I don't have a reliable source, but I've heard that the original script had the machines use us for our processing power, not our energy, but that the studio thought it sounded too complicated and had them change it. Of course, changing it makes it make ZERO sense.

Some not so reliable sources I found:

https://www.reddit.com/r/matrix/comments/q2i0by/is_it_true_t...

https://www.reddit.com/r/plotholes/comments/11khig/comment/c...