What I said about generating energy from humans is actually a basic law of the universe.
No energy can be created or destroyed, it can only be converted from one form to another.
It is actually impossible to “generate” energy all you do is convert it. Eg. A power station converts energy stored in fossil fuels into electricity or solar panels convert solar energy into electricity.
The heat humans generate isnt magic and comes from nowhere, the heat we generate and all the energy our bodies can possibly output comes from food. If you create and supply all the food to the human(the machines do from dead humans) it is impossible to make any energy gain at all.
In some cases this rule is hard to accept, for example an Atomic bomb does not create any energy, it merely converts nuclear energy(within atoms of urnanium) into heat, light and sound.
If the machines had grown food for the humans e.g. plants then they could have generated energy as plants come from energy in the soil and from the sun but they were feeding humans to the humans, even if they could get 100% of the energy out that the put in(this is also impossible but thats another law) they wouldnt generate any energy.
The “combined with a form of nuclear fission” thing in the matrix was just put there to cover the mistake, they needed humans to be enslaved for the plot.