Solar panel degradation is tiny nowadays. For monocrystalline cells the degradation rate is around 0.36% per year, which means in 10 years your panels will still be outputting about 96.5% of what they output brand new. Polycrystalline cells are around 0.64% degradation per year.

Source: https://www.nrel.gov/docs/fy12osti/51664.pdf

Solar generally comes with a warranty, in the range of 80% of nameplate capacity after 20-25 years. There is some degredation but at 20 years that is in line with utility replacement rates.

Well, they've been there for five years and it's not noticeably degraded, so ...