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
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
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
Interesting idea. Not sure if I buy it though. Could be true. I bought sunpower panels that must be about 10 years old I'm assuming, and they produce over 90%. Came out to around $0.14/watt. The way I measured it is their peak power devided by the number of panels to get their current wattage vs rated.
I was so impressed that I ordered more, anywhere from around the same price to $0.30/watt. I don't even know what I'm going to do with all these panels
(prices are calculated at current wattage it is producing, not rated watts. Also includes tax and shipping costs, but not the tax rebate. I will be applying for the tax rebate)
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See hedges post above. It's a 10% hit but price isn't 80% less. It's actually a similar price per watts
fratermus
Solar Enthusiast
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The tax credit factor in that post is the tail that wags the dog. It would be interesting to see the % of solar DIYers on this forum that have received (or are even eligible for) a tax credit for solar.
In the Solar Systems section the Vehicle Mounted Systems subforum is by far the most active, ~2.5x the threads/posts of the runner-up, Residential Solar. I will concede that the residential folks have the advantage in total watts.
Hedges
I See Electromagnetic Fields!
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Any DIY PV on principal residence would be eligible. And would take it unless zero tax owed.
In a normal market, used PV would be 25% the price of new. Right now we're experiencing something like the inverted yield curve (higher interest paid on short CDs vs. long) in that some used panels are priced higher than some brand new.
e.g. Qcells and REC costing $0.27/W delivered, whereas New East Solar cost me $0.19 (including California tax.)
The price difference does make us ask if quality/durability follows suit.
But I've bought Sunny Island for $0.25/W and Sunny Boy for $0.10/W, new in the box. I understand and seek out liquidation sales.
Even "cracked backsheet" and "snail trail" panels have been offered at maybe 30% less than those N E Solar.
"If you don't mind oats that have already been through the horse ..."
In a normal market, used PV would be 25% the price of new. Right now we're experiencing something like the inverted yield curve (higher interest paid on short CDs vs. long) in that some used panels are priced higher than some brand new.
e.g. Qcells and REC costing $0.27/W delivered, whereas New East Solar cost me $0.19 (including California tax.)
The price difference does make us ask if quality/durability follows suit.
But I've bought Sunny Island for $0.25/W and Sunny Boy for $0.10/W, new in the box. I understand and seek out liquidation sales.
Even "cracked backsheet" and "snail trail" panels have been offered at maybe 30% less than those N E Solar.
"If you don't mind oats that have already been through the horse ..."
marionw
Solar Addict
Tax credits generally cannot be claimed on used solar panels except for the cost of installation into an existing system.
It must be claimed on the original installation of the equipment: If you remove the panels and put them on another property or install used solar panels, you cannot reclaim the tax credit. It must be the first time the panels are used. You can, however, claim the tax credit for any costs incurred when adding solar panels to an existing system.
www.solarreviews.com
It must be claimed on the original installation of the equipment: If you remove the panels and put them on another property or install used solar panels, you cannot reclaim the tax credit. It must be the first time the panels are used. You can, however, claim the tax credit for any costs incurred when adding solar panels to an existing system.
2024 Federal Solar Tax Credit: Expert Insights to Help You Save
The 2024 30% federal solar tax credit can help you save big on solar installation. This complete guide explains how it works and how to make the most of it.
www.solarreviews.com
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