The solar server’s array is planned around eight used 300 W panels from local listings at about $75 each. That’s roughly 25 cents a watt. Panels lose output slowly over the years, so a used one with no damage often has a lot of life left.
Buying used is also the greener choice. Most of a solar panel’s environmental cost is paid when it is made: the glass, the aluminium frame, the silicon refining. A panel pulled off a roof during an upgrade and put back to work on someone’s shed or server keeps that investment producing instead of sending it to a landfill or a recycler years early.
How much life is left
The best long-term evidence comes from a 2013 review by the U.S. National Renewable Energy Laboratory, which gathered nearly 2,000 measured degradation rates from panels in the field. The median was about 0.5% of output lost per year. At that rate a ten-year-old panel still makes around 95% of its original rating, and a twenty-year-old one around 90%. Many manufacturers warrant 80% or more at 25 years.
So age alone isn’t the problem. Damage is. A panel that has been cracked, cooked or soaked can lose far more than the slow, predictable decline, and some faults get worse quickly once moisture is inside. The checks below are about finding those.
Look first
Before any meter comes out, walk around the panel in good light and look at both sides.
- Cracked glass, even hairline cracks. Water gets in. Run a hand lightly over the face (carefully) and look at it at a low angle.
- Yellowing, bubbles, or a back sheet that’s peeling away from the cells. These mean the layers that keep moisture out are failing.
- Burn marks or melted plastic on the junction box. A sign that something inside has been overheating, often a failing bypass diode or a bad connection.
- Thin dark lines across cells, sometimes called snail trails. They can mean micro-cracks in the silicon underneath.
- Bent frames, brittle cables, damaged connectors. Connectors can be replaced; a twisted frame often means the glass has been stressed too.
Also read the label. It should list the rated power (Pmax), open-circuit voltage (Voc), short-circuit current (Isc), and usually a manufacture date. Take a photo of it; you’ll want the numbers for the next step.
Then measure
Test in full sun, with the panel facing the sun. Midday on a clear day is best. Readings on a hazy morning will be low for reasons that have nothing to do with the panel.
- Open-circuit voltage (Voc). Set the meter to DC volts and touch the probes to the two leads. It should come in close to the Voc printed on the label. Voltage depends mostly on the cells being intact, not on how bright it is, so a reading well below the label (say, a third lower) suggests a dead cell string or a failed bypass diode.
- Short-circuit current (Isc). Set the meter to its DC amps range, with the red probe in the amps jack, and touch the leads briefly. In strong sun you should see a good share of the label’s Isc. Check first that your meter’s amp range is rated above the panel’s Isc. Current tracks sunlight closely, so this is the number that drops on a cloudy day.
Low readings on a bright day point to a bad cell or bypass diode. Walk away, or haggle.
A few cautions. A panel in sun is always live; it can’t be switched off. Keep the leads apart, don’t touch bare metal, and never disconnect a panel from a working system while current is flowing, because the arc can damage connectors. If you’re testing several panels, write each panel’s readings on a piece of tape on its back. It makes the haggling easier and tells you later which ones to wire together.
Ask
- Where it came from and how old it is. The date is usually on the label.
- Whether it was ever cracked, dropped or left lying flat in the rain.
- Whether you can test it before paying. A seller who says no has answered the question.
It’s also worth asking whether the panels came off the same system. Panels from one array are usually the same model, with matching voltage and current, which makes them simple to wire together. A mixed lot can still work, but panels wired in series are held back by the weakest one.
When a panel fails the test
A panel with a cracked cell or a bad diode isn’t necessarily scrap. Some can still run a small load like a fan or a trickle charger. If it really is done, find a recycler rather than the trash. Panels are mostly glass and aluminium, and both are worth recovering.
Further reading
- Photovoltaic Degradation Rates: An Analytical Review, NREL, 2013. The source for the 0.5% per year median.
- PVWatts Calculator, NREL. Estimate what a given set of panels will produce where you live.
- iFixit. Repair guides, including for multimeters and connectors.
- A solar server’s power budget, what these panels are for.