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.
The CEC Inverter Test Protocol describes a standard way to measure the output characteristics of a photovoltaic inverter. The results of the tests are summarized for every inverter in the CEC database on their website.
An example of a CEC test result is illustrated in the following figure.
The inverter efficiency is measured at six power levels (10%, 20%, 30%, 50%, 75%, and 100% of rated output AC power) and at three DC voltage levels (Vmin, Vnom, and Vmax) for a total of 18 measurements.
For the purpose of rating and comparing inverter efficiencies, the protocol suggests a weighted average efficiency, which is calculated as a weighted sum of these 18 values. The weighting factors are listed in the table below by AC power level. The weights do not change as a function of DC voltage.
| Inverter AC Power Level | Weighting Factor |
| 10% | 0.04 |
| 20% | 0.05 |
| 30% | 0.12 |
| 50% | 0.21 |
| 75% | 0.53 |
| 100% | 0.05 |
