Is the solar system on the roof worthwhile?

we decided against it for 2 reasons:

a) amortization not in 20 years
b) as described by Cijay, the tax effort is not insignificant. That means: monthly VAT declaration, under the small business regulation at least once a year. But then no possibility to deduct input tax (VAT)... in other words: you are actually punished twice.

Many I know who installed photovoltaics only found out afterwards that they also have to file a tax return. The effort alone for the few € savings discouraged me.

In the end, we went the other way: paid attention to A++ when selecting devices, replaced everything with LEDs, did not leave lights on unnecessarily, turned off standby devices completely, unplugged washing machine and dryer after finishing, etc.
And looked for a cheap provider.

The cheapest electricity is the one you don’t need!

Standby devices completely off, unplug washing machine and dryer after use, etc.

While the previous points are certainly reasonable, I would consider these measures exaggerated. These repeatedly criticized standby consumptions are not monetarily relevant at all.

We also decided against a photovoltaic system. The payback would be given in just under 20 years because we have a roof area of 16 x 10 meters facing directly south. But nothing should "come in between." And against a possibly expected minimal profit are the following disadvantages: appearance of the roof, maintenance effort, space requirements, heat generation. Tax declaration and maybe additional fire hazard.

While the previous points are certainly reasonable, I would consider these measures to be excessive. These repeatedly criticized standby consumptions are not monetarily relevant at all..

I always thought so too, until I measured...

We had standby consumption totaling 35-40 watts with all devices.
Calculate that... 35x24x365/1000 = 306.60 kWh per year standby * 0.25 ct/kWh = €76.65 per year.
With our 8.4 kWp output and our own consumption, we would have saved about €350-400 in electricity costs per year. That makes almost 1/4 of the savings through photovoltaics. For the other 3/4, I compare my own working time for the necessary tax declaration, so that in the end, almost no savings remain. Especially not if you also calculate that photovoltaics have to be insured... in the end you have to be careful that it doesn’t become a loss-making business.

You can also dramatize it. A VAT return is not worth 5 minutes at that scale if you know how to do it. Of course, you have to be willing to deal with it. But yes, photovoltaics mean time expenditure during operation, solar thermal is more of a no-brainer, whereas heating maintenance involves costs (which are not tax-deductible...)

It's also more about being forced to do something. And a VAT return in 5 minutes, please show me that

There are costs involved in heating maintenance (not tax-deductible for that...)

Heating maintenance can very well be claimed for tax purposes (under certain conditions and not unlimited, of course).