We recommend to buy solar panels of good qualtiy. If you is really looking for panels that can be used in 20-25 years without have massive loss so we recommend you to buy quality solar panels. Because normal quality solar panels have a about 1% or less yearly loss of energy each year. So it mean if you use them 20 years so they will have a loss percent of 20%. So a 100 Watt panel will produce 20% less then what it produced when it was new. 100 watt minus 20 it is equal 80 watt. After a 20 years usage in direct sunshine a solar panel will produce 80 watt.
If a panel just begin to produce less power just after 1-2 years usage so it is not a good quality. You have just loss your money on bad panels.