Someone tell me I'm not just being a dumb noob before I go post a bug report... my base is constantly draining the fuel tank even when the generator's off and I'm running on solar, could that be something other than a bug?
Well, I think I found the problem - decided to pick up & replace my tank and generator, and the tank fell out in an uninteractable drop box, so I had to make another one and now I've got no problem. Must've sprung a leak...
There's a bug / quirk with solar capacitors that after they've been installed and your save reloaded they can actually consume fuel when they run out of battery charge. Because of it there's no real practical reason to ever shut off a fuel-powered generator ("idling" generators have perfect efficiency). They know about the issue but need to revamp power generation in general to fix it.