190l vs. 309l hot water storage tank for 4 people

One more point: In addition to the mentioned arguments, the water tank can definitely also be used as a storage for excess photovoltaic power. My cousin uses it exactly that way consciously – and heats the hot water significantly with the "excess peak." Therefore, in my opinion, definitely go for the large tank!

I would also take the offer with the larger storage. Because heating the water with a heat pump does not happen as quickly as with a gas heater. If several people shower one after the other, there is enough hot water prepared. Heating 200 liters can take 1 hour with an air-to-water heat pump

But it won't be faster with a 300l storage. We have no problems with four people (two of whom are still rather small) with a 180l integrated storage. The storage is set to 47 degrees, i.e. if necessary, you can turn it higher at the cost of efficiency. Overloading via photovoltaic is nice and good, but please not via heating rod but via the heat pump. It would be a total waste of money to directly heat the electricity.