What size solar system do you need? Size from your bill, not a sales script.
Almost every seller's answer is "go as big as your roof allows" — which is also, conveniently, the answer that maximises the sale. The honest method starts with your electricity bill: your daily kilowatt-hours, when you actually use them, and what you're genuinely planning next. Sometimes that means bigger. Often it means smaller than the quote.
Reviewed by the Mission Green Energy Team · Updated July 2026
The honest
short answer.
The right size comes from your bill: your average daily kilowatt-hours, how much of it you use in daylight, and what you're honestly planning next. Not from a rule of thumb, and not from "panels are cheap, go maximum".
Here's the industry's open secret: "go as big as possible" is the easiest sizing advice to give, because it never needs your bill, never needs a site visit worth the name — and always maximises the sale. It's sometimes right. But it's a script, not an answer.
The honest, bill-first method has three steps:
- Find your daily usage. Your bill prints your average daily consumption in kilowatt-hours (kWh). That's your baseline — the number every honest recommendation is built on.
- Split it into day vs evening. Solar only offsets what you use while the sun is up (or what a battery stores). A home that's busy at 1pm needs a different system to one that's empty until 7pm, even on identical bills.
- Size to self-consumption first. A kilowatt-hour you use yourself beats one you export — feed-in tariffs are low, and exports can be trimmed further in some states (more below). Then add honest headroom only for plans that are real: an EV, electrification, a battery.
Do that, and the answer lands wherever it lands — sometimes a 13kW+ system genuinely earns its keep, and sometimes the honest answer is smaller than the quote in your inbox. Whether solar stacks up at all for your home is a separate question we answer straight in is solar worth it in 2026?, and the payback maths lives in our solar payback guide.
Why does every quote say
"go as big as possible"?
Because bigger systems mean bigger invoices — and "panels are cheap" is the one line that's true enough to be persuasive and vague enough to never be checked against your usage.
To be fair to the script: panel hardware has become a smaller share of the total job, and there are real cases where maximising the roof is right. The problem isn't that "go big" is always wrong — it's that it's always said, to everyone, regardless of their bill. When the same answer fits every customer, it isn't advice. It's a price point.
Watch for the compounding-quote trick in particular: an oversized solar array is used to justify an oversized battery on the same quote ("you'll have all this surplus — you'll need somewhere to put it"). Each inflated component becomes the justification for the other, and the total quietly climbs. If a quote jumps straight to a big system plus a big battery without ever asking for your bill, that's the tell. Our honest battery guide covers when storage genuinely pays — and when it doesn't.
The honest sequence is the reverse: usage first, system second, battery (if at all) third — each sized to the one before it, not to the invoice.
How do you size
from your bill?
Two numbers do most of the work: your average daily kilowatt-hours, and how that splits between daytime and evening. Both come from a recent bill and your smart-meter data — free, and already yours.
Step 1 — find your daily kWh. Most Australian electricity bills print an "average daily usage" figure in kilowatt-hours, usually near a usage graph. Grab a recent bill and note it. If you can, check a summer and a winter bill too — heating and cooling can swing usage a long way between seasons, and sizing off one mild-season bill is a classic way to end up with the wrong system.
Step 2 — work out your day vs evening split. If you have a smart meter, your retailer's app or portal usually shows usage by time of day. The daytime share is what solar can offset directly; the evening share needs a battery or the grid. A household with someone home through the day — working from home, retirees, young kids, a pool pump — self-consumes a lot and gets strong value per panel. A home that's empty until dinner time self-consumes little, and extra panels mostly export.
Step 3 — layer on your real plans. An EV, a heat pump, induction cooking, a battery — genuine plans justify genuine headroom (we cover when below). Vague ones don't.
That's the whole method. It takes fifteen minutes with your own data, and it's exactly what we model — properly, against your roof and network — in a free assessment.
Why size for self-consumption
before exports?
Because a kilowatt-hour you use yourself avoids buying one at your full import rate, while an exported one earns only the feed-in tariff — and feed-in tariffs are now low.
This is the asymmetry that decides sizing. Every kilowatt-hour of solar you consume on the spot is a kilowatt-hour you didn't buy from the grid at your import rate. Every kilowatt-hour you export earns only your retailer's feed-in tariff — and those have fallen a long way from the golden years. As a reference point, IPART's voluntary all-day benchmark for NSW in 2026-27 is just 3.4 to 6.5 cents per kWh — a guide, not a mandate, but it tells you where export value sits. Grid power costs several times that. Used beats exported, every time.
It gets sharper in two states: New South Wales and South Australia now have two-way export pricing — the so-called "sun tax" — where networks can charge for midday exports (typically passed through, or not, by your retailer). The real-world dollar impact is modest for typical systems, but it further trims the value of building a system whose main output is midday exports. We've unpacked it properly, without the panic, in our sun tax explainer.
