Panels fade slowly and predictably, and the famous 25-year warranty is actually two separate promises: one that the physical panel will not fail, and one that its output will not fall below a published curve. The curve is printed right on the datasheet, so you can compute the guaranteed floor for any year of the panel's life. Knowing the difference between those two promises, and what neither one covers, is what separates an honest answer from a brochure.
Efficiency: what the percentage actually means
Module efficiency is the share of sunlight power the panel converts to electricity. The 395W Canadian Solar we stock is rated 20.2 percent, and the number checks out from the same sheet. The panel measures 1722 by 1134 mm, about 1.95 square meters. Full STC sun delivers 1000 watts per square meter, so roughly 1,953W of light lands on the glass. Take 20.2 percent of that and you get 394W. The rating, rebuilt from area and efficiency.
Here is what efficiency buys, and what it does not. A more efficient panel makes more watts per square meter, so it buys roof space: fewer panels, or more power on the same roof. What it does NOT buy is more energy per rated watt. A 395W panel at 20.2 percent and a 395W panel at 18 percent produce the same power in the same sun; the second one is just physically bigger. On a tight roof, efficiency is worth paying for. On open acreage with a ground mount, cheap watts usually beat dense watts.
Degradation: the slow fade
Panels lose a little capacity every year. The mechanisms live in the cell chemistry (this sheet advertises LID and LeTID mitigation, two of the named culprits), but the practical fact is the rate, and the warranty puts a ceiling on it. For this panel: first-year power degradation no more than 2 percent, and no more than 0.55 percent per year after that.
Turn those two numbers into the guaranteed floor:
- Year 1: at least 98 percent of nameplate. For the 395, about 387W.
- Year 10: 98 percent minus 9 more years at 0.55, which is 93.05 percent. About 368W.
- Year 25: 98 percent minus 24 years at 0.55, which is 84.8 percent. About 335W.
Read those as a floor, never a forecast. The warranty numbers are the worst the manufacturer will stand behind, and a well-made panel typically fades slower than its warranty line. How much slower for this specific model is not on the sheet, so we will not put a number on it.
Degradation is also measurable, which is the whole reason a used-panel market can exist. Put a panel in full sun with a meter on it and compare output to nameplate. Our new vs used guide treats that measured-versus-label check as the core of judging a secondhand panel. A panel cannot hide its history the way a battery can.
The two warranties, pulled apart
Promise one is the product warranty. This sheet carries a 25-year product warranty on materials and workmanship. It covers the panel as an object: delamination, frame failure, junction box faults, the panel physically breaking down under normal use. Read the fine print even here. This sheet says the 25-year product term applies to panels installed on residential rooftops in certain regions, subject to the warranty statement's terms. A ground mount or a commercial job may carry a different product term. The warranty statement, never the datasheet headline, is the binding document.
Promise two is the performance warranty. That is the degradation curve above, the guarantee that a working panel still produces at least the floor value for its age. A panel making 90 percent at year 10 is inside warranty even though it is down 10 percent, because the promise was the curve, never the nameplate.
What people miss: a claim under either promise gets you what the warranty statement says, and usually that means a repaired or replaced panel or the power shortfall made good, per the manufacturer's terms. Whether labor, removal, or shipping are covered varies by manufacturer and by document. Nobody should state those terms from memory, us included. Pull the current warranty statement for the exact model and read it before counting on what a claim would pay.
One more term that bites: transfer. Solar warranties are generally written for the original purchaser, and few manufacturers transfer them to a second owner. That is half the price gap between new and used panels. Assume a used panel carries no warranty unless the paperwork in front of you says otherwise.
What this means when you are shopping
The honest comparison writes itself from the two promises. A new panel gives you a published degradation floor you can check, a product warranty in your own name, and a manufacturer to call. A used panel gives you no warranty, but a measurable machine: test it against nameplate and pay for what the meter says. A 10-year-old 395-class panel measuring near 368W is aging inside the warranty line of a new one. That is the same math this guide just did, and anyone selling you used panels should be willing to run it in front of you. We do.
Try this at home
If you are weighing a used panel, compute its warranty floor first. Take the model's first-year percentage and annual rate off the manufacturer's datasheet, run the subtraction for the panel's age, and you have the number a new panel of that vintage would be guaranteed to beat. Then ask the seller for a metered test in full sun and compare. Above the floor is a panel aging gracefully. Well below it deserves a lower price or a pass.
Sources
- Canadian Solar HiKu6 All-Black datasheet, models CS6R-380 to 405MS-HL, v1.1C25, Aug 2022: module efficiency 20.2 percent for the 395W class, dimensions 1722 by 1134 mm, the 25-year product warranty language and its residential-rooftop caveat, the performance warranty rates (2 percent first year, 0.55 percent per year after), and the LID/LeTID mitigation claim.
- The floor values (387W, 368W, 335W, and the year-15 figure in the quiz) are arithmetic from those two warranty percentages, stated as the warranted floor, never as expected output.
- The 1,953W incident-light figure is arithmetic from the datasheet dimensions and the STC definition of 1000 watts per square meter.
- Warranty transfer limits and the measured-output test for used panels: our guide New vs used solar equipment.
- Typical real-world degradation rates for this specific model are not stated on the sheet, so this guide gives none. What a warranty claim pays varies by manufacturer: check the current warranty statement for the exact model, every time.