Source of efficiency?

This chart makes me think you're making things up: https://secure.wikimedia.org/wikipedia/en/wiki/File:PVeff%28...

I'll dig around and see if I can find the source where I read this, but otherwise it looks like I was wrong. My bad.

To back that chart up, photosynthesis generally works at around 5% efficiency.

Which agricultural plant gets 5% efficient photosynthesis ? That's huge. 2% is quite a lot.

http://en.wikipedia.org/wiki/Photosynthetic_efficiency

sugarcane?

Perhaps he means if you're using solar cells followed by Fisher-Tropsch to make actual fuel.

I'm pretty sure plants beat that.

But the base efficiency of plants is horrible : 2% is relatively high in plant species (solar energy input -> produced plant matter output). The most efficient plants in the world are close to 4.5% efficient.

From a pure energy efficiency perspective, it's not. A more relevant parameter might be energy return per human energy input, where letting plants do their thing and then using them is pretty darn efficient.