Charlie, basal human metabolism is about 100 Watts at rest. Parts of photosynthesis are remarkably efficient -- the photosystems themselves push close to 100% -- but it takes about ten photons to fix one molecule of CO2. (James, here energy = h * frequency raises its ugly head.) Because of that, the theoretical max is about 25%. Turns out of cultivated plants, sugar cane is the winner, at 8% efficiency. Corn (maize) is about 2%, and random wild plant life on average is rather below 1%.

(And wash your mouth out with soap for saying 'advanced nanotech', Charlie. Plant life _is_ advanced nanotech. Well, except for the ribulose bisphosphate carboxylase glitch, but that probably couldn't be helped even with Drexlerian nanomagic.)

Carlos