If you were to measure the sun it puts out about 8k to 10K foot candles on a summer day. I have no clue what that is in lumens, but I can get a reading for you tomorrow for the winter here in GA in lux or foot candles.

Joe has it correct though. Plants use a certain light to grow. They mainly use reds and blues. Reds I think induce flowering and blues induce foliage growth. IIRC You could have a very bright light, but if it is not producing the correct wave lengths for the plants it is un-usable to the plant. Although that can be over come by making the exposure of the light longer because alot of lights do give off most of the wave lengths. Some just put off more than others in certain wave lengths.