25 year performance warranties for solar panels are the industry standard.
However - it's very important to understand the difference between a solar panel performance warranty and a solar panel product warranty.
Most solar panels these days have a product warranty of 10-12 years (some super-high-end brands such as Sunpower carry a 25-year product warranty), and a performance warranty of 25 years.
The product warranty is to cover defective materials or workmanship in the manufacture of the panel. It is provided by the manufacturer of the panel, not the solar installation company.
It is critical to realise that most companies that talk up a “25 year warranty” are talking about the performance warranty, which is essentially a warranty that will guarantee that a solar panel will only degrade in performance by a small % per year.
This means that any faults due to poor workmanship or materials can be argued by the panel manufacturer to be only covered under the product warranty.
A panel performance warranty is virtually worthless compared to the all-important product warranty, but some unscrupulous installers will imply that the 25-year performance warranty will protect a homeowner from defective panels.
Interestingly - under Australian Consumer Law you may be covered for 25 years no matter what the small print of the panel performance warranty says.