I provide supplemental lighting in the greenhouse until mid April when the days are longer. I have them on timers so the plants receive about 14 hours of light per day. We don't put on the shade cloth until late May, early June so we do get decent natural light in the winter, just not enough of it.
Karyn

How do you arrange the lights so they don't block the natural sunlught? I have a light setup in my garage I just didn't want to move them into the greenhouse.

The lights are suspended from the supports on the top of the greenhouse (the ones that the baskets are hanging from). I don't have any problems with them blocking the natural light but my greenhouses are pretty big.
Karyn
{{gwi:221419}}

I start my seedlings in the basement under lights and in March I move them to a portable greenhouse on the deck. I am in western NY - NO sun! - and they have always done great without any extra lighting. Just remember that even though it looks overcast and gray to you, the natural light outside is much more intense than the flourescent lights inside.

I am not planning to give any supplemental light in greenhouse but I am growing plants for shade so it is not a problem.
Lenght of the day will mean a lot for commercial grower since blooming time will depend on it. If you going to grow your plants for your pleasure regular greenhouse is good.