Kfw 270 Photovoltaic financing independent from construction financing

I am also currently looking at the 270 from KFW, but the conditions are simply much worse compared to a normal construction loan... does anyone of you know why?
The advantage of paying for the photovoltaic system with a loan is that you can deduct the interest in the photovoltaic tax return (provided you do it commercially).
Does anyone of you know how to prove the interest to the bank? So how does the tax office see which loan was used to pay the photovoltaic invoice? Send the payment slips along?

You don't have to prove anything at first. You report the interest in the EÜR as you believe they were incurred for the photovoltaic system, and that's it. As long as you don't receive an external audit, you don't have to prove or explain anything.

So how does the tax office view which loan was used to pay the photovoltaic invoice?

Often the bank transfers directly from the loan to the construction companies or here to the installer of the system. Then you have a very direct verifiable allocation.