Solar Payback: What to Check Before You Trust the Number

Updated 15 September 2026 · Everyday

A Solar Payback Calculator gives a fast answer, but the answer is only as good as what goes in. These are the mistakes we see most often with solar payback calculations in Australia, and how to avoid them.

1. Stacking percentages

Percentage changes compound, so they can’t simply be added or subtracted.

2. Ignoring supply charges

Electricity bills include a daily supply charge on top of usage.

3. Forgetting public holidays

Day counts don’t know about state public holidays, so allow for them in deadlines.

4. Using the wrong units

Australia uses litres per 100 km for fuel and cents per kWh for electricity. Converting from other units introduces errors.

5. Overlooking the detail that matters most here

Feed-in tariffs have fallen, so using solar power yourself during the day is worth far more than exporting it.

How the payback period changes with system size

A common mistake is getting one input slightly wrong. Here is what happens to the payback period when “System size (kW)” is off by up to 20% in either direction, with everything else held steady.

System size (kW)	Payback period
5.28	4.2 years
5.94	3.7 years
6.6	3.4 years
7.26	3.1 years
7.92	2.8 years

At 5.28 the result is 4.2 years; at 7.92 it is 2.8 years. If a small change in this input moves the answer a lot, it is worth double-checking that figure before you rely on the result.

A quick sense check

Savings = self-used kWh × grid tariff + exported kWh × feed-in tariff. Payback = cost ÷ annual savings. With typical inputs the calculator returns payback period of 3.4 years. If your own result looks wildly different, check that each figure is in the right unit and period (weekly, monthly or yearly) before drawing conclusions.