For all of these processes, the first staging post for the energy is in a chem-
ical molecule such as a carbohydrate in a green plant. We can therefore
estimate the power obtainable from any and all of these processes by es-
timating how much power could pass through that first staging post. All
subsequent steps involving tractors, animals, chemical facilities, land-
fill sites, or power stations can only lose energy. So the power at the first
staging post is an upper bound on the power available from all plant-based
power solutions.
So, let’s simply estimate the power at the first staging post. (In Chapter
D
we’ll go into more detail, estimating the maximum contribution of each
process.) The average harvestable power of sunlight in Britain is 100 W/m2.
The most efficient plants in Europe are about 2%-efficient at turning solar
energy into carbohydrates, which would suggest that plants might deliver
2 W/m2; however, their efficiency drops at higher light levels, and the best
performance of any energy crops in Europe is closer to 0.5 W/m2. Let’s
cover 75% of the country with quality green stuff. That’s 3000 m2 per
person devoted to bio-energy. This is the same as the British land area