This is sort of vague, but repair parts in general. We have found over the years that many bits and bobs of our various machines and setups (such as our cobbled-together solar charging setups) break sooner than expected, and many of the replacement parts you can source online break very quickly as well, especially if you try to economize with cheaper parts, but it’s not always clear which brands are good and which are crummy. And it really burns when you spend 4x as much on an allegedly high-quality American-made part only to have it break just as quickly. When we find a brand that doesn’t break quickly, sometimes it later disappears, becomes expensive, or they change the manufacturing process so it becomes crummy too. So it can be worth stocking up on a few spares for later use or other projects. Ultimately not using parts with electronic or plastic components would be ideal, but that’s obviously very limiting.

Re: your question on solar generators, we’ve owned a couple of the plug and play ones and they’ve been convenient but not the most durable, and not easily repairable. I think you’re better off building your own; it’s a bit cheaper to begin with but the real savings comes when some minor part malfunctions and you don’t have to eat the cost of the battery.