Old post (200603) but still relevant AFAIK (if we move out of liquid fuel for transporation and go for batteries).

http://fatknowledge.blogspot.com/2006/03/sugar-cane-vs-solar-panels.html

 Sugar Cane vs. Solar Panels

As I was reading the crazy ideas about the future in The Singularity is Near, I had a crazy idea of my own: what if humans could be solar powered? Right now, humans use plants to capture the sunlight's energy and convert it into carbohydrates which we can digest and use to power our bodies. What if we could cut out the middle man? What if we could wear a solar paneled hat or body suit that would capture the sunlight and turn it into the glucose that the body needs for energy? And if it were possible, how large of a solar paneled hat would you need?

And this lead me a second question: which collects more usable energy, an acre of sugar cane or an acre of solar panels?

So I ran the numbers. I chose sugar cane because it produces the largest number of calories per acre of any agricultural product. From sucrose.com we learn that you can harvest 10 tons of sugar per hectare = 10,000 kg/10,000m^2 = 1 kg /m^2. This means a square meter of sugar can yields a kilogram of sugar a year. As there are 4 calories (or kcal) per gram this gives you 4,000 kcal/m^2.

At this site we learn that a square meter of 11% efficient solar panels collects around 550Wh a day in Austin Texas. So for a whole year that gives you 550 * 365 = 200 kWh/yr/m^2.

Then as everyone knows (or knows how to use Google Calculator knows) 1 kcal = 1.16222222 watt hour. So our 4,000 kcals of sugar = 4.65 kWh. That means the 11% efficient solar panel generates 200/4.65 = 50 times more usable energy a year than the sugar cane does. And as you can now get 22% efficient solar panels that would go up to 100 times.

Solar panels are therefore 2 magnitudes of order better at creating usable energy per m^2 than sugar cane
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