The sizing consequence is simple: build the system around the power you'll actually use — directly during the day, or via a battery in the evening — and treat export earnings as a bonus, not the business case. A system sized to farm feed-in credits is a system sized for a tariff that no longer exists.
What size suits
which household?
These are honest starting points, not answers — the same ranges we publish in our FAQs. Your bill, roof and plans move every one of them.
6.6kW
The long-time standard Australian install, and still a genuinely good fit for smaller households with typical usage. If you're a couple with no EV or electrification plans, be sceptical of anyone insisting you need double this "because panels are cheap".
Around 10kW
A common honest landing zone for a family home — enough to cover strong daytime usage with surplus for a future battery or EV, without paying for capacity that mostly exports at a few cents.
13kW+
Big households, all-electric homes and EV charging genuinely justify larger arrays — this is where "go big" stops being a script and starts being correct. The usage is real, so the capacity gets used.
When is bigger
genuinely worse?
Not just "not optimal" — genuinely worse or wasted. These are the cases where the extra kilowatts cost real money and earn almost nothing back, and a sales script will never mention them.
Network export limits
Your local network (DNSP) sets export limits, and your installer must design to them. If your exports are capped, the surplus from an oversized array has nowhere to go — you paid for generation you can't sell. See what happens when the network knocks back an application.
Nobody home in the day
If the house is empty until evening, there's little self-consumption for extra panels to serve. Most of the additional output exports at a low feed-in rate — the weakest possible return on the money.
Shaded or complex roofs
Once the good, unshaded faces are used, extra panels go onto worse ones — shaded, poorly oriented, or steep and costly to access. They generate less per panel and can drag the economics of the whole addition. Check your roof's suitability before chasing capacity.
The compounding-quote trick
An oversized array is used to justify an oversized battery — "you'll need somewhere to put all that surplus" — and each inflates the other. If the battery only makes sense because the solar is too big, neither is sized for you.
Export-heavy in a sun-tax state
In NSW and SA, two-way export pricing can charge for midday exports (if your retailer passes it through). It's not a reason to avoid solar — but it's one more reason a system built mainly to export is built on the wrong maths. How the sun tax actually works.
Paying for idle capacity
Every kilowatt past the point of useful generation still costs full money up front. If it neither gets used nor exports at a worthwhile rate, that cash did more for you in your offset account — or in a right-sized battery. Run it through our payback guide.
When is bigger
the right call?
Plenty of homes should go bigger than today's bill suggests — when the future usage is real. These are the legitimate cases for honest headroom.
An EV is coming
An electric car can be one of the biggest loads your home ever adds, and charging it from your own roof is the cheapest way to run it. If an EV is genuinely in your next few years — not "maybe someday" — sizing above today's usage is sensible, not salesy.
Electrification plans
Swapping gas for a heat pump, induction cooktop and reverse-cycle heating moves real load onto your electricity bill. If you're on that path, size for the all-electric home you're building, not the gas-assisted one you're leaving. See our electrification options.
A battery in the design
A battery needs genuine daytime surplus to charge from, so modest oversizing of the array is legitimate when storage is part of the plan. The order matters though: the battery is sized to your evening usage, and the solar to the battery — not the other way around. When a battery is worth it.
One more thing that surprises people: it's normal practice for installers to fit more panel capacity than the inverter's rating. Panels rarely produce their full rated output all at once, so pairing a larger array with a smaller inverter is a standard, legitimate design convention — not a trick, and not a fault in your quote. Network rules typically govern the inverter side, which is one reason panel capacity and inverter capacity are different numbers on your proposal. If the ratio looks unusual, ask your installer to explain the design — a good one will, gladly.
The test for all of it is the same: headroom for a plan is honest; headroom for a hypothetical is margin. If the EV, the electrification or the battery is real, build for it. If it's a line the salesperson added, don't pay for it.
Do rebates change
the sizing maths?
They soften the cost of going bigger — the federal STC incentive scales with system size — but they don't change whether the extra capacity ever gets used. A discount on waste is still waste.
The federal incentive for rooftop solar comes as STCs (small-scale technology certificates), and because they're issued in proportion to your system's capacity, a bigger system earns a bigger upfront discount — typically $2,000–$4,000+ across common residential sizes, with the certificate value floating on the STC market and the scheme stepping down each year until it ends in 2030. (Source: Clean Energy Regulator, cer.gov.au.) The full state-by-state picture is on our rebates & incentives page.
Two honest notes on how this plays in a sales conversation:
- "Stepping up a size barely costs more once the rebate's counted" — verify it, don't accept it. The claim is sometimes true, because the incentive scales with size. But ask for both quotes in writing and compare what the extra capacity actually earns on your usage — not on the assumption that every generated kilowatt-hour is worth your full import rate. Exported surplus earns cents.
