The Aqara Wall Thermostat W200 C-Wire Adapter is a specialized power accessory designed to bridge the gap for modern smart homes with older HVAC wiring. While many legacy thermostats operate on simple battery power or low-voltage signals, the Aqara W200 requires a constant 24-Volt C-wire connection to support its advanced features, such as the high-resolution 4 in. color touchscreen, integrated mm Wave presence sensing, and its role as a Matter-over-Thread hub. This adapter acts as a retrofit solution for homes that only have 4 wires (typically R, G, Y, and W) at the thermostat wall plate, allowing the device to draw steady power without the expensive and invasive process of running new electrical lines through the walls. The Thermostat C-Wire Adapter (also known as a common wire adapter, power extender kit, or C-wire transformer) provides a continuous source of power to a smart thermostat when your HVAC system doesn't already have a dedicated C-wire (common wire). Color: White.