I'm not familiar with that specific thermostat but if it works like most, the thermostat gets it's power from the outside compressor unit. Otherwise, the thermostat would have to be wired or plugged into the power system.
Shut off the fuses on the outside unit, remove the cover and follow the thermostat cable into the unit. You should find a printed circuit board with a step-down transformer that converts the voltage down to a control voltage. If there is a fuse on the board, that would be it.