Some ship equipment e.g. the Jem'Hadar Combat Impulse Engines, refer to being "Efficient at Low Power Levels".
Is "low" 35? 25?
Cheers in advance
It depends on what you are looking at.
In terms of speed, Combat Impulse Engines are better at Engine Power of <<50. In the 50-55, rang, all engines very near to one another in terms of max speed. At lower Engine Power, the Combat Impulse Engine's max speed will be better by an increasingly wider margin.
For more details see my Mini-Engine Guide. A link is in my sig below.
And for the weapon power bonus of the engine. There is a fixed power setting bonus. And the other is an Efficiency power bonus. It's a sliding power setting bonus for Weapon Power Setting from 75 down to 25.
For (a lot more) details on Ship Power see my Beginner's Guide to Ship's Power. The link is in my sig below.
To summarize without all of the confusing extra info, a "bonus at low power" gives a bonus that increases based on how much below 75 your power setting is for the system in question--the lower the power is set to, the higher the bonus.