>Originally the W. brothers' story was that the humans were being harvested for their mental processing power… not "battery power"

The best source that I am aware of about this, was in an audio commentary on the Matrix Ultimate Bluray Collection.

But a few online sauces:

matrix.wikia.com/wiki/Goliath

tvtropes.org/pmwiki/pmwiki.php/Main/HumanResources