Worked Examples and Comparison Checklists
Solar savings worked example: self-use, feed-in credits and payback
Work out what rooftop solar saves on an Australian bill — self-consumed kWh, export credits, a lower feed-in tariff scenario and simple payback — with every step of the maths shown.
Rooftop solar cuts your bill in two separate ways: every kWh you use as it is generated saves the full usage rate you would otherwise pay, and every kWh you export earns the feed-in tariff, which is usually a small fraction of that rate. To estimate the yearly saving, split the system's output into self-used and exported kWh, value each at the right rate, and add them. Then compare the result with the system's net cost to get a rough payback period. With feed-in tariffs low in 2026 and no minimum in Victoria since 1 July 2025, the self-use share drives most of the saving. This worked example uses illustrative numbers throughout.
Step 1: Estimate yearly output
A common rule of thumb for many Australian capital cities is that each kW of panels produces roughly 3.5 to 4.5 kWh a day on average across the year, depending on location, orientation, tilt and shading. Illustrative assumption for this example:
- System: 6.6 kW of panels
- Average daily output: 6.6 × 3.8 = 25.1 kWh
- Yearly output: 25.1 × 365 ≈ 9,150 kWh
For your own home, use the installer's modelled output for your roof, or your inverter's actual history if you already have solar.
Step 2: Split self-use and exports
Self-consumption depends on whether people are home during the day and what runs then. Three illustrative households:
| Household | Self-use share | Self-used kWh | Exported kWh |
|---|---|---|---|
| Out all day | 20% | 1,830 | 7,320 |
| Mixed | 35% | 3,203 | 5,947 |
| Home all day, timers set | 50% | 4,575 | 4,575 |
Step 3: Value each part
Illustrative rates: usage rate 31 c/kWh (flat), feed-in tariff 4 c/kWh.
| Household | Self-use value | Export value | Total yearly saving |
|---|---|---|---|
| Out all day | 1,830 × $0.31 = $567.30 | 7,320 × $0.04 = $292.80 | $860.10 |
| Mixed | 3,203 × $0.31 = $992.93 | 5,947 × $0.04 = $237.88 | $1,230.81 |
| Home all day | 4,575 × $0.31 = $1,418.25 | 4,575 × $0.04 = $183.00 | $1,601.25 |
The same system saves almost twice as much for the household that uses half its output itself.
Step 4: What if the feed-in tariff falls?
Retailers can change feed-in tariffs. Repeat the "Mixed" case at a 1 c/kWh feed-in tariff:
- Export value: 5,947 × $0.01 = $59.47
- Total: $992.93 + $59.47 = $1,052.40, down $178.41 a year
Now raise self-use from 35% to 45% (4,118 kWh self-used, 5,032 exported) at the same 1 c rate:
- 4,118 × $0.31 = $1,276.58; 5,032 × $0.01 = $50.32; total $1,326.90
Shifting 10% more of your output into home use more than offsets the lower feed-in tariff. See solar feed-in tariffs in 2026 for how rates are set.
Step 5: Time-of-use and free midday power
On a time-of-use plan, self-used solar replaces whatever rate applies at midday, which may be a shoulder or solar soak rate rather than the peak rate. On a free-midday plan such as the Solar Sharer Offer or Victoria's Midday Power Saver, grid power in the free window costs nothing, so solar used in that window saves no usage cost at all — its value then is the feed-in credit you give up by not exporting. For many solar owners that makes the free-midday plan less valuable than for households without panels. Model both with the flat versus time-of-use worked example and read the Solar Sharer guide.
Step 6: Simple payback
Illustrative system price: $7,800 installed, with an upfront discount from small-scale technology certificates (STCs) of $2,300, so net cost $5,500. Using the Mixed household's $1,230.81 a year:
- Simple payback ≈ $5,500 ÷ $1,230.81 ≈ 4.5 years
- If prices rise 2% a year, payback comes a little sooner; if the feed-in tariff falls to 1 c, it stretches to about $5,500 ÷ $1,052.40 ≈ 5.2 years
The solar payback calculator handles price growth and a comparison horizon. STC values change with market prices and the scheme steps down each year until it ends in 2030, so take the discount from your written quote, not an estimate.
Choosing a plan once you have solar
Compare plans on total annual cost including exports, as explained in the step-by-step comparison:
annual cost = supply + grid kWh × usage rate − exported kWh × feed-in tariff
A plan with a slightly higher usage rate but much higher feed-in tariff suits a household that exports a lot; a plan with low usage rates suits one that still buys a lot of grid power.
Limits of this method
- Output varies by season; winter exports may be small.
- Inverter export limits (set by the network) can cap exports on some connections.
- Panel degradation, shading from new buildings or trees and system faults reduce output.
- Batteries change the split again but have their own cost and lifespan.
For any major purchase, get several written quotes from accredited installers and check the warranty terms.
FAQ
Is it better to export or use my solar?
At typical 2026 rates, using it at home is usually worth several times more per kWh than exporting it.
Does a bigger system always pay back faster?
Not if most of the extra output is exported at a low feed-in tariff. Size the system around daytime use and future plans such as an EV or heat pump.
Where do I find my self-consumption share?
Your bill shows exports; your inverter or monitoring app shows total generation. Self-use = generation − exports.
Do I get the feed-in tariff automatically?
Only on a plan that includes one. Check your plan's fact sheet and your bill's solar credit line.
Sources
- Clean Energy Regulator — small-scale technology certificates and accredited installers
- Energy Made Easy — comparing plans with solar
- Essential Services Commission — Victorian feed-in tariff information
- Victorian Energy Compare — Victorian solar plan comparisons
