Author Topic: Ebay "Camping Solar Panels" listings with impossibly high power ratings  (Read 1547 times)

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Offline ozzee

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I'm in the market for some solar on my upcoming but now postoned due to covid19 camping trip.

Blindly I decided to get some "flexible 300W" panels. I did a quick current measurement and found they barely produce 130W. I'm now on my fourth purchase of panels and it's not looking good.  I had one listing saying it was 24% efficient but there's no other production panels saying 24% efficiency.

I escalated this to Ebay and they basically said "we're working on it" and failed to shut down any of the listings as far as I can tell.

Is this a common problem?  I have roof panels and they produce close to their rated power even in mid July.

I've referred this to NSW Fair Traiding so hopefully I'll get an answer in a couple of days.

Maybe Dave can do a Camping Solar Panel shootout and see what Ebay does about it then...

BCF has similar panels but their power ratings seem to be much more reasonable and likely correct. Their prices are less competitive but at least you can hope to get what you paid for.
 

Online BrokenYugo

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It can generally be assumed that Chinese watts are about half the size of a regular one.
 
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Offline beanflying

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Current measurement under what Voltage conditions for what panel/controller?

With Solar you need to look at a complete spec at best efficiency point and also ensuring you are facing the Sun and putting in a nominal 1000W/m2 before deciding if a given panel lives up to a claim or not. Simply stating a Current while the rest of your system (charger/controller/battery etc) is pulling the Panel below MPP wont really give you a complete answer.

Even doing all that properly I would tend to still be 20-30%+ skeptical about evilbay claims for performance.
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Offline ozzee

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One instance:

Looking at the power leads connected directly to the panel. A fully illuminated panel perpendicular to incident midday sunlight with a rated short circuit current of 17A delivers 7.8A. I'm pretty sure that being in the middle of winter does not make a 55% difference in power generation. The nominated max power voltage is 18.4V so the upper limit power is 144W, probably more like 125W when you take into account the drop in current from short circuit to 18.4V. These panels are listed as 300W and they're about 1m2 in size (the active power generating PV area a little smaller.) Hence, at 300W these panels would need to be around 30% efficient based on the listing's provided specifications. And; they're German certified, I forgot to mention that.

Maybe the sun shines brighter in China?

 

Offline beanflying

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If you are looking at the leads you are not looking at a short circuit you are looking at two low value resistors plus whatever connectors and fuses are fitted. While it won't make a lot of difference it is still is important how you test in particular when you get above a few Amps.

In particular at or around zero volts the burden voltage of your meter will never read close to accurate as it too is a resistor (unless you are using a hall effect or similar one). Depending on your meter and the leads on it your actual error might be a lot https://zone.ni.com/reference/en-XX/help/370384V-01/dmm/burden_voltage/ Typical DMM leads unless very high grade above a few amps are never going to give a decent result even at higher voltages (non/near shorted).

The main point is here testing method and equipment and how it is used is important. Typically an external shunt should be used and clamped across the panel and then the voltage across that shunt measured to get a current reading.
« Last Edit: July 14, 2021, 08:18:27 am by beanflying »
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Offline beanflying

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Typical shunts like these. The smaller one is 100A rated and the larger 500A. So you place these inline and measure Voltage in mV in this case using your DMM. Generally you would use one a fair bit higher rated than your actual current so you get less temperature issues.
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Offline ozzee

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If you wanted a very precise measurement, then you would employ a hall effect or other current sensor, however, in this case, even if the drop across the leads is 5V say, it will make no appreciable difference to the current. Typically, a solar panel looks like a current source until it gets close to its maximum power point in which case there is of the order of a 10% drop in current.

We're talking more than a 50% difference in expected current. I'm pretty sure a simple 20A multimeter should provide adequate precision. I did test it using 2 different multimeters, both provided almost identical current readings. But even then, the improbably high efficiency should be a dead givaway the specs are dodgy.
 

