That's not an accurate comparison. You are then using that 20% to power grow lights for photosynthesis. How efficient is photosynthesis using grow lights? Even if it's twice as efficient, you're then looking at 4% of 20% which is 0.8%.

Sure, this can be used used land not suitable for agriculture. I only see this really being helpful in cities that have old industrial buildings that are sitting vacant (reuse existing infrastructure and reduce transportation).