Finally we have tgot a solar hot water!
It's running since a week and all the week we didn't have much sunshine.
But I decided tio run the thing in the extreme way: The electric booster is out.
Why: it will save us nearly 100% of electricity for hot water. With the little sunshine we've got there is still some lukewarm water - enough to shower (but not these wasteful 4minutes showers).
For dishwashing sometimes I heat 1 or 2 litres in the electric water heater - far more economic than heating more than 200 l on the roof.