Sizing comes down to two figures — how many kilowatts (kW) of solar panels go on your roof, and how many kilowatt-hours (kWh) of battery storage, if any, sit beside them. This page shows you how to estimate both for your own household, working from nothing more than your power bill. It is the back-of-the-napkin version: enough to walk into quotes with your own number. When you want a precise, roof-specific answer, we point you to the best free tool in the country to get one.
The quick answer
For most Australian homes, the answer lands in a fairly narrow range. Solar usually falls between 6.6 kW and 10 kW — 6.6 kW has long been the default starting point, while 10 kW and above is increasingly common as homes add air conditioning, EVs and electric hot water. A battery is either skipped, or sized at around 10–13.5 kWh to store the day’s surplus for the evening; smaller 5 kWh units are increasingly treated as the bare minimum rather than a sensible target.
One rule of thumb before we go further: a solar system generates, averaged across the year, roughly four times its size in kWh per day. A 6.6 kW system yields about 26 kWh on an average day; a 10 kW system about 40 kWh. A typical Australian home uses somewhere around 16–20 kWh a day. The mismatch is deliberate — you generate in the daytime, use much of your power after dark, and the gap is what a battery or the grid bridges.
The rest of this page is about finding your number rather than the average one. It comes down to three things: how much energy you use, when you use it, and what is about to change.
How precise do you need to be?
There are two honest ways to reach your numbers, and they suit different people.
The back-of-the-napkin method — the one this page walks you through — uses your bill, a few rules of thumb and about five minutes. It gets you a solid ballpark: enough to know whether you are shopping for a 6.6 kW or a 10 kW system, with or without a battery, and enough to tell a fair quote from a padded one. For most people deciding whether solar is worth it, that is all the precision this stage needs.
The precise method matches your actual half-hourly usage against the real solar radiation and shading on your specific roof. Doing that by hand is genuinely hard, which is the honest reason most guides gesture at it and move on. The good news is you do not have to, because there is a free, independent, properly built tool that does exactly this.
That tool is SunSPOT. It was built by engineers at UNSW’s School of Photovoltaic and Renewable Energy Engineering for the Australian PV Institute, with Australian Government support, and it is free and available right across the country. What sets it apart is its independence: it exists to help you understand what solar can do for you, not to sell you anything — unusual in a field where most “free” calculators are really there to capture your details and generate sales leads. Under the hood it draws on Bureau of Meteorology weather data to model the solar radiation on your roof across a typical year, accounts for your roof’s tilt, orientation and shading, and lets you enter your usage from your bill to suggest the system size that gives the best return. You get the most accurate result by uploading twelve months of interval data from your retailer, but even a quick run with your bill figures beats a salesperson’s guess.
The rest of this page gives you the napkin version. If you would rather skip to a tailored answer, run your address through SunSPOT and come back to sanity-check it against the rules of thumb below.
Start with your usage
Everything downstream depends on two numbers from your electricity bill: how much you use, and when.
Your annual usage in kilowatt-hours is the headline figure. Most bills show an average daily usage for the billing period — multiply that by 365 for a rough year, or better, add up the kWh from your last four quarterly bills so you capture every season. As a reference point, the national benchmark for a typical home sits around 6,500 kWh a year, but real households vary enormously: a single-person flat might use 2,400 kWh, while a large all-electric home with a pool and ducted air conditioning can run past 12,000 kWh.
The number that matters even more — and the one most people skip — is when you use power. Solar only generates in daylight. If most of your usage happens between roughly 9am and 4pm (someone home during the day, a pool pump, daytime cooling), then solar alone offsets a big share of your bill and a battery matters less. If the house empties out during the day and the cooking, heating, lights and devices all run from 5pm onwards, your panels will be exporting cheaply at midday while you buy expensive power at night — and that pattern is the case for a battery.
You do not need to be exact yet. A rough split — “we use maybe a third of our power in the day and two-thirds at night” — is enough to size sensibly. If you want the precise picture, your retailer’s app or smart-meter data shows your usage hour by hour, and that same interval data is what feeds the precise tool above.
Sizing your solar (kW)
Solar sizing works backwards from how much each kilowatt of panels generates where you live.
Across Australia, 1 kW of north-facing panels produces, averaged across the year, somewhere between about 3.5 and 5 kWh per day — lower in the cooler, cloudier south, higher in the sunny north and inland. These figures come from the Clean Energy Council’s guidelines and already fold in real-world losses like heat, wiring and inverter efficiency, so they reflect what actually reaches your meter rather than lab conditions.
| City | Avg daily output per kW | 6.6 kW system | 10 kW system |
|---|---|---|---|
| Hobart | ~3.5 kWh | ~23 kWh/day | ~35 kWh/day |
| Melbourne | ~3.6 kWh | ~24 kWh/day | ~36 kWh/day |
| Sydney | ~3.9 kWh | ~26 kWh/day | ~39 kWh/day |
| Canberra | ~4.3 kWh | ~28 kWh/day | ~43 kWh/day |
| Adelaide | ~4.4 kWh | ~29 kWh/day | ~44 kWh/day |
| Brisbane | ~4.5 kWh | ~30 kWh/day | ~45 kWh/day |
| Perth | ~4.6 kWh | ~30 kWh/day | ~46 kWh/day |
| Darwin | ~5.0 kWh | ~33 kWh/day | ~50 kWh/day |
These are annual averages. Winter output in the southern states can run 40–50% below the summer peak, so a system sized only to your summer generation will fall short in July.
To size your solar, take your average daily usage, decide how much of it you want to cover, and divide by your city’s per-kW figure. A Melbourne home using 20 kWh a day that wants to cover its full daily usage needs roughly 5.5 kW as a floor (20 ÷ 3.6) — which is why 6.6 kW is such a common starting point, and why most installers nudge you a little above your bare minimum.
