I read over the plan and I found that I needed a location with:
lots of sun;
at least 15 feet off the ground (to protect against predators); and
ideally a water source nearby (so the mother bat doesn't have to leave her young for too long).
Interestingly, bats are less attracted to bat houses mounted on trees. There's a few reasons for this:
It's too easy for predators to get bats as they exit
The branches causing obstructions to exiting bats which drop down then up into flight
It's too shady from branches above.
Bat houses mounted on buildings retain heat better and are less accessible to predators. You can put them on a pole though. Luckily my townhouse is three stories high and has a sunny side. It's also near a stream. So I felt I probably had a good site.