Module-based microinverters, AC modules, and DC optimizers can help systems make the most of the solar resource by maximizing each PV module’s individual performance. But is the added expense and complexity worth it?
Microinverters, AC modules, and DC optimizers—module-level power electronics (MLPEs)—are gaining in popularity for their ability to squeeze the maximum energy out of a PV system, especially in sites with partial shading. Here’s what you need to know to determine if MLPEs are right for your system and situation.
Microinverters are small, self-contained inverters, ranging from 200 to 400 W AC, that are paired with a PV module to produce grid-tied AC. They mount on the PV module’s frame or the rack where the module is attached. The microinverters’ outputs are wired in parallel by their shared AC power cable.
A PV module must match the microinverter’s input specifications for voltage range and/or number of PV cells in the module (i.e., 60, 72, or 96 cells). Micros have some mounting flexibility, and may be attached to the PV frame or mounting rack.
An AC module is a PV module with a factory-attached microinverter—a close cousin of the microinverter, but with some important differences. AC modules are tested and certified to Underwriters Laboratories (UL) standards as a complete product. They bear three certifications—one for the PV module; one for the inverter; and a third for the pair as a complete product stating the required limitations (like maximum number of AC modules that can be paralleled).
To remain in compliance with the product’s UL certification, failed AC modules must be replaced as a complete unit, regardless of whether the inverter or the PV module is at fault. Field replacement of either item alone invalidates the product’s UL safety certification.
Microinverters that are not sold as part of an AC module are required to incorporate a ground-fault detector and interrupter circuit to turn off the inverter should an unwanted current path develop within the PV module. AC modules are exempt from this requirement. The National Electrical Code (NEC) also differentiates AC modules from microinverters—Section 690.6 of the NEC states: “The requirements of Article 690 pertaining to PV source circuits (the DC side of the PV module) shall not apply to AC modules.” This simplifies and lowers the cost of installation of AC modules compared to microinverters.
Another advantage with AC modules is that because the inverter doesn’t have to be mounted separately, installation time is reduced. Additionally, there’s a single point of warranty contact for both the PV module and the inverter.