I try to compare my system performance with the PVGIS estimate on a monthly basis and use the percentage difference to estimate the total payback time. I compare this against the system cost multiplied by 2.4% per year over 6 years as a rough guide on how much it would have returned if invested in savings for that time, as these were the net fixed savings rates available when I had the system installed.
My system has been installed for 18 months now and I estimate the payback period (assuming no further outlay) to be 6.8 years. This calculation also includes an estimated yearly cost saving of £92 on consumed electricity.
The installation suffers from some shading issues towards the end of each day which I didn't know how to factor into the PVGIS calculation. However, for the first time in 15 months, it's finally ahead of the PVGIS prediction again.
My spreadsheet has got a bit complicated now and needs a bit of an overhaul. I'm not sure how accurate my method is and would be interested to see how others track performance and use this to estimate future generation as well as the payback period.