Offline beanflying

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If your voltage drop across the meter and leads is 5V then you don't have a short circuit to measure a figure for ;)

Have a good read of the link above on burden voltage. Dave also has done some specific videos on it if you have a search. It can get you very large errors depending on the meters and method. Also even 20A rated DMM's will unlikely have 20A rated leads and in particular you need to have very good contacts at that sort of current (Bolted/Screwed and lugged generally).

Still no argument here about the claims being bogus either but if you want to make a case your figures and methodology need to be accurate.

Also worth a read https://www.fluke.com/en-us/learn/blog/electrical/can-you-live-with-the-burden
« Last Edit: July 14, 2021, 09:33:55 am by beanflying »
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Online Gyro

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It can generally be assumed that Chinese watts are about half the size of a regular one.

I don't know what the fuss is about - it's still enough to power a 1kW PMPO amplifier. ;D
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Online tunk

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May explain some of the difference - the following page says that the
average dayly power in Sydney in july is less than half that of december:
http://www.solarelectricityhandbook.com/solar-irradiance.html
 

Offline thm_w

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Why are you on your fourth purchase? If you know 1m2 is not enough to produce the required wattage, buy larger panels or more of them.
That or buy from a reputable brand, then you know the power rating is realistic.

Here is a 0.1m2 panel claiming to be 300W: https://www.ebay.com/itm/373618922094

If you used 3 of these legit units then thats 1.85m2: https://www.lensunsolar.com/index.php?route=product/product&product_id=406
 

Offline Someone

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May explain some of the difference - the following page says that the
average dayly power in Sydney in july is less than half that of december:
http://www.solarelectricityhandbook.com/solar-irradiance.html
The majority of solar exposure data is in that form of kWh/m2/day, typically averaged over some number of measurements. This includes variables such as weather (cloudy/sunny) and number of hours of daylight. So this is misleading if you are trying to compare midday perfect clear day spot measurements (as the OP is wondering about).

There are seasonal variations in the daily peak on clearest day, and they're larger than you might expect. But data is much harder to find. This is why panels are tested  with a simultaneous measure of the actual solar insolation at the time of test, at the location of test.
 

Offline ozzee

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Yeah, I read that (http://www.solarelectricityhandbook.com/solar-irradiance.html) and it talks about light on the ground but doesn't talk about energy density (i.e. perpendicular to the sun). My roof solar panels don't see anything like 50% reduction. They show about 20% lower but, they're angle of incidence is horrible in the winter. Let's assume I aim panels very poorly and I get only 80% of the rated power, that still means I should expect 240W from a 300W panel. If these "300W" panels were sold as 150W panels, I'd probably shrug my shoulders and move on with my purchase.
 

Offline ozzee

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I actually did measure the voltage across the multimeter leads on the batch of 350W panels and it was 1.22V for one multimeter pulling 9.6A and .98V for the other pulling 9.8A (same current).

So let's assume there is another 2V across all the other parts of the circuit (probably much lower than that). So, the current (9.5A) should be within 2% all they way to probably 15V, there's no way there was 15V across the leads. So in the case of solar cells, you are measuring the current to within +-2% of a real short circut even if you have a multimeter with a horrible shunt resistor.

Besides, for sake of erring on the side of being optimistic, I just used the measured quasi short circuit current as the maximum power current (which it probably is much less, maybe 10% so) and you will find that the power even optimistically is less than 50% of the specified panel power.

So I give you the point, it would be nice to be accurate, but in this particular circumstance, it's not making a material difference in the conclusion if you have a few volts (< 10V) across the panel when you're measuring for the short circuit current.
 

Offline beanflying

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You need to go back to basics before you include testing losses is the point.

Take the nominal 1000 W/Sqm at the equator at midday as ideal with no atmospheric issues. This is generally accepted as about 70% of what hits the atmosphere.

