Performance warranty or so called limited power warranty is to warrant the degradation of the cells over 25 years. The manufacturer may state that the performance will only reduce by say 10% after the first 10 years and then another 10% over the next 15 years. You will find that this warranty is fairly standard across most manufacturers.
Solar Panels will degrade over time and this warranty serves to tell the consumer what percentage rate a manufacturer’s module efficiency will drop by over time. This may be in the form of a performance warranty which will state that after 10 years and then 25 years – a solar panel manufacturer will guarantee that their solar panel will produce X amount of power as a percentage of peak minimum or a Linear power warranty which plots a linear curve of power loss over 25 years.