When I say "cost", I'm referring to economic cost, in contrast to what Germany paid to create their system. Not getting something for nothing, just something without infrastructure expense.To my physicist's brain, that sounds like trying to get something for nothing, which never ends well.
We're talking about a potential post-scarcity scenario, which is going to require a type of technology we don't have. But we do have some very good ideas about how biological or nanotechnological machines could work, and if they truly are built of atoms, the machines are not going to be something that will be "serviced".There are always costs, no matter how hard you to try to hide them. Even if no humans are involved in producing the energy, something has to do it. Something has to build the machinery and maintain it, and all those layers of machines use energy which is the cost of production. Somewhere, something has to put work into producing the energy. If you do miraculously manage to harness large percentages of this ambient energy then you need to expect environmental impacts as the natural transport of energy and materials around the planet gets changed. There is no such thing as a free lunch, let alone a free energy system.
As far as energy goes, I'm not saying there is no energy expense, I'm saying that the amount of energy required to make a smart phone with an assembler at home out of recycled garbage is orders of magnitude smaller than the energy used to produce and distribute a phone the traditional way. The drop in energy consumption from the change in production will be drastic, even if people become hoarders.
But aside from that, the energy required to use assemblers to cover every rooftop and roadway with solar cells would also be relatively modest compared to the energy cost of pavement or shingles. All the power we could need could be gathered via solar that is normally "utilized" to decompose asphalt by trapping heat. Instead of that energy heating those surfaces, they could produce power that wouldn't be radiated into the environment but locked up in production. The net utilization of sunlight would never exceed the amount of energy that is already heating the earth. So while energy consumption for production drops due to on-site production efficiencies, the gathering of energy could also be exceptionally low impact. Even, as I mentioned, reversing some global warming issues if enough earth heating is tapped off to replace abandoned non-renewable sources.
None of this is crazy - it is how the natural world uses chlorophyll and protein assembler machines to build all the life around us. We would just be using a similar process to build stuff.