Apart from line voltage models, all of the HomeKit compatible thermostats run over Wi-Fi. That, combined with a propensity for color touch screens and on-board smarts mean that batteries simply aren’t going to cut it. As such, these all require a c-wire for power. Many models do come with a power adapter to allow you to use existing wires from your HVAC system if you don’t have a dedicated c-wire available, and there are other options you can try to provide one in the worst-case scenario.