Most HVAC thermostats operate using voltage from:

Most HVAC thermostats operate using voltage from the secondary of the control transformer. This is important because the control transformer steps down the high voltage from the main power supply (typically 120 or 240 volts) to a lower voltage, usually around 24 volts AC, which is safe for the thermostat and compatible with the control circuit of the HVAC system.

The use of low voltage is crucial for a reliable, efficient, and safe operation of the thermostat, reducing the risk of electrical hazards. Additionally, this lower voltage allows for the use of small wiring and components, making installation easier and more economical.

While some thermostats may use batteries, this is less common in standard HVAC systems as battery power could lead to maintenance issues or unexpected failures due to dead batteries. High voltage systems are generally not used directly with thermostats due to safety concerns, and the main power supply relates to the input to the control transformer rather than the thermostat itself.