“The National Renewable Energy Laboratory (NREL) performed a meta-analysis of studies that examined the long term degradation rates of various PV panels. They found that the 1% per year rule was somewhat pessimistic for panels made prior to the year 2000, and today’s panels, with better technology and improved manufacturing techniques, have even more stamina than their predecessors. For monocrystalline silicon, the most commonly used panel for commercial and residential PV, the degradation rate is less than 0.5% for panels made before 2000, and less than 0.4% for panels made after 2000. That means that a panel manufactured today should produce 92% of its original power after 20 years, quite a bit higher than the 80% estimated by the 1% rule.

Crystalline silicon modules located in extreme climates showed high degradation rates. For very cold climates, panels subjected to heavy wind and snow loads suffered the most. On the other hand, panels in similar climates that were installed in a facade, eliminating the snow load, had very low rates of degradation. At the other extreme, panels in desert climates exhibited large decreases in production over time – close to 1% per year – mainly due to high levels of UV exposure.

Panels in more moderate climates such as the northern United States had degradation rates as low as 0.2% per year. Those panels could retain 96% of their production capabilities after 20 years.

operate.http://www.nrel.gov/docs/fy12osti/51664.pdf

So, 20 year old panels should be outputting 80% to 96% of original, depending on where and how they are installed.