Your panels are warrantied for 25 years. Your output won't stay flat.

A 25-year warranty is not 25 years of the same output.

Every solar panel loses a small share of its output as it ages. This is called degradation, and it is normal. It is not a defect. But most quotes and payback slides show a flat generation figure for every year, which quietly overstates the later years. If you are weighing a 25-year investment, it is worth knowing what the real curve looks like.

What degradation actually means.

Panels lose output through slow changes in the cells and materials: light-induced loss in the first months, then gradual wear from heat, UV and humidity. Output is usually measured against the panel's rated power. A panel that starts at 100% and degrades by a small percentage each year will still produce power at year 25, just less of it.

Read the performance warranty, not just the product warranty.

Panels usually carry two separate warranties. The product warranty covers manufacturing defects, often for a shorter period. The performance warranty is the one about degradation: it states the minimum output the manufacturer guarantees at set points in time, commonly with a larger drop allowed in year one and a smaller yearly drop after that. The exact numbers vary by manufacturer and model, so do not rely on a general figure. Find the datasheet for your specific panel and read the linear power warranty table.

Three things to check in the document:

• The guaranteed output at year 1, year 10 and year 25, as a percentage of rated power.
• Who handles a claim: the manufacturer, the importer or your installer, and in what form (replacement, repair or extra panels).
• Whether the warranty is transferable if you sell the house. Our guide on selling a solar home covers the paperwork to hand over.

What it does to your savings.

Here is a simple illustration, using made-up round numbers, not a forecast. Suppose a system generates 4,000 kWh in its first full year, and output falls by 0.5% of the original figure each year. By year 10 it would be near 3,800 kWh, and by year 25 near 3,500 kWh. The loss is real, but gradual. Over the full life, the total shortfall against a flat assumption would be modest, though not zero. Your actual rate will depend on the panel technology, heat, soiling and quality of installation.

The practical point: when an installer shows a 25-year savings table, ask whether it includes degradation. If it does not, mentally trim the later years. Our tariff and payback guide explains why rising BESCOM charges can partly offset that loss, but that is a tariff assumption, not a certainty.

Normal ageing versus a fault.

Degradation is slow and smooth. If your output falls sharply in a single month, that is not ageing. It points to soiling, shading, a failed string or an inverter issue. Use our monthly underperformance check first, and compare like months across years, since monsoon months will always look lower.

How to track it yourself.

Each October, note the lifetime kWh from your inverter and compare the same clear-weather month with last year. A very small year-on-year dip is expected. A large one deserves a call to your installer. Keep these yearly readings with your warranty papers. If you ever need to raise a performance claim, a consistent record is far more useful than memory. Regular cleaning also keeps soiling from being mistaken for degradation.

What to ask before you sign.

Ask for the panel datasheet by model number, the performance warranty table, and a written statement of who services a claim in Bengaluru. If the installer cannot produce these, treat that as a signal about after-sales support.