Vertical farming doesn't work out in general. Plants tend to be limited by light, so when you go up you shadow horizontal land. There is some efficiency gain because solar panels -> LED can convert light wave lengths, but I don't know if it is enough to overcome the losses in the system plus the cost of the building.
Vertical farms are all niches: lettuce is high value, and spoils fast. Thus they can get closer/faster to market by going vertical. They convert electric (coal and natural gas) to light, so they are not very environmentally sound.
From the numbers ive seen (which could be off), you get like 250 watts per square meter of solar panels, but you need atleast 350-500 watts per square meter of the highest efficiency currently available grow LEDs. And that is assuming you grow them in a building with higher than natural air temperatures because "wasted" wavelengths aren't really wasted because plants need them to elevate leaf temperatures significantly higher than ambient temperatures. So it has heating costs also.
Also, the reason they grow lettuce for these tests and not more calorie dense or nutrient packed plants is because lettuce requires little light and little nutrients, most of its weight and volume is just water and air. Growing wheat or potatoes or other primary food crops requires a lot more light and a lot more nutrients than these demonstration crops.
Vertical farms are all niches: lettuce is high value, and spoils fast. Thus they can get closer/faster to market by going vertical. They convert electric (coal and natural gas) to light, so they are not very environmentally sound.