As Adraius says, it's less straightforward than it might seem, because almost any item that has upkeep has been designed to give you potential benefits as long as it's active, even if you're not actively using them at that moment.
Other items(like some hacking utilities) can even bring you too very low matter without you noticing.
Not true. There is no hackware that consumes any energy or matter, by design. (Which is why the game won't even let you turn them off if you wanted to--they and a number of other types of items, e.g. storage, passive armor, power amplifiers, recalibrators, and a ton of other passive stuff, don't have any resource costs and can stay on indefinitely. (Did you mean something else here? There are extremely few items that actually consume matter--were you referring to swapping costs? If so, and you like to swap a lot, that's a different issue and can be overcome with Integration Mediators.)
When not moving, hovering units also waste your energy.
But they can give you a bonus to dodge if you have Maneuvering Thrusters or Reaction Control Systems, which a lot of hovering robots will tend to attach.
I do agree with you on Targeting Computers.
I think the numbers are generally fudgeable enough that you shouldn't usually need to worry about a little extra lost energy to perform well enough. I do occasionally realize the need when things get down to the wire, but that shouldn't be happening around every corner. This is probably a bigger issue for those using hover/flying builds since there is a more delicate mass-energy-heat balance at play.
An auto-disable feature would probably end up being really confusing and annoying most of the time, and would only really apply to a small minority of parts, a few computers and perhaps sensors. What you want on and off at a given time comes down to a lot of situational details and your own tactics at the time.
Also, I'm not sure if it's the game's intent so have people constantly flipping parts on and off to get the most benefit - even automatically. Maybe a better solution would be to have some parts not consume energy/produce heat under certain circumstances, rather than fully disabling the part?
This is a possible compromise for a minority of items, but then the numbers don't always add up and that is cause for confusion...
For example one exception could be Targeting Computers--they only deduct energy
while you're firing, because they don't otherwise do anything for you. This would require a special indicator in the stats, because their energy cost would no longer be considered "upkeep," per se, which is another complication from a UI perspective.
What other parts does this (unquestionably) apply to? All things considered I don't think there are quite so many.
