Quick definition
AC coupling means your solar panels and your battery each run through their own inverter, and the two inverters meet on the home's standard AC circuit rather than sharing one DC bus.
It's the setup most installers use when adding a battery to a home that already has solar, since the existing solar inverter doesn't need to be touched.
At a glance: round-trip efficiency loss typically runs 2 to 4% higher than DC coupling, since energy converts DC to AC and back to DC an extra time on its way into the battery.
Why it matters
Most homes that add a battery aren't building a solar system from scratch. They already have a working inverter tied to the grid, and ripping it out to install a single combined solar-plus-battery inverter adds cost and risk for little gain. For the numbers behind AC coupling, the solar inverter size calculator in our Inverters & Equipment section walks through the calculation step by step.
AC coupling solves that. The battery gets its own inverter, wired into the home's electrical panel the same way a generator would be. The two systems talk to each other through software, not through shared DC wiring, so the original solar installation stays exactly as it was.
Say a home has a 7 kW solar inverter installed three years ago. Adding a battery later means power flows from the panels through the solar inverter as AC, into the house, and any surplus routes through the battery inverter to charge the battery, converting back to DC. During an outage, the battery inverter converts stored DC power back to AC to run the home. That's two extra conversion steps compared to a DC-coupled system built as one unit from day one, which is where the efficiency gap comes from; NREL's own cost and performance data puts AC-coupled round-trip efficiency measurably below DC-coupled for exactly this reason.
The tradeoff is simple: AC coupling costs a little efficiency, DC coupling costs more flexibility and usually requires the original inverter to be compatible or replaced. For retrofits, AC coupling almost always wins on total installed cost. The Department of Energy's overview of inverter fundamentals covers why this two-inverter handoff is treated as a standard, code-recognized architecture rather than a workaround.
Where you'll see this
If you're comparing whether to AC couple or DC couple a new battery, the AC coupled vs DC coupled sizing tool walks through both paths side by side using your actual system numbers.
Once you know you're going the AC-coupling route, the battery inverter pairing size calculator helps match a battery inverter's rated output to your existing solar inverter and your home's panel capacity. If you're not yet sure your original inverter has room to keep working alongside a new one, the solar inverter size calculator is worth checking first.
This setup comes up most often in solar-plus-storage retrofits, in homes adding backup power after an outage, and in any quote where the installer proposes a separate battery inverter alongside panels you already own.
Frequently asked
Yes, slightly. Power crosses two separate inverters on its way into the battery, so you lose a bit more to conversion, typically a few percentage points a year compared to a DC-coupled system.
That's the main reason installers choose it. Your existing solar inverter stays untouched, and the battery inverter connects on the AC side as its own independent unit, which keeps the retrofit simpler and avoids voiding your solar inverter's warranty.
Yes, but it needs the right hardware. The battery inverter has to support forming its own grid signal during an outage, since the solar inverter alone will shut off automatically for safety once utility power drops.
Yes. AC coupling depends on the battery having its own dedicated inverter, separate from the solar inverter, so the two can each convert power independently and meet on the home's AC wiring.