- Don't buy capacity to harvest certificates. The STCs discount the hardware; they don't consume the electricity for you. If the extra kilowatts wouldn't have passed the bill-first test at full price, a discount doesn't change the answer — it just makes the wrong answer cheaper.
So — what size should you get?
Here's what we'd tell a friend: pull a recent bill before you talk to anyone, and make every quote justify its kilowatts against your numbers.
Find your average daily kWh, look at how it splits between day and evening, and be honest about what's actually coming — EV, electrification, battery, or none of the above. If you're a 1–2 person household with no big plans, a 6.6kW system is often genuinely right, whatever the quote says. A typical family usually lands around 10kW. Large, all-electric or EV-charging homes justify 13kW and beyond — that's where maximising the roof is the honest advice, and we'll happily give it. What we won't do is hand every caller the same "go maximum" line, because the same answer for everyone isn't advice.
The honest way to settle it is to model your bill against your roof and your network's rules — which is exactly what a free assessment does, and what our AI advisor Jouli is built to sanity-check for you first.
Solar system sizing:
your questions, answered.
Start with your bill, not a rule of thumb. Find your average daily usage in kilowatt-hours — it is printed on most Australian electricity bills — and think about how much of that power you use during the day versus in the evening. As indicative ranges only: a 6.6kW system suits 1 to 2 people, around 10kW suits a typical family, and 13kW or larger suits big households, all-electric homes, or those charging an electric vehicle. But two homes with the same headline usage can need very different systems depending on when they use power, roof space and shading, and any network export limits. The honest way to size a system is to model your actual bill — which is what a free Mission Green assessment does before recommending anything.
For many smaller households, yes. A 6.6kW system suits around 1 to 2 people with typical usage, and it has been a standard Australian install for years for a reason. It stops being enough when your usage is high, when you are planning an EV, a battery or full electrification, or when a large family drives daily consumption well past what 6.6kW of panels can cover. It can also be more than you need: a low-usage home that is empty during the day may self-consume so little that most of the generation is exported for a low feed-in tariff. Check your average daily kilowatt-hours on a recent bill first — that number, not your neighbour's system or a sales script, tells you whether 6.6kW is enough for your home.
No — and be wary of anyone who says it is, because 'go maximum' is also the answer that maximises the sale. Bigger is genuinely worse or wasted when your network's export limit caps what you can send to the grid, when nobody is home during the day to use the power, when your roof is shaded or complex so the extra panels underperform, or when an oversized solar quote is being used to justify an oversized battery on top of it. Bigger is genuinely right when you have an EV coming, electrification plans, or a battery that needs daytime surplus to charge. The difference is whether the extra kilowatts serve your usage or the seller's margin — model it from your bill and the answer falls out.
Look for the average daily usage figure, shown in kilowatt-hours (kWh) per day — most Australian electricity bills print it, often next to a usage graph. Check a summer bill and a winter bill if you can, because usage swings with the seasons. Then work out when you use power: if you have a smart meter, your retailer's app or portal usually shows usage by time of day, which reveals how much you use while the sun is up versus in the evening. Daytime usage is what solar offsets directly; evening usage needs a battery or the grid. Those two numbers — daily kilowatt-hours and the day-versus-evening split — are the honest starting point for sizing, and they come from your bill, not a brochure.
Often yes, within reason — this is the legitimate case for going bigger. An EV can be one of the largest loads a home ever adds, full electrification of heating, hot water and cooking raises usage, and a battery needs genuine daytime surplus to charge from. If any of those are realistically in your next few years, sizing above today's usage is sensible, and it is also normal industry practice for installers to fit more panel capacity than the inverter's rating. The caveat: future-proofing should be a plan, not a sales line. If there is no EV, battery or electrification on your horizon, the extra capacity mostly exports at a low feed-in tariff — and network export limits can cap even that. Size for the future you actually intend, not a hypothetical one.
Four practical limits. First, your network: the local distributor (DNSP) sets export limits and rules on system and inverter capacity — single-phase homes typically face tighter limits than three-phase — and your installer must design to them; the network can also knock back or condition an application. Second, your roof: the usable, unshaded area determines how many panels will actually perform, and a shaded or complex roof can make extra panels poor value. Third, your switchboard and wiring sometimes need upgrading to support a larger system, which adds cost. Fourth, the economics: past the point where extra generation is either self-consumed or exported at a worthwhile rate, additional capacity adds cost without adding much return. A good installer checks all four against your site and your bill before quoting a size.
Where these figures come from.
Incentive, tariff and export-pricing figures on this page are drawn from official primary sources and were current as at 2026. Rates and rules change — confirm at the source before relying on a figure.