To be truly Smart, a Smart Thermostat needs to monitor the temperature continuously, so they need a constant power supply, to achieve this, you need a common (C) wire. A few Smart Thermostats can be powered by battery; however, there is a cost to replacing these on a fairly regular basis. Mains-powered Smart Thermostats also have a battery for backup in case the power goes out, so you don’t lose all the smartness the thermostat has gained over a prolonged period.