The engines subsystem affects your speed and maneuverability (and consequently, your ability to evade certain AOE debuffs such as gravity well.)
The shields subsystem affects both your shields’ innate ‘hardness’ (how much damage they can reduce through resistance) and regeneration rates. The weapons subsystem affects the damage output of all directed energy weapons (cannons, beams). A starship has four major subsystems – weapons, shields, engines, and auxilliary – and the amount of power that is available to each of these subsystems directly determines how effective each area of your ship is. Subsystem power is one of the least-taught and most important parts of a ship’s performance, as it affects absolutely every last facet of its key characteristics, abilities, and skills that you use.
‘Efficiency’, as a term of game mechanics, refers to a starship’s effective overall subsystem power levels against the amount of base investment that is needed to produce that number. Soon I’m going to be posting my build for the USS Ramillies, but before that, I want to talk about an aspect of power management that some may not realise.