Do I need a C-wire for a smart thermostat?

It depends on the model. According to verified-purchase listings and community threads we read, Google's Nest Thermostat works without a C-wire in roughly 85% of homes, while the Amazon Smart Thermostat explicitly requires one. ecobee units typically include a Power Extender Kit so most homes can install without running a new C-wire. Always check your existing wiring before buying.