Sizing, Worked Examples and Quote Checklists
Adding a battery to existing solar panels: inverter checks, AC vs DC coupling and the federal rules
How to add a home battery to an existing rooftop solar system in Australia — VPP-capable inverter rule, AC- vs DC-coupled options, export limits, old systems and quote questions.
You can add a battery to an existing rooftop solar system and still get the federal Cheaper Home Batteries discount, provided the solar system is no more than 100 kW and meets state electrical safety rules. The key check is the inverter the battery will connect to: under the Clean Energy Regulator's rules it must be VPP-capable and CEC-listed (or have been listed when your solar was commissioned). If it is not, it has to be replaced with a listed, VPP-capable inverter, which adds cost. In practice you choose between an AC-coupled battery that has its own inverter and leaves your solar inverter alone, and a hybrid inverter that replaces your solar inverter and manages panels and battery together. The right choice depends mostly on the age and type of your existing inverter.
For sizing, start with the battery sizing guide. For the eligibility rules in full, see eligibility: approved list and VPP-ready.
What the federal rules say about existing solar
From the CER's solar batteries page (checked 27 October 2026):
| Rule | Detail |
|---|---|
| Solar size | Battery can be installed with a new or existing PV system of no more than 100 kW |
| Existing panels | Must meet state and territory electrical safety rules; do not have to be on the current CEC list |
| Inverter the battery connects to | Must be VPP-capable, and CEC-listed now or when your solar was commissioned |
| Other inverters on site | Do not need to be VPP-capable |
| If the inverter fails the test | Replace it with a CEC-listed, VPP-capable inverter |
| Stackable batteries | Only newly added modules are claimed; the whole system is re-certified |
AC-coupled or hybrid: how they differ
| Feature | AC-coupled battery | Hybrid inverter replacing old inverter |
|---|---|---|
| What changes | Battery with its own inverter connects to the switchboard | Old solar inverter removed; hybrid manages panels and battery |
| Existing solar inverter | Stays in place | Replaced |
| Losses | Solar energy is converted to AC then back to DC to charge, so slightly more conversion losses | Solar can charge the battery on the DC side, often with fewer conversions |
| Cost tendency | No inverter replacement, but battery unit includes an inverter | Hybrid inverter cost added |
| Best when | Solar inverter is fairly new, reliable and the battery won't connect to it | Solar inverter is old, faulty or not VPP-capable |
| Watch out for | Two devices to maintain and monitor | Panel string voltage and current must suit the new inverter |
An AC-coupled battery usually connects to the AC switchboard rather than to the existing solar inverter, which can matter for the VPP-capable rule. Ask the installer to state in writing which inverter the battery connects to and that it meets the rule.
Questions to check about your existing system
- Age and condition of the inverter. Inverters generally have shorter lives than panels. If yours is old, a hybrid replacement may make sense now rather than paying for a new inverter separately later.
- Inverter model. Find it on the label or in the installation paperwork and check whether it is VPP-capable and listed.
- Panel layout and strings. For a hybrid, the installer must check the strings suit the new inverter's input range.
- Export limit. Your network may limit exports. If your system is regularly curtailed, a battery captures energy that is currently wasted.
- Phase supply. Single- or three-phase supply affects which inverter and battery combinations work.
- Switchboard space and condition. Older boards may need an upgrade.
- Feed-in tariff arrangements. If you are on a legacy premium feed-in tariff, check whether changing the inverter or adding a battery affects eligibility with your retailer before going ahead. Most legacy schemes have now ended, but check if you still receive one.
Illustrative cost comparison
The following figures are illustrative only, for a 13.5 kWh usable battery installed in late 2026 on a home with a 6.6 kW system.
| Scenario | Gross price | Federal discount (91 STCs × $36) | Net price |
|---|---|---|---|
| A. AC-coupled, inverter stays | $13,500 | −$3,276 | $10,224 |
| B. Hybrid inverter replaces 10-year-old inverter | $15,200 | −$3,276 | $11,924 |
| C. AC-coupled now, old inverter fails in 2 years and is replaced for an assumed $2,000 | $15,500 | −$3,276 | $12,224 |
Scenario B costs more upfront than A, but if your existing inverter is near the end of its life, scenario C shows the risk of paying for both. Ask installers to quote both options when your inverter is older than about 8 to 10 years, and compare against your actual inverter's warranty and condition.
Export limits and curtailment
When a battery is added, the installer may need to update your network connection agreement. Some distributors apply export limits or dynamic export controls to new or altered systems. This can be a hidden cost or a hidden benefit:
- Cost: a lower export limit reduces income from surplus solar.
- Benefit: a battery stores energy that would otherwise be curtailed.
Ask the installer what approval they will lodge, whether any export limit will change and whether the fee is included. The CER requires retailers to give written information about feed-in tariffs and export limits before installation.
Getting the quotes right
| Quote item | What to look for |
|---|---|
| Inverter the battery connects to | Model named, VPP-capable, listed |
| Coupling | AC-coupled or hybrid, stated |
| Existing system check | Written note of panel and inverter condition |
| Switchboard work | Included or priced separately |
| Network application | Who lodges, cost, any export limit change |
| Warranties | Battery, new inverter (if any), and workmanship |
| Monitoring | Whether both inverters show in one app |
Use the quote comparison guide and the installed cost estimator to test each option.
FAQ
Can I add a battery to a very old solar system?
Yes, as long as the system meets state electrical safety rules and is no more than 100 kW. If the battery connects to the old inverter and it is not VPP-capable, the inverter must be replaced.
Will adding a battery void my solar panel warranty?
It should not, if an accredited installer does the work correctly. Ask the installer and check your panel and inverter warranty terms; keep all compliance paperwork.
Can a different installer add the battery?
Yes. Any SAA-accredited installer with battery endorsement can do the work. Your original installer's workmanship warranty may not cover changes made by someone else.
Sources
- Clean Energy Regulator — solar batteries rules for existing systems and inverters
- Clean Energy Council — approved battery and inverter lists
- Solar Accreditation Australia — installer accreditation lookup
- Energy Made Easy — compare plans after adding a battery
