I fear there is no such system where this applies. The tech stack on any old netbook is so advanced and complex that there is nobody on this planet who fully understands it.

Being theoretically able to read the code is certainly better than not being able to, but it’s not the same as having actually read and understood all the relevant code to the point where you can be somewhat confident that there’s no backdoor in it.

(And even if someone had the time and mental capacity to do that, at some point when going through the stack you always hit a proprietary layer. Be that drivers, the bootloader, component firmware or the hardware itself.)

*insert obligatory xz utils reference*

Vienna has around 68km² of roof surface.

But there are two nice facts that make the whole calculation easier:

• Resources used in a city don’t need to be produced inside the city. Imagine we had to grow all the crops and lifestock consumed in a city also inside of the city borders. Or gas, oil, coal, water or any other resource. Cities can only exist by importing resources from the outside. And luckily power cables are a thing, so it’s quite easy to import electricity from outside of the city.
• Solar isn’t the only kind of renewable energy. The area north east of Vienna is dotted with wind turbines. There’s a huge amount of hydropower all over Austria. In fact, 87.5% of the electricity generated in Austria is generated by renewable energy, and the remaining 12.5% are pretty much only to cover the times when renewables don’t output enough power.

So the whole point is kinda moot. If we would put solar on all rooftops in Vienna, it would be enough to cover for the entire remaining fossil-based electricity generation in Austria if we had the means to store that excess energy.