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Emergency Reserve is a generator program which passively generates +5 PWR if you have at most 3 PWR at the start of the turn. It has a cooldown of 3 turns. Emergency Reserve can be bought from some Server Terminals for 300 credits.
Emergency Reserve never triggers on the first turn of a mission, even when playing with 0 starting PWR as a custom difficulty setting.
Emergency Reserve's name suggests that it is useful for emergencies when you are low on PWR — and it can be, but if you are only triggering it occasionally, then it would normally be better to have a different generator to avoid those emergencies in the first place.
Emergency Reserve is best when you can trigger it consistently, every 3 turns, or otherwise as frequently as possible. In this case, Emergency Reserve generates an average of +5/3 ≈ +1.67 PWR per turn, making it one of the more efficient generators. This is easier to achieve in low-PWR loadouts, but can also be achieved if you time your use of PWR well.
If you also have Cycle installed, then it will reset your PWR to 3 before Emergency Reserve's effect, so you never miss out on the PWR generated by Emergency Reserve.