For a long time I had a goal of building a greenhouse to raise coral. Before I got the chance to build it along came LED lights and made the potential savings very small. The green house I was going to build was designed and built in Alberta somewhere. It had one sloped wall that was at 90 degrees to the sun at noon on January 1st. This whole wall was glazed, but could be covered with insulation at night. If the angle to the sun was wrong you could lose much of the energy during the coldest part of the year. So if you expect to get any real benefit from the roof window, it has to be set up correctly. I would rather have a very snug shed and light it with LEDS. It will be cheaper to operate.