Adding Panels to an Existing Solar System: $500 to $659 per kW, the Three Real Options and the STC Rule (October 2026)

What adding panels to an existing system costs in October 2026

Adding panels costs from about $500 per added kilowatt when the existing inverter has room, to about $659 per kilowatt when a new 5 kW inverter is part of the job, at trade prices for the hardware, one crew day at award rates, GST, and the STC discount at $39 per certificate on the added kilowatts only. Those are component floors we build below, not quotes. The alternative, a second system priced as a new one, is $6,774 for 6.6 kW on the national standard offer, 1.03 per watt.

Most requests to add panels do not end with panels added to the old string. The inverter is usually full, the old panels are no longer made, and the installer who touches the array takes on its compliance. The three options that actually get quoted are a separate second system, a new inverter with the extra panels, or microinverter panels AC-coupled beside the old array. Before any of them, pull a month of inverter monitoring: if the system already exports most of what it makes, a battery is the better buy, and the last section works that comparison.

Why your old string usually cannot take more panels

The inverter is full. Under the 133 per cent design rule the array can be at most 1.33 times the inverter rating, so a 5 kW inverter carries 6.65 kW of panels. If it already has 6.6 kW, the headroom is 0.05 kW, which is no panel at all; if it has 5 kW, the headroom is 1.65 kW, three panels at 440 W. A 3 kW inverter with 3 kW of panels can take 0.99 kW, which is 2 panels. The inverter's own datasheet limit is usually higher (a common 5 kW unit accepts 7.5 kWp, a ratio of 1.5), but the certificate claim follows the design rule, not the datasheet.

The panels do not match. Panels on one string run at the current of the weakest panel and add their voltages. A 2015 panel and a 2026 panel differ in current, voltage, size and frame, so mixing them on one string pulls the new panels down to the old ones and may push the string voltage past the inverter's input window. New panels need their own string on a spare input, and a two-input inverter that already has two strings has none. Old models are discontinued, so like-for-like extension is rarely possible, and a second brand on the roof can affect how the panel maker treats a warranty claim.

The compliance passes to whoever touches it. An installer who alters an array must certify the whole installation against the current wiring and PV standards, and an array from before the current isolator, cabling and earthing rules usually fails that check without rework. That is why many installers decline the job and quote a replacement instead. Replacement is the right call where panels are damaged, degraded or under about 250 W each, because the roof space is worth more with 440 W modules on it.

Option 1: a second, separate system

A second system is a new inverter and a new array on their own circuit, priced as a new install: $4,130 to $4,890 for 3 kW, $5,550 to $6,630 for 5 kW and $6,774 to $8,084 for 6.6 kW on the national standard and premium offers, after STCs. It earns full certificates on its own capacity, leaves the old system untouched, and is the option most installers will quote.

The limit is the distributor's cap on total inverter capacity at the meter, which counts both inverters. 10 distributors carry a 10 kVA single-phase cap in our data (Essential Energy, Jemena, AusNet Services, Energex*, Ergon Energy*, SA Power Networks, Horizon Power, TasNetworks, Evoenergy, Power and Water Corporation*; the asterisk marks a figure taken from an industry summary because the distributor's site blocked us on 27 October 2026). An existing 5 kW inverter plus a new 5 kW inverter sits exactly on that cap, and 5 kW plus 6.6 kW of new panels on a second 5 kW inverter still fits because the cap is on inverters, not panels. Ausgrid, United Energy and Western Power publish higher single-phase caps. AusNet sizes the connection on the combined rating of every solar and battery inverter on site, and most of the other published caps are also written as aggregate capacity at the connection point.

Option 2: replace the inverter and add panels

Replacing a 5 kW inverter with a larger one lifts the headroom: a new 8 kW unit carries 10.64 kW of panels and a 10 kW unit 13.30 kW. The new panels go on their own string, the old array stays on the other input, and the whole system is re-certified. A budget 5 kW single-phase string inverter lists at $1,090 ex GST and an 8 to 10 kW three-phase unit at $2,129 to $4,124 ex GST; a single-phase 8 or 10 kW unit needs the distributor to accept it, which the caps above mostly do, and an export limiter at $200 to $500 where the export cap is 5 kW.

The certificates follow only the new panels. The regulator's own wording for an upgrade with a new inverter is that "the existing panels are not included in the new claim for STCs" and that "installers must confirm and re-certify the entire system"; the STC section below works the counts.

Option 3: microinverter or AC-coupled panels

Panels with a microinverter each, or a small second string inverter, connect on the AC side and never touch the old DC array. They suit a shaded or odd roof face and a system whose inverter has no spare input. The cost is the premium: microinverter systems are listed at $2,800 to $5,500 supplied and installed inside a new residential system against $1,300 to $2,600 for a budget string inverter, and the sourced shading premium is $1,500 to $2,000 per system. No Australian price list we could read on 27 October 2026 publishes a per-unit microinverter price, so we do not print one.

