If you're looking for active, I've got one of these installed in my backyard office (96 square feet) that does pretty well. It's quiet and solar powered. Been pleased with it so far.
http://www.amazon.com/ECO-WORTHY-So...&redirect=true&ref_=oh_aui_detailpage_o04_s00
No bigger than what I have that might be a good idea. I might look into something like that for our dog house too.
