Used solar panel produce less... ?
- Thread starter garyc57
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sunshine_eggo
Happy Breffast!
OffGridInTheCity
Solar Wizard
I bought 9yr old, used panels from Santan Solar and they tested 95%+ original specs. If you buy some, simply point at good angle / clear sky to the sun and 1) measure voltage with voltmeter and then 2) short them and measure amps with clamp meter and then 3) volts * amps = watts to confirm output.
That would not measure real MPP output.
Bluedog225
Texas
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They have some nifty solar panel testers on amazon. Work great.
OffGridInTheCity
Solar Wizard
Not sure what this has to do with the question about how well used panels work.
It's true, but your power output formula is ridiculously wrong.
Open circuit voltage x short circuit current..? Try to measure them contemporarily!
OffGridInTheCity
Solar Wizard
Not 'ridiculously' wrong at all, but a simple a way to confirm the panel is working. Open circuit vs load is only a few volts - say 10v * 8a = 80w. The method will expose bad diodes as well. Even better is to get a known good panel and do the same test on both and compare the used against the known good one.
When I bought 40 x used panels from Santan Solar I used the volts * amp method and got 95% of original specs AND THEN.... after hooking them up it confirmed by the actual MPPT charge controller output that they were good panels (90%+) and worth putting up.
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Texas-Mark
Solar Addict
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Measuring current directly across a panel output is going to tell you the max current the panel can output (assuming you are doing this in full sun). Perfectly valid way to test used panels.
It is ridiculously wrong because when you measure open voltage, current is zero, so power is 0.
When you measure voltage and current at mppt, you get max power.
Obviously, your stupid panel doesn't know anything about that mppt and responds to your power draw either fulfilling it or with a voltage collapse.
When you measure voltage and current at mppt, you get max power.
Obviously, your stupid panel doesn't know anything about that mppt and responds to your power draw either fulfilling it or with a voltage collapse.
Your formula is greatly over estimating the panel power output. It assumes power is Voc * Isc.
For example, Canadian Solar CS6R-395MS-HL panel as these specs under STC:
Voc = 36.6 volts
Isc = 13.77 amps
Power = 504 watts (this is wrong)
When operating at max power point:
Vmp = 30.6 volts
Imp = 12.91 amps
Power = 395 watts
Your method over estimates the panel power by 28%. You don't get Voc and Isc at the same time.
Only truly viable way to measure panel performance is when attached to an MPPT or panel test meter, and you have an irradiance meter to measure the solar input. You can then scale the panel rating by the irradiance and compare the operating voltage and current to the panel rated power.
Mike C.
Texas-Mark
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MPPT current is never going to be more than directly measuring current out of the panel outputs with an amp meter. And for the purposes of testing used (or new) panels, this works just fine.
I wrote max power, not max current.
And I wrote that formula Pmax = Voc * Isc is ridiculous because it does not exist!
It's like measuring max flow of water when the valve is closed
Bluedog225
Texas
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We’re usually pretty civil in this forum. Be nice to keep it that way.
Rednecktek
Solar Wizard
Keep in mind that panel warranties are usually 25yr, which means that after 25 years in service, they'll only be at least 80% of rating. Dust, leaves, smog, bird krap, those will all affect a panel more than being a little old...
Especially for the price! Paying 80% less the cost of a new panel for a 10% hit in potential output? Yes please!
Especially for the price! Paying 80% less the cost of a new panel for a 10% hit in potential output? Yes please!
sunshine_eggo
Happy Breffast!
I believe the intent, but not the results. Too many examples of underperforming used panels on the order of 10-15 years old.
+1000%
MichaelK
Solar Wizard
I did a side by side comparision under load of my new retail purchased Renogy panels, and a set of Astronergy panels I bought used. The used panels beat the new panels by 7% output. I'd say it's more a function of the brand than just the age. Quality used panels will outperform mediocre new panels.
Hedges
I See Electromagnetic Fields!
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Isc would be a simple and good way to determine degradation. Imp and power would be proportional to that.
It doesn't necessarily detect damage. I had a few where Voc and Isc were in family, but Vmp x Imp was less than half of good panels.
Quality undamaged panels will have great output even used. If price is right, a good deal.
Have to compare to 70% (after tax credit) of cheap new panels (assuming they are quality, something we aren't sure about), which we've bought between $0.17 and $0.30/W
Some panels have degraded badly. That could happen to new ones you buy. I looked for brands that held up well in reports of HAST highly accelerated stress tests, although not same model.
It doesn't necessarily detect damage. I had a few where Voc and Isc were in family, but Vmp x Imp was less than half of good panels.
Quality undamaged panels will have great output even used. If price is right, a good deal.
Have to compare to 70% (after tax credit) of cheap new panels (assuming they are quality, something we aren't sure about), which we've bought between $0.17 and $0.30/W
Some panels have degraded badly. That could happen to new ones you buy. I looked for brands that held up well in reports of HAST highly accelerated stress tests, although not same model.
Most common degradation is caused of low quality laminates (mainly EVA) and adhesives reacting to UV, humidity and heat. This degradation starts quite early, within first years. So 5 years old panel with no signs of degradation/discoloration could actually be safer investment than new panel.
If you calculate like that, then nobody should ever buy used panels.
New panels for $0.17-$0.30/watt minus 30%, vs. used panels not getting any rebates, makes no sense to buy used panels for same price or more
