Maximum Power Point Tracking (MPPT). All grid-tied inverters employ some kind of MPPT, which optimizes the power output of the PV array. Most inverters will simply follow one MPP for the entire array as it changes throughout the day, but some models can follow two series-string MPPs for a slightly better energy harvest for arrays with multiple orientations. Microinverters, AC modules, and DC optimizers all offer the ability to track individual module MPPs (discussed later).
The inverter’s AC output voltage must match the building or service voltage where it is interconnected. Most U.S. residences are single-phase 120/240 VAC, while commercial buildings may have three-phase service. Many inverters are field-configurable for different output voltages, but, for a few, this must be specified up-front.
Other Options. An inverter may include, or have an option for, integrated disconnects (DC and/or AC), series fusing, data monitoring, remote displays, and a Web interface.
Not all integrated disconnects satisfy National Electrical Code requirements for PV array disconnecting (DC) or PV system disconnects (AC). But having a disconnect box that is distinct from the inverter allows the power to be shut off and the inverter removed without exposing energized parts or allowing water infiltration if the inverter needs to be replaced or serviced. If the inverter does not include separable NEC-approved disconnects, external ones may need to be installed.
Data monitoring options may be as simple as logging inverter input and output voltage, current, power, and energy; or as complex as reading irradiance levels, array and ambient temperatures, monitoring household usage, and tracking individual module performance. Some inverters offer remote displays, and some offer Web-based monitoring. Being able to check your PV system’s performance helps identify problems and minimize lost PV production.
Microinverters. Small grid-tied inverters that are paired with each module in the system—aka microinverters—are one of the newest inverter technologies available. Mounted on the rack underneath the module, or integrated into an “AC module,” the power conversion takes place right at the PV module so there is no need for high-voltage DC wire runs.
Microinverters provide MPPT for individual modules, which not only increases energy harvest but also keeps shading on one module from affecting an entire string of modules. Since there is no high-voltage DC, and the AC output is automatically stopped with removal of grid AC, safety for firefighters is increased. Microinverters track individual module performance, so module-level data monitoring is available, and can help pinpoint modules that are not performing to specifications for warranty replacement.
While microinverters are currently more expensive than string inverters on a purely dollar-per-DC-watt basis, there are other factors to consider. There is slightly more labor involved in mounting a microinverter underneath every PV module (except in the case of AC modules), but there will be less wiring, labor, and wall space needed, since there are no large inverters or DC disconnects to install. On very small PV systems (less than 1.5 kW) it’s often more cost-effective to use microinverters—with a string inverter, you’ll end up paying for unneeded power capability, since most are not available in smaller sizes. Even though most microinverters carry a 25-year warranty, replacing a failed unit may be difficult and costly, especially on large roof-mounted arrays.
Transformerless Inverters. The NEC now allows these, and more manufacturers are making them available in the U.S. market. The main difference between transformer-based and transformerless inverters is that the latter do not use a DC system ground, although the system still has full equipment grounding (see “Ungrounded PV Systems” in HP150). Tranformerless inverters can be more efficient than their transformer-based counterparts (up to about 2% for models available in the United States) and include enhanced ground-fault protection. The inverters weigh less, making installation easier, and they tend to cost less than their transformer-based counterparts.