Solar installers will happily size a system for you, and the answer is usually "a bit bigger than you think". That's not always wrong, but it helps to do the sum yourself first, so you can tell a sensible quote from an upsell. You need your power bill and about ten minutes.
Step 1: Find your daily usage in kWh
Your bill shows how many kilowatt-hours you used over the billing period. Divide by the number of days:
daily use (kWh) = kWh on the bill ÷ days in the billing period
Look at a summer bill and a winter bill if you can; usage often changes a lot between seasons. Many retailers' apps also show a full year of usage. If kWh vs kW is still fuzzy, this three-minute explainer sorts it out, and our guide to reading your bill shows where the number hides.
If your bill is monthly, divide by 30: 1,000 kWh a month is about 33 kWh a day.
Step 2: Find your local solar yield
Each kilowatt of panels produces a certain number of kWh per day, on average, depending on where you live. For Australian capitals it's about 3.6 kWh in Melbourne to 4.4 kWh in Perth; in the UK about 2.3–2.7; in the US anywhere from about 3 to 5. Our 6.6 kW output guide has the table and the sources.
Step 3: Do the basic sum
system size (kW) = daily use (kWh) ÷ yield (kWh per kW per day)
For a home using 20 kWh a day in Sydney (yield 3.9):
20 ÷ 3.9 = 5.1 kW of panels would make, on average across a year, as much energy as the house uses.
For a home using 1,000 kWh a month (33 kWh a day) in a sunny US state (yield 4.5):
33 ÷ 4.5 = 7.3 kW.
That's the size that matches your usage on paper. Now for the part most quick calculators skip.
Step 4: Adjust for when you use power
Solar only produces during the day. Without a battery, you can only use your solar directly while the sun is up; the rest is exported to the grid. And the price you're paid for exports, the feed-in tariff, is usually a fraction of the price you pay to buy power. In much of Australia, for example, you might pay 30–40 cents to buy a kWh and get 0–10 cents for exporting one.
So the real question is how much of your solar you'll use yourself:
- Home during the day (working from home, retired, pool pump, daytime laundry): you'll use a good share of it directly. A system sized to your usage, or a bit more, makes sense.
- Out all day, heavy evening use: much of your solar will be exported at a low price. A bigger system mostly earns feed-in credit. It can still pay off, but more slowly, unless you add a battery or shift some use into the day.
Easy ways to shift use into the day: set the dishwasher, washing machine and hot water timer to run around midday, pre-cool or pre-heat the house in the afternoon, and charge an EV in the middle of the day.
Bigger is often still sensible
Panels are now cheap compared with the fixed cost of an installation: scaffolding, labour, the inverter and the paperwork. Adding a few more panels while the installer is there costs much less than adding them later. That's why many installers recommend going larger, and why the 6.6 kW system has become standard in Australia.
What limits you in practice:
- Roof space. A modern 440 W panel is about 1.7 by 1.1 metres (roughly 5.6 by 3.7 ft). 6.6 kW is 15 panels, about 30 m² (320 sq ft) of usable, unshaded roof.
- Export limits. Many networks limit how much a home can export at once, often around 5 kW on a single-phase connection in Australia. Panels beyond that only help if you use the power yourself.
- Inverter ratio. In Australia, panel capacity can be up to 133% of the inverter's rating, which is why 6.6 kW of panels usually goes with a 5 kW inverter.
Planning for an EV, heat pump or battery
If you're likely to buy an electric car, add its charging to your daily use. A car driving 40 km (25 miles) a day at 16 kWh/100 km needs about 6.4 kWh a day. In Sydney that's another 1.6 kW of panels (6.4 ÷ 3.9). The catch: the car has to be home and plugged in during the day to use that solar directly. See how long it takes to charge an EV at home for the details.
The same goes for swapping gas heating or hot water to electric heat pumps: add the extra kWh first, then size.
A worked example
A family in Brisbane uses 22 kWh a day: about 8 kWh during daylight and 14 kWh in the evening and overnight. They plan to buy an EV within two years, which would add about 6 kWh a day.
- Today: 22 ÷ 4.2 = 5.2 kW matches their usage on paper. But only about 8 kWh a day would be used directly.
- With the EV, charged at home during the day: (22 + 6) ÷ 4.2 = 6.7 kW.
- Sensible quote to ask for: about 6.6 kW of panels on a 5 kW inverter. Ask for a battery-ready inverter, or a quote for adding a battery later.
Ask any installer to show you the estimated yearly output for your roof, its direction and its shading, rather than a generic figure. If their estimate is far above the sums here, ask why.