How about something more along the lines of "Auxiliary Batteries". They have a specific amount of energy (instead of the multipliers).
Examples:
For Fighter and Bomber Classes
Auxiliary Batteries Mk I gives an additional 500u to the base power output. Mk II would be 1,000u, Mk III would be 1,500u and so on and so forth until MK VIII at 4,000u
(like with Armor Upgrades, the equipment is cargo dependent)
For Gunboats and Higher
Auxiliary Generators Mk I generates 2,000u of extra power, Mk II generates 3,000u, and so on and so forth until Mk VII at 8,000u
(like with Capital Armor Upgrade, the equipment is cargo dependent)
-OR-
we could go another route:
Reactor Upgrade Type A - Multiplies recharge rate by 1.25
Reactor Upgrade Type B - Multiplies recharge rate by 1.50
Reactor Upgrade Type C - Multiplies recharge rate by 1.75
Reactor Upgrade Type D - Multiplies recharge rate by 2.00
Thats my suggestion, or at least, my thoughts. what do you think?