1. IIRC, an adult human metabolism dissipates on the order of 400 watts at rest -- the increase when working isn't that great. I'm not sure there isn't something missing here in the Kcal calculation ...
2. Again IIRC (ancient biology/biochem lessons surfacing here) photosynthesis is bloody inefficient -- less than 1% of the incident photoelectric energy landing on a chlorophyl molecule ends up being used for ADP->ATP synthesis. When you add the not-unreasonable requirement that plants spend the majority of their metabolic energy on processes not directly contributory to human nutrition (e.g. synthesis of indigestible lignified cell walls, required to stop them flopping around in the dirt), the available energy drops further.
So as long as we're using a plant-based biochemistry, I figure you need to add three orders of magnitude to your estimate. Advanced nanotech might shave two orders of magnitude off of this, but I'm not sure how.
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2. Again IIRC (ancient biology/biochem lessons surfacing here) photosynthesis is bloody inefficient -- less than 1% of the incident photoelectric energy landing on a chlorophyl molecule ends up being used for ADP->ATP synthesis. When you add the not-unreasonable requirement that plants spend the majority of their metabolic energy on processes not directly contributory to human nutrition (e.g. synthesis of indigestible lignified cell walls, required to stop them flopping around in the dirt), the available energy drops further.
So as long as we're using a plant-based biochemistry, I figure you need to add three orders of magnitude to your estimate. Advanced nanotech might shave two orders of magnitude off of this, but I'm not sure how.