I've had good luck with bat houses, and bad luck. The best was a rather tall, slender bat house, probably 8x8x30 tall, with probably 20 dowels inside that go about half way across inside the house. That one was mounted about 20' up in a pine tree, at the edge of a patch of trees. No major water supply nearby. Dozens of bats.
The worst was my current bat house, probably 4 Deep, 20 wide and 30 tall. Also with dowels inside. Mounted on the side of our pump house, for 6 years now. Nary a bat.
Obviously my predictions on location and design have a lot to be desired. We do still have bats - dozens of them nightly, but they aren't roosting in my fine looking bat house. I have no idea where they roost, but they are certainly the world's best mosquito control.