Proper storage for heat pump with photovoltaics?

Makes a difference to use the heating element for 5 minutes or to keep a 600l buffer tank charged 365 days a year

Yes, a huge one. Heating rod for 5 minutes every day (which we probably won't manage with the mentioned dimensions) could be more expensive from the grid than being able to heat the hot water entirely via photovoltaics for 270 days.

But you would really have to calculate that. The truth definitely won't be as black and one-sided as you describe it here.