A colony ship is built around some star or other. (Maybe later in the story it's a dying red giant, that could account for energy issues.) Though energy is essentially limitless (if you have enough time), how fast you can get it (power) is not. The limit is set by the area of the solar panels and their efficency. Because of the limit, the builders (maybe a mythical first generation of inhabitants?) designed the system so that the power priority is to life-sustaining processes. For example, maintaining the spin (and therefore gravity), purifying air, light for hydroponics. However, it was built to be overrideable, and a few generations ago the colonists/inhabitants voted to do so to sustain some aspect of their lifestyle. The current generation is now seeing the effects of the poor power management.
So I decided I'd have a little fun with this. I've got the outline up on PBwiki here, and if you feel like contributing, go right ahead!