I use an ecobee and they pretty much don't work without power (C-wire). At least with them, they provide/include a C-wire adapter for installations that don't have add'l conductors.Right. A “Dumb” thermostat can get by with internal batteries. But if you want a “Smart” thermostat you can control or monitor via wifi, a C wire is typically needed. Nest thermostats suggest they can get by without one but reviews on that claim are very mixed.