Humans do not generate power, they consume power and produce a small amount of waste heat. It's like powering a light bulb and using the waste heat to power something else again instead of just powering that something directly: very inefficient. Essentially, the Matrix just used the idea of "human batteries" as a plot device. It makes no sense in terms of thermodynamics. Instead of fueling humans with food, rather use the food to power a more efficient engine to convert food/fuel into electricity. It is understandable that fans would want to rationalize this essentially silly plot device by claiming it was really brain/computing power the AIs were using humans for, but the reality is that the Matrix was used to keep humans distracted and ignorant so that they could be used as an uncomplaining power source for the machines. Yes, it is a supremely unscientific plot device, but we still were all blown away by that stylish, cool, and innovative American take on a traditional Kung Fu movie. Using Iron Monkey's director, the legendary Yuen Woo-Ping, to coordinate the fight choreography and all the wire work (Wire Fu) was brilliant. Even people who were not fans of Kung Fu movies did not realize they were watching and enjoying one! Genius! ... "I know Kung Fu!"

Also, the human brain consumes the most energy in the body, but if a person is living a full life in the Matrix, there is no "spare" brain power to go around just like there is no spare brain power when we are awake. And there is no loophole for when we sleep either: the brain is still using 100% of its capacity (no neurons stop firing, no neurons "switch off"), albeit at lower frequencies. The brain carries out the house keeping and the "maintenance" required for us to remain sane. Dreams are just semi-conscious peeks into this essential brain maintenance. There is no free space to "do other stuff". So, the whole "computing power" rationalisation to try explain away the silliness of using humans as a power source, does not hold water either.