Comment by Avicebron

Err (150kg)(3500kcal/kg) = (5.25E5 kcal/E_koku)/(365 days) = ~1500 kcal/day. So not great working in the field or fighting for the average guy. But I feel like this is one of those things where the average lord didn't count how many fish you caught or what have you. So I bet if you had miso, fish, and like some vegetables from the outside your house, you would probably survive alright.

EDIT: the actual point is that it's probably not literal. Ease of quantifying and transporting dried rice over long distances without refrigeration probably had a lot of how they ended up with a Koku as useful measurement.