In my sustainable agriculture course, I read that modern grain varieties put 60% of their photosynthesized energy into growing the fruit (the edible portion of the grain). Wild varieties put more like 3% into the fruit. There's your 20x difference in yield. But it comes at the expense of every other survival mechanism of the plant: modern grain varieties yield so much grain because they don't have working systems for anything, be it support, storm resistance, disease resistance, or general hardiness. They give us food because we supply them with a good amount of fossil-fuel derived work.

It's somehow similar venture capital. It's a resource produced in older times that allows you to get temporary advantage compared to somebody that uses just in-time generated resources.

This, among other reasons, is why grain ethanol was such a scam.

The worst mechanical processes, by contrast, usually get at least 10-15% efficiency. That's what photovoltaics can do, for example. Gasoline engines have a thermal efficiency of about 30%, diesel about 45%. Granted, you lose energy in the extraction/refining/transportation steps, but it's still an order of magnitude more efficient than photosynthesis.

The primary [photosynthetic] reactions have close to 100% quantum efficiency (i.e., one quantum of light leads to one electron transfer); and under most ideal conditions, the overall energy efficiency can reach 35%. Due to losses at all steps in biochemistry, one has been able to get only about 1 to 2% energy efficiency in most crop plants. Sugarcane is an exception as it can have almost 8% efficiency.