Product review (5 stars): Great for powering thermostats if no "C" wire present
This transformer works well for powering thermostats in situations where there is no "C" wire present at the thermostat location.  You can mount it in a single gang electrical box in the wall (should be metal); I made a bracket to hold it securely in the box.  Then run 110 VAC from a nearby outlet box into the thermostat box to the primary side of the transformer; run 24 VAC from the secondary side of the transformer to the thermostat (all wires inside the wall).  Works like a champ.