AC modules and microinverters share many characteristics, but they are different from both an National Electrical Code (NEC) and an installation perspective. Microinverters are field-installed, one inverter per module, while AC modules have factory-integrated inverters and have one warranty for the complete assembly. AC modules are listed to both UL standard 1741 (for inverters) and UL 1703 (for PV modules). AC modules generally have a standard 25-year performance warranty, like PV modules, while microinverter warranties vary from 10 to 25 years.
There are only a few microinverters currently available in the United States. Those include Enecsys’ Gen 240/300-60-MP, in 240 and 300 W AC output power versions, that can be paired with 60-cell modules; Enphase Energy’s M215 and M250 models, producing 215 W or 250 W AC output power, which are also compatible with 60-cell modules; PowerOne’s Aurora microinverter (250 W or 300 W versions, which accommodate 60-, 72-, and 96-cell modules); and SMA America’s Sunny Boy 240-US (used with 60-cell modules; see “Gear” in this issue).
AC modules are available from Canadian Solar, ET Solar, Mage Solar, SunPower, Talesun, and Westinghouse Solar. The microinverter that is part of an AC module is usually made by a third party (like SolarBridge Technologies) but is typically sold under the module manufacturer’s label. For information about AC modules and the NEC, see “Code Corner” in this issue.
Grid-tied string inverters are more efficient than ever. Go Solar California’s eligible equipment list (gosolarcalifornia.org) includes more than 30 models with a weighted efficiency value of 98% or greater. Inverters operate more or less efficiently at different power output levels, so weighted efficiency is an average across a range of power, with estimated percentage of time spent in a given power range factored into the equation. Model specification sheets list “peak efficiency” as a higher value than weighted efficiency, as peak efficiency is the highest efficiency an inverter can reach, regardless of power output.
One technological development that helps increase efficiencies is the move to transformerless inverters (also called nonisolated inverters). Instead of relying on an iron-core or high-efficiency transformer, transformerless inverters convert DC to AC through rapid electronic switching, with no isolation between the DC and AC conductors. These inverters are not only more efficient, but weigh less and feature superior ground-fault protection that detects faults more reliably and at lower current levels. Some can be enabled to check the insulation resistance of DC conductors each day before beginning operation—helping reduce the potential of fires due to undetected ground faults. Some, including SMA America’s transformerless inverters, include DC arc-fault protection, as required in the 2011 NEC.
Many inverter manufacturers are offering these inverters, often distinguished by “TL” in the model number. Although neither current-carrying conductor on the DC side is bonded to ground (there’s no DC system ground or DC grounding electrode conductor necessary), they still must have all of the equipment grounding required of any PV array (see “Ungrounded PV Systems” in HP150). The fourth-generation Enphase M250 is an ungrounded microinverter and does not require a DC-grounding electrode conductor.
SMA America’s new Secure Power Supply (SPS) feature is available on SB 3000, 4000, and 5000 TL-US inverters. In the past, capturing power from a utility-interactive array during a power outage was only possible if the system had a battery bank and inverter with stand-alone capabilities. While SMA’s SPS-enabled inverters are primarily utility-interactive, each includes a manually switched outlet that allows stand-alone daytime power during utility outages. When the sun is shining at a high-enough irradiance level, the inverter can provide up to 1,500 watts through the outlet. The power supply can be used for small appliances or to charge cellphones or computers, but there is no energy storage for use at night or during cloudy weather. The amount of power available will fluctuate based on the sun’s intensity on the array.