Mild oversizing usually pays off, and this is one of the few places where bigger is genuinely better in current economics. Panels are the cheapest part of the system per unit of output; winter and cloudy days drag your average down; and any surplus either charges a battery or earns a feed-in credit. The real limits on going bigger are your roof and your network. As a rough guide, a 6.6 kW system needs about 30 square metres of unshaded, north-facing roof — orientation and shading change that, which is covered in Solar Panels and your roof. Your distribution network also caps how much you can export, often around 5 kW for a typical home, so a large system behind a low export limit spills midday production unless a battery soaks it up.
For a precise, roof-specific figure rather than a city average, this is the moment to run SunSPOT. For what specific systems include and cost, see the size pages:
Sizing your battery (kWh)
A battery is sized to your evening and overnight usage — the part of the day the sun cannot cover.
Start from the figure you estimated earlier: how many kWh you use outside daylight. For a home using 18 kWh a day that runs roughly two-thirds of its load after dark, that is about 12 kWh to get through to morning. This is why 10 kWh and 13.5 kWh are the most common residential sizes — they carry a typical evening, overnight and early morning with a little to spare, without paying for capacity you will rarely fill.
Two rules keep the maths sensible. First, do not size the battery bigger than your solar can refill: a 13.5 kWh battery is only useful if your panels produce a 13.5 kWh surplus on top of daytime use to charge it. Battery and solar size move together — oversizing one without the other wastes money. As a pairing guide, 6.6 kW of solar comfortably fills a 10 kWh battery in most conditions, while 13.5 kWh and larger batteries are better matched to 10 kW or more of panels.
Second, size to your usage, not to the biggest unit on offer. Without a battery, a typical home self-consumes only about 30–40% of what its panels generate and exports the rest; adding a 10 kWh battery lifts that to roughly 70–80%. Most of the financial benefit comes from that first jump, so a battery that routinely cycles only half its capacity earns its keep slowly.
Whether a battery is worth it at all is partly a savings question and partly about wanting backup in a blackout — and it leans heavily on your feed-in tariff and tariff structure, covered in the next section. For costs and payback, see How much do solar & batteries cost? and How much can solar save on your electricity bill?. For specific pairings, see the combination pages:
What changes your number
A handful of factors push your figures up or down from the household average. Work through the ones that apply to you.
Daytime occupancy. Someone home during the day, or appliances on timers, shifts usage into solar hours — favouring more solar and less battery. An empty house from 8am to 6pm pushes the other way, toward storage.
Your tariff and feed-in rate. Feed-in tariffs — what your retailer pays for exported power — have fallen to just a few cents per kWh in most states, well below what you pay to buy power back. The lower your feed-in rate and the higher your usage charges, the more valuable it is to store and use your own solar rather than export it, which strengthens the case for a battery. On a time-of-use tariff with an expensive evening peak, that case is stronger again. The current landscape is covered in Rebates and Incentives.
Export limits. Many networks cap household exports, often near 5 kW. If you want a large system behind a low limit, a battery lets you keep the midday surplus instead of spilling it.
Future loads. Size for the home you will have in a few years, not just today’s. The big ones: an EV adds roughly 3,500–4,500 kWh a year (around 15,000 km of driving), a heat-pump hot water system replacing gas adds about 1,800 kWh, and a pool pump or ducted heating and cooling can each add thousands. Every one of these is a reason to size solar up now, while adding panels at install is cheaper than expanding later.
Backup intent. If keeping the lights on during a blackout matters to you, that shifts the conversation — not every battery provides backup, and the ones that do may need to be sized and wired for the specific circuits you want to keep running, separate from the pure savings maths.
The general pattern: size up when you have daytime-shiftable load, electrification on the horizon, or a low feed-in rate; stay lean when your usage is modest, steady and already concentrated in daylight.
Worked examples
Three households, to help you find the one closest to yours.
The weekday-empty family home. Two working adults, two kids, house empty 8am–4pm, everything running from 5pm: cooking, laundry, heating or cooling, devices. Usage around 18–20 kWh a day, heavily skewed to the evening. Solar exports most of its midday output, so this home benefits from both: roughly 6.6–8 kW of solar paired with a 10–13.5 kWh battery, sized to carry the evening through to morning.
The home-all-day household. Someone works from home or is retired; a pool pump, daytime washing and air conditioning run while the sun is up. Usage might be a similar 18 kWh a day, but much of it falls in daylight. Here solar does most of the work alone: a generous 8–10 kW of solar, with a battery as an optional later add-on rather than a priority, since less of the consumption needs time-shifting.
The electrifying upgrader. A smaller bill today — say 12–14 kWh a day — but an EV arriving next year and a plan to swap gas hot water and cooking for electric. Today’s usage understates tomorrow’s. This home should size for the future: 10 kW of solar now, while it is cheap to add, with a 13.5 kWh or larger battery added alongside the EV to soak up cheap daytime charging.
If one of these resembles your home, the matching system-size pages walk through the specific equipment and costs. If none quite fits, that is exactly what SunSPOT is for.
From estimate to quotes
You now have a working estimate: a solar size in kW, and a battery size in kWh (or a decision to skip the battery for now). That estimate is what turns quoting from guesswork into comparison.
Before you request quotes, run your address through SunSPOT once to confirm your figure against your real roof — it takes a couple of minutes and gives you a number you can hold an installer to. Then carry that number into every quote: ask each company to price the size you specified, so you are comparing like with like rather than letting each one size the job to suit its own pitch.
From here the buying process moves on to requesting and comparing quotes — see How much do solar & batteries cost? for what your sized system should cost, then the questions to ask once the quotes start arriving. The important shift is that you are now driving the conversation rather than following it.
