You don't have to register a business, though. Call to the tax office, "I'm doing photovoltaics, which tax number?" Answer: "Just your normal tax number."
Correct, but only almost always. Due to tedious private and business changes, I had to apply for a new, additional tax number. For this, the first form for the "business," estimated to be 27 pages long. Nevertheless, after overcoming this hurdle (I hate paperwork and have all my tax matters done for 25 years) and applying for the required Elster certificate, I could then submit the first VAT pre-registrations online. Of course, initially the fields were filled out incorrectly. But the nice gentleman from the tax office then calls you, tells you where to correct something, done. Now I just have to read values from the inverter and wall box beside the electricity meter to record self-consumed and sold electricity, multiply by the electricity price, and enter it. The form is then copied and adjusted quarterly, which only takes 5 minutes for each 3 months. For this, I was able to deduct VAT for the entire system including battery and wall boxes. After 5 years, switch to hobby status – done. A small pitfall for owners of several photovoltaic systems: They are linked, and only one type of assessment can be chosen for all together (no matter how old the systems are).