If a customer already owns a solar array and now wants storage, the panels and the roof do not change — the question is how the battery connects to what is already on the wall. There are two established answers. AC coupling keeps the existing PV inverter and adds a separate battery inverter on the AC side. DC coupling replaces the PV inverter with a hybrid unit and puts the battery on the same DC bus as the array. Which one is correct depends on the age and type of the inverter already installed, what "backup" actually has to cover, and how much of the existing array you are prepared to re-string.
Here is the framework we use when installers ask us this before quoting a retrofit.
The two topologies in one minute
Both topologies end with the same thing: a battery that charges from solar and discharges into the house. They differ in where the conversion happens and how many inverters are on the wall.
| AC coupling | DC coupling | |
|---|---|---|
| Inverters on site | Existing PV inverter + battery inverter | One hybrid inverter |
| Where the battery sits | AC bus | DC bus, shared with the array |
| Existing PV inverter | Stays in service | Usually retired |
| Typical retrofit trigger | Healthy inverter, keep it | Aging inverter, or new build |
| Monitoring | Two systems to reconcile | One system |
| Array re-stringing | Not required | Usually required |
AC coupling: keep the PV inverter, add a battery
AC coupling is the least invasive route. The array, its strings and the existing grid-tie inverter stay exactly as they are. A second inverter — the battery inverter — is added on the AC side, where it charges the battery from AC and discharges back into the house.
The critical requirement is that the added inverter can form the grid. A standard grid-tie inverter shuts down when the grid disappears because it has no AC reference to follow. OutBack Power's AC-coupling application note describes exactly this mechanism: the grid-tie inverter is designed to convert PV only while the utility grid is active, and adding a battery-based inverter is what allows it to remain active during a grid outage (source). In other words, AC coupling only delivers backup if the battery inverter is a grid-forming unit that can create an intentional island.
Purpose-built hardware exists for this. GoodWe, for example, describes its SBP G2 series as an AC-coupled retrofit solution that upgrades an existing single- or three-phase on-grid PV system into a storage system by adding a battery, and states compatibility with low-voltage batteries in the 40–60 V range (source).
Watch-outs we flag to installers:
- Double conversion losses. Solar is converted DC→AC by the PV inverter, then AC→DC to charge the battery, then DC→AC again on discharge. DC coupling avoids one of those conversions.
- Two control authorities. Export limiting and zero-export rules have to be respected by both devices working together, not by one.
- Two monitoring portals. Unless both devices speak the same platform, the customer ends up with two apps.
- Island balance. In backup mode the island is formed by the battery inverter, so the PV running in that island has to stay within what the battery inverter can absorb and control. Size this deliberately; do not assume the whole array can run off-grid.
DC coupling: one hybrid inverter, battery on the DC bus
DC coupling puts the array and the battery on the same DC bus behind a single hybrid inverter. For a new installation this is almost always the cleaner design, and Sol-Ark's AC-coupling application note is explicit that DC coupling is generally recommended, citing production-control advantages and tighter grid-export control (source).
For a retrofit, DC coupling means retiring the existing PV inverter and re-terminating the array into the hybrid. That is more work, and it is only possible if the array's string configuration fits the hybrid's MPPT window. It is usually the right call when:
- the existing inverter is at or near end of life, so it would have to be replaced anyway;
- the customer wants one monitoring platform and one warranty relationship;
- the site needs tighter control over export and production.
What to verify before committing:
- String voltage and current against the hybrid's MPPT window, evaluated at the site's record low temperature.
- MPPT count — the array has to be re-grouped to fit the inputs available.
- Whether the array is DC-reworkable at all. Microinverter systems and arrays with proprietary per-panel electronics generally cannot be re-terminated into a hybrid without replacing that electronics too.
Which one fits the project?
| Question | Points to AC coupling | Points to DC coupling |
|---|---|---|
| How old is the existing inverter? | New or mid-life, in warranty | Near or past end of life |
| Does the customer expect whole-home backup? | Yes, with a grid-forming battery inverter specified | Yes, with tighter production control |
| How much array re-work is acceptable? | None | Re-stringing is fine |
| How many monitoring platforms will the customer accept? | Two is acceptable | One only |
| What kind of array is it? | Microinverters or proprietary electronics | Conventional string array |
| What is the budget shape? | Lower labour, more components | Higher labour, fewer components |
Five checks before you quote
- Age, model and warranty of the existing inverter. This single fact usually decides the topology. If it is healthy and covered, AC coupling protects that investment.
- The array's electrical window. Only relevant for DC coupling, but it is the hard constraint — measure it against the hybrid's input range rather than assuming.
- What "backup" means here. A backed-up loads panel and whole-home backup are different scopes, and they change the inverter rating you need.
- Battery bus voltage against the inverter's supported range. Low-voltage 48 / 51.2 V packs and high-voltage stacks are not interchangeable. Our hybrid inverter range is explicit about this: Huawei SUN2000 pairs with high-voltage battery stacks, while Deye SG-series units support low-voltage LiFePO4 and off-grid operation.
- Certification and grid-connection rules for the destination market. Certification applies to the specific SKU and the specific market, not to a product family name. Confirm the certification pack against the project country and the chosen inverter brand before order release — we state the same caveat on our own product pages rather than letting a series name imply a listing.
Matching the battery to the topology you chose
The battery is downstream of this decision, not upstream of it.
- Low-voltage AC-coupled retrofits and low-voltage hybrids want a 48 / 51.2 V pack. The Storage Wall series covers 5.12, 10.24 and 14.34 kWh units on that architecture, with an IP65 outdoor variant for sites where the garage is already full.
- High-voltage hybrids want stacked or rack modules. The stacked series is offered in low-voltage and high-voltage (up to 1000 VDC) bus versions, so the same module family can follow either path.
If the format is still open — wall, stacked, rack or floor cabinet — decide that on the site, not on the datasheet. We set out that decision separately in how to match battery format to the install site, and the capacity calculation itself in how to size a home battery.
The steps that stay the same
Whatever the topology, three things do not change: the inverter's BMS communication protocol has to be confirmed against the battery before you quote (the questions to ask), the site itself has to be prepared for weight, ventilation and cable routing, and the customer's expectation of what runs during an outage has to be written down and agreed in advance.
If you are quoting a retrofit and want a second opinion on the topology, send us the inverter already on the wall, the array's string layout and what the customer expects to keep running during an outage. We will tell you which coupling we would use and which of our battery formats fits it — get in touch and we reply within 24 hours.
Frequently asked questions
Can you add a battery to an existing solar system?
Usually yes. If the existing array and inverter are in good condition, AC coupling lets you add storage without touching the array. If the inverter is near end of life or the array needs re-work anyway, DC coupling with a hybrid inverter is the cleaner long-term design.
Is AC coupling or DC coupling better?
Neither is universally better. DC coupling is generally recommended for new systems, because everything sits behind one inverter with one control authority. AC coupling is faster and less invasive to add when the existing PV inverter is healthy and staying in service.
Does an AC-coupled battery provide backup during a grid outage?
Only if the added battery inverter is a grid-forming unit. A standard grid-tie PV inverter shuts down without an AC reference, so the battery inverter has to create the island that the PV inverter can then follow.
Do I have to replace my existing solar inverter to add a battery?
Not for AC coupling — that is the point of it. You do for DC coupling, because the hybrid inverter takes over the array's DC input.