There have been promising (if still far off) developments as of the last few years regarding using mushrooms as a source of electricity.
Per CNN (Jack Guy), circa 2018:
"Now, a team of US researchers say they’ve found a way to make environmentally friendly energy using bionic mushrooms covered in bacteria.
Clusters of energy-producing cyanobacteria were attached to a typical button mushroom using 3D-printing technology, alongside an electrode network to harness the power they produce."
Per Medium (Charlotte Ely), circa 2021:
" The use of mushrooms as a replacement for graphite in batteries becomes even more compelling when you consider that these organic tissues have a high concentration of potassium which, over time, causes an increase in the porosity of the carbon nanoribbons and thus enhances its capacity to store charge. In short, a mushroom-run battery will become even more powerful with time, whilst its graphite counterpart will get weaker and experience a decrease in run-time."
All a ways off, and of course, it isn't currently possible to power something as hungry as a PC with them, but if this is a hypothetical post-apocalyptic fictional future we are talking about here, then mushrooms may be the key to almost completely organic electricity production for low-power systems.
Where I live, I'm not sure that any one source of power would be enough. I'd probably have to have several sources linked to long-term battery storage.
Probably solar for most of it, with a wind turbine adding to it when weather permits. I might move closer to a creek to get some minor amount of hydroelectric power in there as well. It's best to have redundancies in case any one source messes up.
The power source I'd use to power my ecological paradise were I stuck on an island would be tidal power, a thick low level wall around the island that would leverage the shifting of the tides to provide accessible and defensible power that would also deny my shores from the uncertain oppression of mysterious forces beyond my island while also providing me consistent power that would be as they say, "Reliable as the tides"!