Air-Water Heat Pump - Buffer Storage Yes or No?

What do you understand by "too little power" expressed in numbers? What does the 300L tank need per day?

1x daily a nice comfortable shower (one person or two people not so long), then daily about 1.1-1.5 kWh. Without use every 1.5 days about 0.8 kWh. Empty tank at 30 degrees 3 kWh (4 people showering very long).

1x daily nice and cozy shower (one person or two people not that long), then about 1.1-1.5 kWh daily. Without usage every 1.5 days about 0.8 kWh. Emptying tank at 30 degrees 3 kWh (4 people showering very long).

Ok, maybe I am on the wrong track in my understanding if it is indeed as you say. I currently consume 3-6 kWh for hot water (190L tank) and assume it would be more with a 300L tank. Every kWh heated with photovoltaic saves real money, and not little at today's/future prices. Hence my mention that it would be more advantageous to place the heating of the larger tank in the photovoltaic times. Even with the 3 kWh you mentioned, that is let's say at least 0.6-1 EUR savings per day compared to grid consumption. That makes 20-30 EUR per month or 200-300 EUR per year just by shifting the heating time to the photovoltaic-relevant time frame.

Every kWh heated with photovoltaics saves real money, and not just a little given today's/future prices. Hence my mention that it would be more advantageous to schedule the heating of the larger storage during photovoltaic times. Even with the 3 kWh you mentioned, it's say at least 0.6-1 EUR savings per day compared to grid consumption. That amounts to 20-30 EUR per month or 200-300 EUR per year just by shifting the heating time to the photovoltaic-relevant time period.

That is the property. Correct. But a) you don't completely empty the storage every day, so the savings are significantly lower, and b) that doesn't work in winter, which reduces the savings even more. By the way, photovoltaic electricity also costs money – no feed-in tariff as well as costs (tax-related and in the profit and loss statement). I rather calculate with 1.5 kWh per day x 365 = approx. 550 kWh. Of that, you make half (?) from photovoltaic electricity, so 275 kWh. Savings at 35 cents electricity costs are then 96 EUR, for which you forgo the feed-in tariff (17 EUR) and you have costs for self-consumption (another 20 EUR) and there remains a profit of 5 EUR per month. Now I calculate this with my current electricity costs of 25 cents and the result becomes even smaller. Or you have heat pump electricity, which makes sense in individual cases.

This is property. correct. But a) you don’t empty the storage completely every day, so the savings are significantly lower

Somehow we are unfortunately talking past each other :)
Your argument in favor of a large storage was that the smaller one is not sufficient. Therefore, my assumption or calculation was that one could assume an empty storage every day.
Besides, I was concerned with the energy difference that would have to be provided for the larger storage compared to the smaller storage... This value is unknown to me, so I am actually trying to understand what difference one could realistically assume here.

and b) that doesn’t work in winter, which makes the savings even lower. By the way, photovoltaic electricity also costs money – no feed-in tariff as well as costs (tax-related and in the income statement).

These costs are already included in my 20 cent difference (currently cheapest electricity price - feed-in tariff).
Taxes etc. are of course omitted, but this is supposed to be only a rough presentation that ultimately varies greatly for each individual. Presumably, the assumption calculation is even too conservative for most.

I rather calculate with 1.5 kWh per day x 365 = approx. 550 kWh. Of that, you make half (?) from photovoltaic power, thus 275 kWh. Savings at 35 cent electricity cost is then €96, for that you miss the feed-in tariff (€17) and have the costs for self-consumption (another €20), and there remains a monthly profit of €5.

Do you actually only consume 1.5 kWh per day for hot water? Or are you only referring to the difference regarding different storage sizes?

Somehow we are unfortunately talking past each other :)

True ;)

Your argument in favor of a large tank was that the smaller one is not sufficient.

Yep. The smaller tank is not sufficient. But not every day. Maybe it is enough for half or 2/3 of the year, but unfortunately often not.

Do you really only consume 1.5kWh per day for hot water?

Yes. That is the absolute average consumption. It was roughly estimated off the top of my head, but last week it was actually about 11kWh (I just checked).

Or are you only referring to the difference based on different tank sizes?

No, not the difference. Maybe the difference is 0.5kWh (rule of three due to size). If you focus only on the difference, the advantage of a photovoltaic system naturally becomes even smaller. Of course, the photovoltaic system "saves" grid consumption. But hot water preparation is only a small part.

Of course, the photovoltaic system "saves" grid consumption. But hot water preparation is only a small part.

It's ok, I understand. Still, in my case, the daily consumption for hot water is around 4 kWh. How you manage the 1.5 kWh even with the larger storage, I still have to understand :)

Most of my 4 kWh/day were previously consumed in the morning and evening, precisely outside the photovoltaic times. Now I have shifted the hot water heating times into the photovoltaic-relevant daytime hours and manage just as well on average, with a simultaneous "significant" saving. In your case, it would indeed be negligible, although I still can't fully comprehend the 1.5 kWh :)