20 years seems to be a common "write-off" timeframe for them, but that might be fueled in part by increasing efficiency as the technology gets better.
That seems pretty bad are they still usable after 20 years? With such a investment long term I would really want to milk them for as long as possible. If they lose a few percent per year that sounds not so good on a already long term investment.
I still think cheap electricity will always win out. Does not matter what it's up against.
Warranty on a panel is typically for 80% output after 20 or 25 Years. However it is normal for panels to loose about 5% in the first 12 Months. They are an ok investment particularly if you can benefit from a generous Feed In Tariff. In the UK there is a massive cut in the Tariff coming in January from 12.47p to 1.63p. There are going to be lot of out of work Solar Installers....
Rich