Inverter efficiency is the ratio of the usable AC output power to the sum of the DC input power and any AC input power. Typical grid-tied inverter efficiencies exceed 95% under most operating conditions. Efficiency changes as a function of AC output power, DC voltage, and sometimes inverter temperature. Sandia National Laboratories and BEW have worked together to develop a test protocol to measure inverter efficiency as a function of AC output power and DC voltage. This protocol has been adopted by the California Energy Commission (CEC) and any inverter used in a CEC approved PV system must be tested by an independent lab to this protocol. The CEC lists test results for thousands of inverters on its webpage.