Questions are often asked about the use and provision of bat boxes.
Tony has offered some words of advice on the subject and there is a pdf leaflet available from the bat Conservation Trust (see below for a download).
The Siting of bat boxes.
It is often stated that they should be in threes on a tree in order to give a selection of aspects for the bats to choose from. Certainly, if one is hoping to get breeding bats in a box, then it should be warmed by the sun. But then there is the possibility that at times it might get too hot. So hence the chance to decamp to a cooler site.
But if you think of Armstrong's Wood, sun is likely to be dappled under the canopy, so this business of three aspects on a tree is a bit arbitrary. It might be more applicable to isolated trees in a hedgerow, but then open-grown trees tend to have a more shady canopy under them, with lower branches anyway!
Of more importance I would think is what sort of habitat you place the box in. People often think that a box placed on the gable end of a house is a suitable alternative to the house for a roost. But it isn't like putting up a box for a Blue Tit, is it? The chances are, if there are buildings around the bats will prefer to roost in the buildings.
And if there is good old mixed deciduous woodland around, there are likely to be plenty of roosting opportunities there. Just think how slow the take-up was at Armstrong's Wood. And there the bats had a choice of 15 boxes for many years. But if your house is surrounded by conifer plantations, then I would expect bats to fall upon any box you put up.
So it really comes down to the reason for putting up a box. If you want to give bats a home, chisel a gap out from the bottom of your weather board/soffit/barge board, and let them get into the loft and the batten gap of your roof. They'd much prefer that to a box.
If you just want to see bats (and of course, if you don't have a licence you can't legally open up a bat box to look inside to see if there is a bat!) it would be preferable to hang up one of my 'beehive' boxes that you can see into from the ground. these also have the advantage, apparently, that they have a quicker take-up than a conventional bird-box-type bat box.
These boxes are basically plywood boxes with sheets of polyurethane (excellent insulation) fixed to the top & sides allowing a narrow gap between the sheets for bats to climb up inside and roost. The polyurethane surface is sufficiently rough for the bats to get a grip. Although it is still important that they are not disturbed too often, it is possible to see up inside and look for occupants.
See below for two files to download, one contains general advice from the Bat Conservation Trust and the other is a description from Tony on how to construct bat boxes using polyurethane foam boarding.