumed yourself costs value-added tax, about 4 cents. With every kWh consumed yourself, you forgo ~11 cents feed-in tariff...

Besides the sunniest and most wintry days, there are also others. Here, storage systems work during the day as well, not just at night, unless you switch on your consumers only one after the other. The daily production on such days may be sufficient in total, but not at the required times. In the end, it doesn’t matter how everyone calculates. The blanket statements that storage systems are “always” or “never” economical are simply nonsense.

Value-added tax on the self-consumed (and consumed due to storage losses) electricity?

To be fair, however, this only applies for 5 years (or is there any reason not to apply the small business regulation after 5 years?). Nevertheless, I generally also consider storage to be not sensible in most cases.