Hi AerBabe,
Nothing to do with power. Its really quite unnecessary. Turning the lights off for T/O and landing is meant to preserve the passengers and cabin crews night vision in the event of an evacuation. However, its a waste of time because in order to have total night vision your eyes need around 30 minutes to become accustomed to the dark. Also I suppose theres the notion of being able to see flames better in the dark with out the internal lights reflecting off the windows, thus making it safer for the crew to evacuate the safest side of the aeroplane.