The energy source is actually very important, because it dictates how much energy you have to invest to gain extra energy. Using draft animals has a very low ra

The energy source is actually very important, because it dictates how much energy you have to invest to gain extra energy. Using draft animals has a very low rate of return. During my grandparents' time, the draft animals used to produce the food consumed about a third of the total farm produce--and this is likely to be a more efficient farm than any that existed a thousand or two thousand years ago. Fundamentally, this means you have wildly different costs for energy. Modern electrical energy costs around ten cents per kilowatt-hour. Gasoline fuel costs in the US right now turn out to around eleven cents per kilowatt-hour basis (although obviously an internal combustion engine isn't the same efficiency as a electric engine). By way of comparison, a single workhorse for an entire working day will put out maybe 6 kWh of energy, and the food input requirements for that workhorse are going to cost far more than 60 cents.