Worked example: 6.6 kW system

Inputs: 6.6 kW system, $6,000 cost, 20 kWh per day household use, 50% self-use, 35 cents per kWh electricity price, 8 cents per kWh feed-in tariff, and 4.2 kWh generated per kW per day.

Annual generation is 6.6 × 4.2 × 365 = 10,117.8 kWh. Half of that, 5,058.9 kWh, is self-used, because 50% self-use of 10,117.8 kWh is below the annual household use of 7,300 kWh. The remaining 5,058.9 kWh is exported.

Yearly saving is 5,058.9 × $0.35 + 5,058.9 × $0.08 = $1,770.62 + $404.71 = $2,175.33. Payback is $6,000 ÷ $2,175.33 = 2.76 years. The 10-year net figure is $2,175.33 × 10 − $6,000 = $15,753.27. These are the figures this calculator returns for those inputs, with cents rounded in the display. If you change self-use to 30%, the saving falls and payback lengthens, which is a useful sensitivity check before you talk to an installer.