So the sunlight->panel->led->light conversion does waste ~90% of the sunlight.
This is made less bad by not lighting plants with white light but purple light, but clearly solar panel on the roof, plants indoor don't make much sense.
Now, the reasoning is completely different if the electricity is produced elsewhere, transporting produce may not be that expensive, but transporting electricity is a lot cheaper.
The main benefit from urban farming is definitely reduced time from harvesting to the plate, and the ability to consume varieties that don't transport well...
Why not? Wasting 90% of the sunlight is still better than using sunlight directly, where the plants themselves only use 4-7% of the sunlight. The plants are wasting all the green-spectrum light, which is where most of the sun's energy is.
Besides, you wouldn't use white LEDs, there's no sense in that. You'd use LEDs tuned for the wavelengths that plants actually use.
At a cost of atleast $2 a per watt for lights (possibly up to $4 a square foot for industrial/commercial electrical rating) and assuming 35 watts per square foot, a 2000 square foot grow space, you are looking at $140,000 in LEDs alone. plus 70KW for 16 hours a day(+10% more for supply inefficiencies), that's about 1200 kilowatt hours of power needed to supply daily. Not sure how many panels that is but it has to be a lot.