The tough part is defining occupancy AND how it should be interpreted. Sure, walking around is easy, but being a couch potato or sitting at a desk is much harder to detect. And then there's the question of just what should or shouldn't be done. Various things would be done quite differently depending on the time of day and the person(s) involved. As in, the wife NOT wanting night time path lighting, but the kids needing it. Lots of other conditionals make it pretty tricky to get it down well enough to be usable to a wide enough audience.