The efficiency at which an inverter can convert DC to AC receives a lot of attention. All inverter manufacturers proudly report the peak efficiency values on their specification sheets—the ability of these machines to operate at efficiency values greater than 95% is impressive. Although knowing the peak efficiency number is helpful, it doesn’t reflect typical operation with varying PV output and utility voltages. Because of this, the California Energy Commission (CEC) requires inverter manufacturers to also provide “weighted” efficiency values, which are just as helpful to installers and end users in other states. Independent testing agencies follow the CEC protocol to test inverters, varying the DC input power and voltage values. The CEC publishes these values, with links on their Web site to the testing data.
Batteryless inverters have higher efficiencies than their battery-based counterparts. But inverter efficiency only tells part of the story. Overall system efficiency should be evaluated and considered for true comparison between the system types. In addition to lower inverter efficiencies, battery-based inverters operate at lower voltages (potentially higher voltage-drop), battery charging losses, and losses associated with the charge controller and inverter operating separately. A batteryless system isn’t without its inefficiencies; battery-based systems just have more that add up.