Your post sounds like there is some strange rule regarding taxation of solar installations, but it is (or rather was) just treated as any other income from commercial operation.

- If you have any income from commercial operation (selling stuff) you are required to declare that, when doing your taxes.

- When selling electricity you're not so much classified as an energy company, but fall under similar taxation rules as a company. This also means you can get the VAT back, you paid when buying panels, inverters etc.

- Since 2023 solar installations smaller than 30 kWp are completely exempt from this taxation scheme. This means you're not paying any VAT, when buying equipment for your solar installations and don't have to declare the revenue.

- You can get around the 30 kWp limit under some circumstances by having multiple separate installations.

- Parent post is talking about an off-grid installation, which would be exempt anyway, because there is no revenue from selling electricity.

The tax exemption from 2023 was a very welcome move by SPD and the Green party to reduce bureaucracy regarding solar installations and I think the 30 kWp is pretty reasonable

Would be interesting to see, how other countries are handling this.