Hi I asked that question myself and I got fairly good aswers in this thread below
http://www.orchidboard.com/community...-any-good.html
Anyway I had one 23watt Daylight going to supplement 2 hours of sunlight 7 to 9am and had great results. Please bear in mind that this is what I think, as I am new to plants and orchids.
I have it in between the plants only about 16cm away.