Added microinverter capacity still counts toward the distributor's inverter cap, still needs a connection application, and still earns certificates on the new panels only. The old inverter's export limit, if one is set, has to be reset to cover the combined output or the new panels will be curtailed first.

Cost per added kilowatt from the components

The arithmetic uses the lowest trade prices in our data: panels at 0.37 dollars per watt ex GST (a 515 W budget module; the budget tier runs to 0.45), tin-roof racking at $76 per kW, one crew day at award minimum rates of $478 for an electrician and an installer, a new inverter where the scenario needs one, 10 per cent GST on the lot, less the Zone 3 certificates on the added kilowatts at $39 each. Two kilowatts on an inverter with room comes to $1,000, or $500 per kW. Four kilowatts with a new 5 kW inverter is $2,634, or $659 per kW; with a three-phase inverter it is $3,777. Adding 6.6 kW with a new inverter is $3,208, $486 per kW, against $6,774 for a complete 6.6 kW system on the index, which shows how much of a new-system price is margin, overheads, balance of system and warranty provision that this floor leaves out.

In Zone 4 the same 4 kW scenario earns 23 certificates, $897, and the net floor is $2,790. The floor excludes cable, isolators, a switchboard change, scaffold for a second storey, the connection application and any margin, so a fair quote is above it; a quote below it is not fitting what it names. Check any quote in hand against it with the quote check.

STCs on an added system: the regulator's rule

New panels earn certificates only on the capacity they add. The Clean Energy Regulator page "Small-scale renewable energy systems" (last updated 18 September 2026) sets out the cases. Adding capacity on the existing inverter is eligible where "the total system capacity after the upgrade has a rating of no more than 100 kW" and "the inverter has sufficient capacity", claimed as an "extension system". Adding capacity with a new inverter is eligible on the new panels, and "the existing panels are not included in the new claim for STCs". A separate system is eligible where "the combined capacity of the systems has a rating of no more than 100 kW". Replacing one, some or all panels on an inverter that has already claimed is "not eligible for STCs". Confirm the case that matches your job on that page before the installer lodges the claim, and do not add panels before the original claim is approved.

So 4 kW added in Zone 3 earns 27 certificates, $1,053, against 45 certificates and $1,755 for 6.6 kW; the rounding down is on the added kilowatts alone, which is why two small additions earn fewer certificates than one larger one. A quote that shows certificates on the whole system, old panels included, is claiming what the rule excludes.

Network re-approval and the export limit

The original connection approval was for the original inverter. Adding an inverter, replacing it with a larger one or adding microinverters is a new application to the distributor, and the pre-approval must come before the work on every network that publishes a process. Evoenergy charges $299 for a residential application that adds capacity to an existing system; Ausgrid charges $24 up to 10 kW and $138 from 10 to 30 kW on single phase, and notes that site alteration charges for an upgrade appear on the retailer's bill; Western Power and Horizon Power publish no fee for a basic application. The table shows how much inverter each network allows beside an existing 5 kW unit on single phase; rows marked with an asterisk (Energex, Ergon Energy, Power and Water Corporation) come from an industry summary because the distributor's site blocked us on 27 October 2026.

The export cap does not grow with the array. On a 5 kW single-phase cap, added panels only pay when the home uses their output in daylight; SA Power Networks' fixed figure is 1.5 kW and TasNetworks' is 10 kW. Where flexible export is offered the cap lifts on most days, and a battery that charges from the added panels removes the question. Your city's figures are on its page, for example Sydney and Melbourne.

What an added kilowatt is worth, by capital

One added kilowatt makes 1,424 kWh a year in Sydney. Exported at the New South Wales feed-in rate of 4 cents it is worth $57 a year; used at home at 34 cents it is worth $484. Against the $659 per kW floor for added capacity with a new inverter, that is 11.6 years to repay if exported and 1.4 if used, and a real quote sits above the floor. The addition only makes sense for a home with a daytime deficit.

When a battery beats more panels

A 10 kWh battery is listed at $9,500 to $12,500 installed before the federal discount of $2,652 (68 certificates at $39), so $6,848 to $9,848 net. If it shifts 10 kWh a day from a 4 cent export to a 34 cent evening import on 300 days a year, an assumption we state rather than measure, it is worth $900 a year in New South Wales and $780 in Victoria, a payback of 7.6 to 10.9 years in New South Wales. Four kilowatts of added panels at the $2,634 floor are worth $228 a year in Sydney if the output is exported and $1,936 if it is all used at home.

The rule that follows: if the monitoring shows the existing system exporting most of its daytime output, the home has no daytime deficit and more panels would export at the feed-in rate, so the battery wins. If the system is export-limited and the home still imports in daylight, or an EV charger or heat pump has arrived, the extra panels are used at the grid rate and win. Many homes do both, panels first because they are cheaper per kWh, then the battery while the federal factor is 6.8 certificates per kWh; it steps down to 5.7 on 1 January 2027. The battery guide and the solar panel costs page carry the full tables.

Adding panels: questions