A device for measuring power, or other circuit parameters, of an alternating current load. The device has an ammeter and voltmeter for measuring voltage across the device and current through the device. The device also has a known impedance element that is switchably connected across the load. The ammeter is used to provide a second current measurement with the impedance so connected. These measurements, together with the known impedance value, are use to calculate power, the power factor, impedance, or some other parameter associated with the load.