Since batteries provide reliable and steady power, even when the electricity goes out, many smart thermostat models do use batteries. This can be anything from AA and AAA to lithium-ion options. When choosing a smart thermostat look for one that uses a rechargeable battery or one that has a high milliamp hour (mAh) rating so it won’t need to be replaced as often. Selecting a model that also connects to a C-wire is a good option in case your batteries wear out.