Hm that's weird. Do you have a stove or shower or toilet? You can turn the utilities off on those and it makes a huge difference. My sim lives off the grid too with +- the same setup + 2 dew collectors and can always watch television or play a game for a good while in the evening before the power goes out. But I need to turn off the utilities on all items that have the option to turn them off when I'm not using them (which aren't that many, the TV and console don't have this option for example). I also figured out that darts use energy (because of the little lights), so do make sure to double check such items - dunno how many your sims has :)
You can also get some bugs and harvest their fuel, then upgrade your television to run on fuel instead of electricity, so you can use that (or bits). Maybe that works for the computer too?
If none of this works it might be a bug :/