Inverter topology specifies the working parts inside the inverter (transformer-based or transformerless). Most past U.S. inverters included an isolation-transformer and were used with grounded PV systems. Recently, transformerless (TL or “non-isolated”) inverters, which require the PV system to be ungrounded, are becoming common in the United States. Benefits of transformerless inverters include higher efficiency, increased ground-fault sensitivity, and cost less to manufacture. However, there are additional National Electrical Code (NEC) requirements for ungrounded systems (see “Ungrounded PV Systems” in HP150).
CEC-rated efficiency is a ratio of output power to input power, as computed by California Energy Commission (CEC) algorithms. Efficiency varies depending on conditions such as ambient air temperature, inverter temperature, and array voltage. An inverter’s efficiency at any moment depends on how much power the inverter is processing. The CEC specification considers how long arrays normally spend at various power levels. Each inverter is tested at several power levels, and at low, medium, and high DC input voltages to calculate an average efficiency value. This weighted efficiency is independently verified and is used by designers for comparing inverters.
NEMA enclosure rating identifies the inverter casing’s level of weather-proofing. NEMA 3R enclosures are intended for either indoor or outdoor mounting and provide some degree of protection from rain, sleet, and snow. NEMA 4X enclosures can also be mounted indoors or outdoors but, with their higher rating, are considered watertight and corrosion-resistant.
MPPT inputs refer to the number of maximum power point trackers (MPPTs) the inverter has. Having multiple MPPT inputs allows parts of the PV array to operate efficiently at different power levels. This is particularly helpful for arrays on multiple roof planes, or when PV strings have different numbers of modules. Multiple MPPT inputs can also be helpful for future array expansion.
Arc-fault protection is a recent PV system requirement, introduced in section 690.11 of the 2011 NEC. It is required for PV arrays operating at 80 VDC or greater installed on or penetrating a building. This is intended to mitigate potential fire hazards from arcing that is more likely with DC circuits.
MPPT voltage window is the voltage range that the inverter accepts to search for the array’s MPP. The wider the window, the more options designers have for using various numbers and types of PV modules (with different output voltages) in their systems. A wide MPPT window also helps an inverter keep the array operating at its maximum under a variety of weather conditions—especially helpful in areas with large seasonal temperature variations. Finally, a lower low-end MPPT point can be advantageous since an array’s voltage will decrease with age.