If you already have panels on the roof, the battery decision is really an inverter decision. There are two ways to bolt storage onto a working solar system, and your existing hardware usually picks one for you.
AC-coupled: add a second box
The battery brings its own inverter and sits alongside your existing one. Solar produces AC as it always did; the battery converts back to DC to charge, then to AC again to discharge.
- Pros — works with almost any existing solar system, no need to touch what already works, straightforward to add later.
- Cons — a small round-trip efficiency loss, and two inverters means two warranties and two things to fail.
DC-coupled: one hybrid inverter
Your existing inverter is replaced with a hybrid unit that handles both panels and battery. Solar charges the battery directly as DC, with fewer conversions.
- Pros — slightly better efficiency, one system, one warranty, tidier switchboard.
- Cons — you're throwing away a functioning inverter, and the labour is higher.
The practical rule
If your inverter is under about five years old and still under warranty, AC-coupling almost always wins. If it's approaching end of life — or already failing — replacing it with a hybrid is often cheaper than doing both jobs separately two years apart.
Things that catch people out
- Export limits. Some networks re-assess your connection when you add storage; you may end up with a lower export cap than you have now.
- Backup circuits. Whole-home backup needs a changeover device and usually costs more than the quote's "backup ready" line suggests.
- Space and heat. Batteries have clearance rules and hate west-facing walls. A garage interior is usually the right answer.
- Existing monitoring. Mixing brands often means two apps and no combined view.
Ask any retrofit quote to state explicitly which coupling method it uses, what happens to your current inverter, and what your export limit will be afterwards.