The return, or Return on Investment (ROI), describes the savings from your PV system in relation to your total investment.
Annual ROI in percent:
ROI over the entire lifetime of a PV system (20 years) in percent
ROI (annual) % = (Annual Savings / Total Investment) × 100
ROI₍20 years₎ (%) = ((Annual Savings × 20) − Total Investment) / Total Investment × 100
The annual savings are composed of the surplus PV electricity fed into the grid and the amount of purchased grid electricity.
For electricity fed into the grid, the current feed-in tariff is €0.07 per kWh.
For grid electricity, the kWh price is the one you enter under Electricity Consumption in the project.