Q&A: Enphase restriction?

Q

I am planning a rooftop PV system, using LG panels and Enphase microinverters. An experienced PV designer told me he thinks the use of Enphase microinverters will preclude the future addition of any battery other than one from Enphase.
I have never heard this before. Is there likely to be any validity in this opinion?
—Rod Sloggett

A

I think there is some validity in that opinion, but it’s overstated.

With microinverters installed on the panels, the cables coming from the roof carry electricity already converted from DC to 230 V AC current. To store this energy in a battery, it needs to be converted back to DC again. This is achieved with ‘AC coupling’, where the battery has its own inverter, connected to your home’s electrical switchboard. Many solar batteries are designed to work this way, but some are intended for ‘DC coupling’ instead. A microinverter solar system can’t use DC coupling, so your battery choices are more limited than if you selected a system with a central inverter.

With a microinverter solar system you should be able to add any AC-coupled battery (including the Enphase battery) and store excess solar for the evening. The battery has a sensor in the switchboard to detect when you’re exporting to the grid, and thus when to charge up.

Things get trickier in a grid blackout. The battery can power the house, if the system is designed and set up to do so. But since it has no communication with the microinverters, the battery may have trouble controlling the panels’ generation to prevent being over-charged. During a daytime blackout, your solar system will probably shut down immediately. If it does keep running, at some point the battery may force it to shut down by increasing your home’s electrical frequency. This process is not graceful and may cause some premature wear on the microinverters and/or affect their warranty. If you select an Enphase battery, this issue doesn’t arise because it’s not designed to operate in a blackout anyway.

Here’s an ATA article that gives more info on AC coupling, DC coupling etc: www.positivecharge.com.au/news/2017/01/just-add-batteries
—Andrew Reddaway