How to expand an existing solar system in Perth: adding more panels

A 6.6kW solar system installed in 2018–2020 was designed for a household without an EV or large battery. If your electricity usage has increased — or you're planning to add a battery or EV charger — expanding solar generation capacity may make sense. Here's how expansion works technically and what your options are.

Why people want to expand existing systems

Common reasons:

• Added an EV (home charging adds 10–20kWh/day, often in the evening — but expanding solar and adding a battery to self-consume EV charging during the day makes financial sense)
• Planning to add a battery (larger solar feeds the battery more effectively)
• Electricity consumption has grown (larger family, new appliances, air conditioning upgrade)
• Current system is undersized relative to available roof space
• Want to maximise DEBS peak window exports (3pm–9pm) to cover evening EV charging from the grid less

Option 1: add panels to the existing inverter (if capacity allows)

When this works: Your existing string inverter may have spare capacity. Most modern inverters allow "DC oversizing" — the CEC allows up to 133% panel capacity to inverter AC capacity. For a 5kW inverter, that's up to 6.65kW of panels. If you originally had 5kW of panels on a 5kW inverter, you can add approximately 1.65kW (4–5 additional panels) on a new string or added to existing strings.

Constraints:

• Inverter must have available MPPT (Maximum Power Point Tracker) inputs — check your inverter specification
• String voltage limits must not be exceeded (adding panels to a string increases string voltage)
• Western Power NCN must be amended — your expanded system needs a new NCN lodgement
• Inverter warranty may be affected by modifications — check with manufacturer

Cost: Adding 4–5 panels and new string cabling: approximately $1,200–$2,500 depending on roof access and string configuration.

Inverter compatibility: Not all panels match well with existing panels. Adding different wattage or technology panels to an existing string can cause mismatch losses. An installer should design the expansion to minimise mismatch — often this means running new panels on a separate string/MPPT input.

Option 2: add a second (additional) inverter

When this works: If your existing inverter is fully loaded (6.6kW panels on a 5kW inverter at the 133% CEC limit), adding more panels requires a second inverter.

System design:

• New panels are connected to a second inverter
• Both inverters connect independently to your switchboard's grid connection
• Combined generation from both inverters appears on your smart meter

Western Power export limit: Your total system export capacity is subject to your Western Power approved export limit. If your existing 5kW inverter is approved for 5kW export, and you add a 3kW second inverter, you may need to apply for a 5kW total (shared) export limit — Western Power may not automatically approve the combined 8kW. Discuss this with your installer before the expansion.

Cost: 3–5kW additional system with new inverter: approximately $2,500–$5,000 depending on panel choice and inverter.

Option 3: replace the inverter and expand the array

When this works: If your existing inverter is approaching the end of its warranty life (10 years) or is already failing, replacing it with a larger or hybrid inverter and simultaneously expanding the panel array may be the most cost-effective path.

Hybrid inverter advantage: A new hybrid inverter replaces your existing string inverter and adds battery capability. At the same time, your panel array can be expanded to match. This effectively modernises your entire system: more panels, hybrid inverter ready for battery, and potentially improved monitoring.

What happens to existing panels: Existing panels connect to the new hybrid inverter on one or more MPPT inputs. New panels connect on additional inputs (or in separate strings). Mixing panel ages and models requires careful system design.

Option 4: add a separate system on a different roof section

When this works: You have additional roof area (east, west, or a second building) that wasn't used in the original installation. This is the cleanest approach — fully independent from the existing system.

System design: New panels + new inverter + new grid connection (combined on the existing smart meter). This is essentially installing a new solar system alongside the existing one, avoiding any compatibility or mismatch complications.

Western Power considerations: Combined capacity of both systems is subject to your property's approved export limit. A new NCN is required for the second system.

Panel compatibility and mismatch

Adding different panels to an existing system: New panels are typically different wattage and may be different technology (e.g. TOPCon vs original PERC) from your 2018–2020 system. Mixing dissimilar panels in the same string creates mismatch losses — the string output is limited by the lower-performing panel.

Best practice:

• New panels on a separate string with their own MPPT input
• Alternatively, if the inverter has only one MPPT input, accepting some mismatch loss is unavoidable — a qualified installer can model the impact

Getting quotes for expansion

When getting expansion quotes:

• Specify your existing system: inverter brand, model, and capacity; panel count, wattage, and installation year
• Ask whether your existing inverter can accommodate additional panels and on which inputs
• Request a system design that minimises panel mismatch
• Ask about Western Power export limit implications
• Confirm a new NCN will be lodged

An installer who hasn't looked at your existing system documentation before quoting is giving you a rough estimate — ask for a site visit or at minimum a review of your existing system specs.

The strongest economic case for solar expansion is when you're simultaneously adding a battery or an EV charger. Larger solar feeds the battery more effectively and increases the portion of EV charging powered by self-generated solar.