Almost all modern panels have a 10 year product warranty and a 25 year production guarantee. The production guarantee for mainstream panels is about a 1% degration per year or 25 % over 25 years. In reality the degradation is usually much less. SunPower which is a premium brand guarantee much less degradation.
A very interesting analytical paper on the subject by NREL can be found here: http://tinyurl.com/qcjl58v