In the recent past, the main reason for me to keep the boat dark at anchor wherever I could was to save on power. A 20-watt bulb at 12 v is an amp and half, over two if you have a 25-watt bulb. With 14 hr of darkness, just say between 23 & 28 amp-hr to burn the light all night. In winter, with clouds, low sun angles, and shorter periods of daylight, my single 85-watt solar panel could gin up only about 20 amp-hr on good day. But now, I have an LED up there, and a Balmar 150-amp alternator. Problem solved.