The quick answer
Most Australian homes are best served by a 6.6kW system, which sits between the 5kW and 10kW options people usually compare. It fits a standard 5kW inverter, suits an average daily usage, fits most roofs, and pays back fastest. You step up to 10kW or more when you have high daytime usage, a big household, an electric vehicle, a pool, or plans for a battery.
The mistake is treating "what size solar" as "how much roof can I fill". The right question is "how much power can I actually use", because solar you export earns almost nothing while solar you use saves you a lot.
Size by your usage, not a guess
Find your average daily usage in kilowatt-hours on your electricity bill. As a rough rule, each kW of panels produces about 4 kWh a day averaged across the year (more in summer and the north, less in winter and the south). Then remember that you will only use a portion of that during daylight unless you shift load or add a battery.
- ✓Under ~12 kWh/day: a 5kW system may be plenty, especially for a smaller household.
- ✓~15 to 25 kWh/day: 6.6kW is the natural fit for most families.
- ✓~25 to 40 kWh/day, or you have an EV, pool or ducted air conditioning: 10kW or more starts to make sense.
5kW vs 6.6kW vs 10kW at a glance
| System | Approx Daily Output | Installed (after STC) | Best For |
|---|---|---|---|
| 5kW | ~18–22 kWh | $4,000–$6,000 | Smaller homes, lower usage, limited roof |
| 6.6kW ★ Most Popular | ~24–28 kWh | $4,500–$7,000 | Average 3–4 bed family home |
| 10kW | ~36–44 kWh | $7,000–$11,000 | High users, EVs, pools, pre-battery |
Output varies widely by state and season. Northern and sunnier states sit at the top of each range, Melbourne and Hobart toward the bottom. See our cost guide for state-by-state output.
Why does 6.6kW keep winning? It pairs with a 5kW inverter under the common oversizing rule, captures a strong STC discount, fits most roofs, and almost always pays back faster than a 10kW system for a household that cannot use all the extra daytime power.
Roof space and orientation
A modern panel is roughly 1.1 by 1.8 metres. As a guide, 6.6kW needs around 30 square metres of usable roof, and 10kW around 45. But raw space is only half the story, orientation and shade decide how much that space is worth.
- ✓North-facing gives the most total daily energy.
- ✓East and west spread generation into the morning and evening, which can actually lift self-consumption for working households.
- ✗Hard south underperforms in Australia and is usually a last resort.
- ✗Shade from trees, chimneys or a neighbour hits output hard. Microinverters or optimisers help, but avoiding shade is better.
Inverter and export limits, the bit people miss
Your network has rules about how much you can push back to the grid, and they shape your system more than most people expect.
- ✓Single-phase homes are often capped at a 5kW inverter and a 5kW export limit, though this varies by network. That is a big reason 6.6kW of panels on a 5kW inverter is so common.
- ✓Three-phase homes can usually go much larger, which is part of why bigger systems suit some properties and not others.
- ✓Export limiting may be required, where the system is set to send only so much to the grid. You still use all you can on site, you just cannot dump unlimited excess.
A good installer checks your phase and your local network rules before quoting a size. If a salesperson quotes a huge system without asking, that is a warning sign covered in our installer guide.
The oversizing trap
A bigger system earns the installer more and earns more STCs, so there is a built-in nudge to upsell you. Sometimes it is right, often it is not.
There is a legitimate kind of oversizing: fitting, say, 6.6kW of panels to a 5kW inverter. Panels rarely hit their rated peak, so a modest oversize (commonly up to about 133% of the inverter) captures more morning and evening energy and is standard good practice.
The trap is the other kind: being sold far more capacity than you can use, justified by "more rebate" and "sell the excess to the grid". With feed-in tariffs at a few cents, exporting a big surplus barely moves your bill, while you paid full price for those extra panels. Unless you have the usage, an EV, or a battery to soak it up, a 10kW system can pay back slower than a 6.6kW one. Size for consumption first.
Sizing for a future battery
If a battery is on your horizon, it changes the maths. A battery lets you store daytime surplus and use it at night, so a larger array that would otherwise just export cheaply suddenly has somewhere useful to go. In that case a 10kW system feeding a battery can make good sense.
If you are not adding a battery now but might later, ask for a battery-ready hybrid inverter and a sensible amount of headroom rather than maxing out the array today. Our guide on whether a battery is worth it right now covers the timing.
Seasonal swings and single vs three phase
The daily output figures earlier are year-round averages, and the seasonal spread matters when you size a system. A 6.6kW array in Melbourne might produce over 30 kWh on a clear summer day but under 12 on a grey winter one. Sizing only for summer leaves you short in winter, while sizing for winter can mean a big summer surplus you export for cents. The honest approach is to size around your annual usage and accept that you will lean on the grid more in winter and export more in summer, unless you add a battery to smooth it out.
The other technical factor that quietly decides your options is whether your home is single or three phase:
- ✓Single phase is most common in older and smaller homes. Many networks cap single-phase systems at a 5kW inverter and a 5kW export limit, which is the single biggest reason 6.6kW of panels on a 5kW inverter is the default. You can sometimes go larger with export limiting.
- ✓Three phase is common in larger and newer homes and is what you often need for high-power appliances. It usually allows a much larger inverter and export allowance, which is why 10kW and bigger systems tend to suit three-phase properties.
You can check which you have by looking at the main switch in your switchboard, or simply ask the installer to confirm during their assessment. If someone quotes you a large system without first establishing your phase and your network's export rules, treat it as a sign they are selling a size rather than designing one. The right process is usage first, then roof and orientation, then phase and network limits, and only then a recommended system size.
Frequently asked questions
Is a 5kW or 10kW solar system better?
Neither is automatically better. 5kW suits smaller homes with lower usage or limited roof space, while 10kW suits big users, EV owners and homes planning a battery. Most Australian households land on 6.6kW, which fits a standard 5kW inverter and pays back fastest.
What size solar system do I need?
Size it to your usage. Check your daily kWh on your bill, allow roughly 4 kWh per day for each kW of panels, and remember you only benefit fully from the solar you use during the day. Under 12 kWh a day suits 5kW, 15 to 25 suits 6.6kW, and 25 plus or an EV points to 10kW.
Why is 6.6kW the most common solar size?
It pairs with a common 5kW inverter under the standard oversizing rule, captures a strong STC discount, fits most roofs and suits average household usage. For homes that cannot use a large daytime surplus, it usually pays back faster than a 10kW system.
Can I install a system bigger than 5kW on single phase?
Often you are limited to a 5kW inverter and export cap on single phase, though it varies by network, which is why 6.6kW of panels on a 5kW inverter is so common. Three-phase homes can usually go much larger. A good installer checks your phase and network rules before quoting.
Is it worth oversizing my solar system?
A modest oversize of panels relative to the inverter, commonly up to about 133%, is standard good practice. But being sold far more capacity than you can use, justified by extra rebate and grid exports, often pays back slower because feed-in tariffs are low. Size for what you actually consume.