Microinverters & AC Modules

While the majority of grid-tied inverters are string inverters, microinverters and “AC modules” (modules prewired with installed microinverters) are available. A few other manufacturers, such as SMA America and Power-One, are expected to introduce microinverters in the near future.

This Enphase M215 is the company’s third-generation product, which includes “single-bolt” mounting to speed installation.

Enecsys recently released its microinverters in the United States.

This AC module is comprised of an AUO Solar module preinstalled with a SolarBridge microinverter.

Exeltech offers its PVAC module.

Microinverters and AC modules are gaining popularity as they offer some benefits over the traditional string inverter. Both can simplify design—for microinverter systems, you simply need to make sure your module is on the compatible module list. DC series string calculations are not required. With both microinverters and AC modules, installation is considered easier and safer by some installers more familiar with standard AC wiring because there is only conventional AC electricity and not high-voltage DC. System output is often improved because the modules operate independently. This is especially helpful when modules have partial shading, differing orientations, or variations in soiling). And it is possible to have an array comprised of different module makes and models. These systems usually offer module-level monitoring, which is a fantastic tool for short-term and long-term performance verification and troubleshooting (see “Potential PV Problems” in HP143). And they make future PV system expansion an easier task (see “Expanding Your PV System” in this issue).

Up-front equipment cost can be higher with microinverter systems, and depends on the system size. A brief Internet search found microinverter pricing at about $0.80 per watt. A 2.5 kW string inverter was priced at $0.70 per watt; a 5 kW string inverter was about $0.55 per watt. These price differences can be manageable in the smaller residential market, due to the added-value items such as module-level monitoring, but with larger systems (greater than 4 kW), the price disparity can be a barrier for microinverter systems.

With more electronics involved (and more things subjected to high temperatures) the potential points of failure increase. If a microinverter or AC module malfunctions, field replacement can be a daunting task, especially for a flush-mounted pitched-roof system, since you’ll likely need to remove several other modules to access the failed unit, sometimes while working on a steep roof. However, if one microinverter or AC module goes down, the output of the other modules in the system isn’t affected. If a string inverter shuts down, the whole array connected to that inverter goes offline.

Whether microinverters save installation time is much debated. They require no combiner box, nor do they have a DC disconnect or a central inverter to deal with, which can save installation time. On the flipside, you’ll be attaching one inverter to your mounting structure for each module (or, for dual microinverters, one for each set of two modules), plus attaching and securing cabling assemblies between each microinverter. AC modules negate the steps of having to mount each inverter and connecting the module DC wiring leads into the inverter.

Debating the merits and drawbacks of microinverters and AC modules versus string inverters will certainly continue as more of these products enter the market. As competition increases, cost should come down and new solutions may be developed to speed installation, such as new cabling assemblies and single-point mounting for microinverters.