What does this even mean? What inputs and outputs are you comparing to get an apples-to-apples comparison between efficiency between fossil fuels and solar?
It's talking about the economic inputs required to create a unit of energy. Literally all you have to fathom is "can we add up what it takes to make a watt of energy from coal, vs. from solar power vs. some other plant, amortized over the lifetime of said plant?"
What does this even mean? What inputs and outputs are you comparing to get an apples-to-apples comparison between efficiency between fossil fuels and solar?