Install photovoltaic power storage yes or no?

Our new building is already quite advanced. Next week, 15 photovoltaic modules with a total output of 5.1 kwp are to be installed. We are getting a heat pump with a capacity of 5.12 kwh. A power storage system was initially not planned because we first wanted to live in it for at least one year to observe our electricity consumption behavior and then possibly retrofit a storage system later or expand the photovoltaic system. Now there is a completely new program according to which power storage systems are subsidized with 40%; but only in connection with a new building. That sounds tempting again. Should we do it? What capacity should the storage system have at least initially? The condition is: "The ratio of system output to usable storage capacity must be at least 1.2 kwp to kWh of the battery storage. Storage capacity exceeding this ratio is not eligible for funding." Have I understood correctly that only a storage system with a capacity of 4.25 kwh would be eligible for funding?

Or is all this way too small? Just "play stuff"?

Basically: save your money and please please install more modules on the roof (if possible fully cover it). When it comes to the storage, it depends on your motivation, high self-sufficiency or maximum return. In the first case, you can do it, in the latter, you have to calculate it. I don't know the 40% subsidy, but in what I know there was always a clause that a maximum of 50% of the system capacity may be fed in and that is total energy waste.

But again, system size brings you much more than the storage, because even in bad weather the basic yield is higher + more remuneration. Also always think about electricity consumption in 10 years, with maybe 2 electric cars that you want to charge over 5kwp you won’t get very far.

Storage is now worthwhile, even without subsidies. Two years ago, it was completely different. So go for it! A 40% grant is amazing.

I would first observe how your habits/consumption are. And I would agree with the statement to put as much photovoltaics on the roof as possible. Either just under 10kwp because of the [Erneuerbare-Energien-Gesetz-Umlage] or over about 12.5kwp since it still pays off despite the surcharge. There is a specialized forum for this. Quite elitist but very competent.

I believe that there are two options, whether to use a storage unit or not.

My opinion:
If you want to be more independent from the grid and more self-sufficient, I would choose a storage unit, even if it is currently not very sensible (costs).

If you choose the cost/benefit factor to determine when the system pays off, then I would (if it is to become a KFW40+ house) choose the storage unit as small as necessary and fill the roof as much as possible.

There is now a new program that subsidizes power storage with 40%; but only in connection with new construction.

Tell me where the subsidy is available. I'm also personally interested. With the subsidy and no other disadvantages, it is almost a must. But please actually cover the entire roof – that makes sense.