Adding Solar Panels to an Existing System: What You Need to Know

When your solar was first installed, it probably matched your household's energy use at the time. But life changes. Maybe you've bought an EV, installed a heat pump hot water system, started working from home, or just added a few more people to the household. Suddenly your 5kW system that used to cover everything is falling short during the day, and your grid imports are creeping back up.

Panel expansion usually comes up when power use has increased, daytime solar isn't covering household demand, a battery is being considered (and you want enough generation to charge it), or new loads like EV charging or pool heating have been added. In most of these cases, the existing panels are performing fine. There just aren't enough of them any more.

This is the question that comes up every time. The short answer: not necessarily, but compatibility still matters.

You don't need the same brand. You don't even need the same wattage. What you do need is for the new panels to work well with your existing system electrically. That means paying attention to voltage and current ratings, how panels are grouped into strings, and whether your inverter can handle the new configuration.

If new panels are wired on the same string as old ones, mismatched specs can drag performance down. The entire string operates at the level of its weakest panel. This is why good installers often run new panels on a separate string or a different MPPT (maximum power point tracker) input on the inverter. That way, old and new panels each operate at their own optimal point without holding each other back.

Your inverter converts the DC electricity from panels into AC power your home can use. It has a maximum input capacity, and in many Australian homes, the inverter is already running close to its limit. Adding more panels to an inverter that's full is like adding more taps to a pipe that's already at capacity. The extra water has nowhere to go.

Before expanding a system, an installer needs to check the inverter's maximum input capacity, how many string inputs (or MPPTs) it has available, whether it's already operating near its limit, and any export limits imposed by your local distributor (DNSP).

Modern panels are significantly higher wattage than what was available even five years ago. A panel from 2019 might be 330W. Today's equivalent is 440W or more. It's tempting to think that swapping to bigger panels or adding high-wattage ones will automatically boost performance. But it's not that straightforward.

Higher-wattage panels won't help much if your inverter can't accept the extra input, the excess power is just exported at a low feed-in tariff, or your household usage doesn't align with when the panels are generating. Panel upgrades work best when they're matched to how your home actually uses power. Adding 3kW of extra capacity that just gets exported at 5c/kWh during the middle of the day isn't a great investment.

This is a question that comes up constantly, and the answer genuinely depends on your situation.

Most of the time, existing panels are fine and you're just adding to them. But there are situations where a partial or full panel replacement is the better path.

Replacement tends to make sense when panels are physically damaged or significantly degraded, the original system used very low-wattage panels (under 250W), roof layout changes require relocating panels, or you're doing a full system redesign with a new inverter and battery. For a 10-year-old system with 250W panels and an undersized inverter, sometimes starting fresh with modern equipment is more cost-effective than trying to bolt new panels onto aging infrastructure. A good installer will tell you honestly which approach delivers better value.

Some system expansions, including adding panels or upgrading inverters, may be eligible for Small-scale Technology Certificates (STCs) under the federal renewable energy scheme. The value of STCs fluctuates, but they typically reduce upfront costs by a meaningful amount.

State-level incentives vary. Some states offer additional rebates for upgrades to existing systems, while others focus their incentives on new installations. Eligibility depends on where you live, the size of the expansion, and how your system is configured.

Because the rules differ between states and change regularly, it's worth checking what applies to your specific situation before making any decisions. You can use our rebate eligibility tool to get a clearer picture of what upgrade incentives may apply to your home.

Adding panels is not about filling every spare inch of roof. It's about understanding what your inverter can handle, matching generation to your actual usage patterns, supporting future upgrades like batteries or EV charging, and avoiding wasted export that earns you next to nothing.

The best starting point is your monitoring data. Look at when you're importing from the grid, how much you're exporting, and whether your current panels are maxing out the inverter during peak hours. That data tells you whether more panels, a battery, an inverter upgrade, or some combination of all three is the right move.

A system review from a qualified installer should cover all of this before anyone starts quoting panel prices. If someone offers you a quote without checking your inverter specs and export limits first, find a different installer.