Sometimes straightforward, often not. This question comes up more and more as systems installed under the Feed-in Tariff age and their owners want batteries or more capacity.
The four things that limit you
1. Inverter capacity Your existing inverter was sized for your existing array. Adding panels may exceed what it can handle, meaning a larger inverter or a second one. See inverter capacity.
2. Grid connection Your system was registered with the network operator at a certain capacity. Increasing it means notifying them again, and if the total crosses the 3.68kW per phase threshold it moves from G98 to G99, needing permission first. See grid connection.
3. Roof space and layout The obvious one, and often the binding constraint. Whatever was left over after the first installation is usually the less good roof.
4. Mixing panel ages New panels have different electrical characteristics from ten-year-old ones. Putting them on the same string means the string is limited by the weaker panels. The usual answer is a separate string or panel-level electronics.
The Feed-in Tariff complication
If you are receiving Feed-in Tariff payments, get advice before changing anything.
The FiT closed to new applicants in 31 Mar 2019, but existing accredited households continue to receive payments. Altering an accredited installation can affect that accreditation, and the payments are generally worth considerably more than a current export tariff.
Talk to your FiT licensee before committing to any change. This is genuinely worth a phone call, because the downside is losing an income stream that may have years to run.
Adding a battery to an existing system
The most common request, and usually more practical than adding panels.
Two routes:
Replace the inverter with a hybrid one Cleaner, handles solar and battery together, but you are discarding a working inverter.
Add an AC-coupled battery The battery has its own inverter and sits alongside the existing system. No need to touch the original installation, which is particularly useful if you are on the Feed-in Tariff. Slightly less efficient because of the extra conversion, and it means two inverters.
For an existing system, AC coupling is often the sensible answer. For a new installation, a hybrid inverter from the start is better. See hybrid systems.
Note the connection point again A battery that can export counts towards the same threshold as your panels, so adding one may require a new notification or application.
And the battery will need to comply with the siting rules that have been mandatory since 15 Oct 2026, even though your original system does not.
Is it better to expand or start again?
Depends on the age of what you have.
Expand if the existing system is relatively young, the inverter has life left, and you are adding a modest amount.
Consider replacing if the system is old, the inverter is near the end of its life, the panels are low-wattage by current standards, and the roof could hold considerably more with modern panels. Scaffolding is going up either way, and that is a large part of the cost.
Get both quoted. The answer is not obvious and it depends on numbers specific to your installation.
What to ask
- Can my existing inverter take more, and by how much?
- Does this need a new network notification, or a G99 application?
- Will the new panels go on a separate string?
- Am I on the Feed-in Tariff, and has anyone checked what this does to it?
- Would replacing the whole system make more sense, and what would that cost?
- Does adding a battery mean a hybrid inverter or AC coupling, and why?