IoTaWatt measures each circuit using a passive sensor that clips around one of the insulated wires of the circuit to measure the magnetic field created by current passing through the wire. The output of the current-transformer (CT) is low voltage and plugs into one of the device's 14 inputs. From the browser based configuration app, you select the model CT that is connected. IoTaWatt knows how to interpret the signal from the device to produce an accurate measure of the power being used at any moment.