How long do solar panels last?

Many module warranties run about 25 years for product defects, with a separate performance promise that output will still be a stated share of nameplate at the end. That is a contract with a manufacturer. It is not a promise that the inverter, the roof, or the installer will last the same time.

Degradation

NREL's review of field data by Jordan and Kurtz found a median module loss around half a percent per year, with a wide spread. Newer products can do better or worse. Half a percent per year is roughly 12 percent over 25 years, which is why performance warranties often land somewhere near 85 to 92 percent of original output. Read the warranty you are offered. Do not treat one brand's number as the industry's number.

What fails sooner

String inverters often carry a shorter warranty than the modules. Budget for a replacement or choose equipment with a warranty you believe the company can honor. Workmanship warranties are the installer's promise. If the installer closes, that promise may be thin. Monitoring is how you notice a failure before a year of production is gone.

Roofs

The roof under the array has its own life. A reroof means the modules come off and go back on. If that cost is not in the original conversation, it shows up later as a surprise. Ask who pays. That question belongs in the proposal checklist.