Shift South to Sydney while the Sun is near Peak North. Haven't done the trig or the maths but at a guess 30% loss? So 700W/sqm as a quesstimate this time of year but do the numbers if you want to make claims of not living up to performance. So roughly 'assuming' claims of the seller are valid peak this time of year it would be reasonable to expect circa 200W assuming the supplier is being completely 'honest'  >:D about the 300W claim.

Add to this your measurement losses which you are understating by a lot and your methods are not taking these seriously when it comes to short circuit current.

If as you stated you were getting 130W from the panel answer us what controller (PWM or MPP or None) were you running and is it running at MPP? What are the losses through the controller likely another 10% loss so you are back to 180W maximum this time of year.

Before long you are looking best/worst case of an exaggeration of 20-25% by the seller/supplier - maybe.



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Offline Ben321

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The spectral response of the panel is also important. That solar intensity measurement of 1000W/m^2  is for ALL wavelengths that can pass through the atmosphere, from UV through IR. Your solar panel likely has a certain spectral response curve, which only makes use out of a portion of the available wavelengths. I wouldn't be surprized if your solar panel could only use 50% of the available wavelengths (and with many of those only slightly contributing to the electrical output of the panel). So maybe a 30% to 40% actual panel efficiency. My guess is that the specs provided by the manufacturer were based on simply multiplying the solar intensity by the surface area of the panel, and NOT taking into account the panel's efficiency, resulting in an artificially high maximum output power being stated in the specs.
 

Offline ozzee

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I assume that the specified power of a panel to be that which can be achieved in ideal natural conditions. i.e. clear sky, i.e. minimal atmospheric attenuation but still what can be achieved in the natural environment. Sure, if I concentrate sunlight via mirrors I'd be able to make any panel push whatever power you want (before melting it that is) but that's not the expectation when a customer sees the power specification.

Sure, If I was seeing around 75% of the specified short circuit current, I might doubt myself, but these panels were producing far less than that.

Covid's put the cabosh on the whole trip now I'm not in a rush to get the panels sorted out.

I am planning to get some older model 300W roof panels and trying them out. My roof panels seem to generate much closer to the specified current (80%) and they're not even oriented optimally so maybe I raid my roof panels :-)
 

Online Kleinstein

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The standard condition for the PV panels are 20°C temperature of the cells and 1000W/m² with an AM1.5 spectrum. That is the wavelengths if the light goes through 1.5 times the normal clean air. So this would be a clear sky day with the sum coming in at some 40° Angle.  So the winter conditions may be relatively close to the standard. The temperature  mainly effects the voltage and has little effect on the current. Here low temperature may actuall reduce the current a little (like a few percent - so still not relevant). Because of temperature even legit panels  don't get the nominal power very often (maybe on a sunny day in spring with some wind).

The current usually does no depend much on the voltage at the panel, as long as the voltage is well below the MPP or open circuit voltage. So the maybe 1 or 2 V drop at the cables and meter does not make a big change. The main point is the angle of incidence (can be calculated from the cosine) and the actual intensity. This can vary a bit with more or less mist and humidity in the air which effects the NIR part and is not directly visible to the eye. So the roof mounted PV panel could help has an indicator.

24% efficiency would be really high end. Possible but only at a price - more like premium grade for maybe a solar plane of solar racer.

Half the claimed current would be around 12% efficiency - more like the polycrstalline grade. It should be visible if the cells are mono or poly type.
One should still be very confident in the meter to measure the current - there are fake panels, but also broken DMMs.
 
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Offline mzzj

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Lots of flexible or ”camping” power panels on ebay that have no way to produce the stated watts as they don’t even have enough surface area to produce the advertised watts.  No need for complicated measurements when 500x1000mm panel claims to output 300w 🧐

90% of the flexible panels from chinese junk sellers are suprisingly … junk with only 30% power. 

 

Offline mzzj

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Forgot to mention that I have been looking for flexible panels also by myself and it seems like good one are lot more expensive than rigid rooftop panels. Not really suprise considering the production volumes.
 


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