In short
- A 20kW system is about 36 panels on 100 to 130 m² of roof.
- In Sydney it makes about 80 kWh of power a day, on average.
- It suits a small business that uses well over 80 kWh a day, mostly in daylight hours.
A 20kW system is the usual size for a small business: an office, a clinic, a café, a workshop or a small shop. It is big enough if your business uses well over 80 kWh a day, mostly between about 9am and 4pm. For a business like that, the system pays for itself in about 2.3 to 5.8 years on our example figures.
| Quick facts | 20kW system |
|---|---|
| Roof space | 100 to 130 m² |
| Number of panels | about 36 |
| Power made per year | about 29,200 kWh (Sydney estimate) |
| Power made per day | about 80 kWh on average |
| Suits | an office, medical clinic, café, workshop or small shop that is busy in the day |
Is 20kW big enough for my business?
Hold the system up against your power bill. If your business uses well over 80 kWh a day, mostly in daylight, it will use almost all of the solar power itself. That is where the value is.
If you use about 80 kWh a day or less, or use much of it in the evening, a lot of the power goes back to the grid for a small credit. A smaller system may then pay back faster.
If your bill shows you would use more than 20kW makes, compare it with a 30kW system. For a general overview, see commercial solar.
How much power does a 20kW system make?
kW is the size of the system, a bit like a car's top speed. kWh is the amount of power it makes over time, like the distance you drive.
We use the Australian Government's YourHome guide, which puts average output in Sydney at about 4.0 kWh a day for each kW of panels. So 20kW makes about 80 kWh a day on average, or about 29,200 kWh a year.
Sites further south, such as Melbourne, make less. Which way your roof faces, how steep it is and any shade will change the real number more than your city does.
Will it work on my power supply?
A 20kW system needs three-phase power (the heavier-duty supply most shops, workshops and offices in commercial buildings have). Power networks set their single-phase limits at household scale.
The good news at this size is the approval. 20kW sits under the 30 kVA line where several networks switch to a more detailed approval process, so the paperwork is usually simpler and quicker than for a 30kW system. The proposal should still say whether your network limits how much power you can send back.
How much does the government discount take off?
The federal discount comes from small-scale technology certificates, or STCs. Your system earns these certificates, and the installer usually claims them and takes their value off your price.
The number of certificates is worked out like this:
kW of panels × your area's rating × years counted
For a system installed in 2026, the scheme counts 5 years. That drops by one each year until the scheme ends in 2030.
In Sydney, Brisbane, Adelaide and Perth (zone 3), the rating is 1.382. So 20 × 1.382 × 5 = 138.2, which gives 138 STCs. In Melbourne (zone 4) the rating is 1.185, so the same system earns 118.
The dollar value moves with the certificate price on the day. Read the STC line on your quote rather than assuming a figure.
In Victoria, the VEU business solar discount starts at 30 kW, so a 20kW system doesn't qualify for it. There is more at commercial solar rebates.
How long will it take to pay for itself?
Commercial Solar Quotes does not publish prices. The three prices below are example quote prices used for the sums, not our prices or a quote. Smaller systems usually cost more per kW than larger ones. Find the row closest to your own quote, after the STC discount.
What we assumed:
- Power made: 20 kW × 4.0 kWh × 365 days = 29,200 kWh a year
- Each kWh you use yourself saves $0.28 you would have paid for grid power
- Each kWh you send back to the grid earns a $0.05 credit
If you use 80% of the solar yourself (busy in the day): 23,360 kWh × $0.28 = $6,541. Add 5,840 kWh sent back × $0.05 = $292. Yearly saving $6,833.
If you use 40% of the solar yourself (busier in the evening): 11,680 kWh × $0.28 = $3,270. Add 17,520 kWh sent back × $0.05 = $876. Yearly saving $4,146.
| Your quote per kW (after the STC discount) | Total price | Yearly saving if you use 80% | Years to pay back | Yearly saving if you use 40% | Years to pay back |
|---|---|---|---|---|---|
| $800 per kW | $16,000 | $6,833 | 2.3 years | $4,146 | 3.9 years |
| $1,000 per kW | $20,000 | $6,833 | 2.9 years | $4,146 | 4.8 years |
| $1,200 per kW | $24,000 | $6,833 | 3.5 years | $4,146 | 5.8 years |
In plain words: a business that uses most of its solar pays the system off much sooner than one that sends most of it back.
These are simple sums. They leave out panels wearing slowly, price changes, loan costs and upkeep, so use them to compare, not to predict. For the wider picture, see commercial solar cost.
Should I add a battery to a 20kW system?
If your business is busiest during the day, usually not. You already use almost all the solar power.
A café or shop that trades into the evening is different. A battery can store some spare midday power for those hours. Whether it pays depends on the gap between what you pay for grid power and what you get for sending it back. See commercial solar batteries.
What to ask your installer
- Is my site three-phase, and if not, what would a supply upgrade cost?
- Does my network limit how much power I can send back to the grid?
- How much of the solar do you expect my business to use itself, and where did that figure come from?
- Is the STC discount shown as its own line on the quote?
- If I lease the building, has the owner agreed to the install?
How much solar you use yourself decides payback more than price, and only your bill shows it. Send a bill and an accredited installer prepares a costed proposal sized to how you actually use power. It is free, with no obligation.