[Suicide Junkie]

Supplies = Power (Usually from Anti-Matter)

In regards to reactors. The supplies represent the anti-matter in the reactor needed to power the reactor - in that sense it generates all the power the ship needs. Once that anti-matter is depleted than the ship is out of power and has no supplies. Anti-Matter Pods will hold extra anti-matter. Impulse engines could provide a little power from their reactors but not much. Ramscoops collect interstellar hydrogen which is just a minor power source.

Does that clear things up?

*Sigh*
No.

You say "supplies = power", but then you describe "supplies = fuel"

If you are going with a supplies = fuel (standard SE4 system), then your ships will always have enough power to run all their systems 100% all the time. Until they run out of fuel, then weapons, shields, engines, etc, all go offline.

Definitions:
Power: Energy (used) per unit time.
Fuel: Total energy capacity.