Thermostats can be powered in three ways:
- A “C” wire that comes through the wall from your HVAC system.
( This is optional, some systems do not have this wire. )
- An external power supply.
The advantage of an external power supply if you do not have a “C” wire is the batteries will deplete much more slowly.
There are quite a few external power supply options, and they all follow similar standards.
Searching in Google or Amazon on the following phrase will display several; “24-volt transformer external thermostat”.
More exact specifications: 24VAC, 0.5 to 1 amp transformer.
They are wired as follows: