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Measuring the Isc of small solar panel

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Peabody:
I bought this solar panel:

https://www.ebay.com/itm/184566218058

The open circuit voitage under full sun is about 5.7V, but when I connnect my ammeter across it to measure the short circuit current, I get about 40mA, which doesn't seem like "1W" to me.  Am I doing the measurement wrong, or is it just an example of Ebay BS?  I get same result with my old analog meter.

If anyone knows of a US domestic source of an inexpensive  5V panel with Isc of 200mA or more, please provide a link.

TheMG:
While it is entirely possible that the panel is overrated, keep in mind that there are many factors at play when it comes to the output from a PV panel.

First of all, they are rated under IDEAL conditions, which you will almost never get. The solar irradiance/potential in your geographical area, weather and smog conditions, time of day, and angle of the solar panel toward the sun, will all affect the maximum output you can get from it, and in most cases that is going to be somewhat less than the maximum rating. So you always want to size your solar panels a bit larger than your energy requirement.

In this case, I would say the panel is very likely overrated. It's physically very small and has an even smaller effective surface area as the individual PV cells occupy only a small amount of the total area! I don't see how you could ever get 1W out of that.

Rough math:
Panel dimensions 132mm x 63mm, total surface area 0.008316 m2

Based on the pictures, I'm estimating only about 40% of that is effective area (actual PV cells): 0.0033264 m2

1 watt / 0.0033264 m2 = 300.625301 watts per square meter

The sun energy reaching the earth: 1360 watts per square meter, only about 70% of this or about 952 W/m2 actually reaches the surface, at most.

That would make your panel theoretically 31.5% efficient in order to produce 1W of power. Even the best of the best solar panels money can buy nowadays, are about 21-23% efficient.

So yes, I'd say that eBay panel is greatly overrated.

Manul:
I like the "2000mAh" part of its description  :palm:

If you tested it under clear sunlight, not angled and not through window glass, I guess it is quite overrated.

Peabody:
Aside from the price, would this be a reasonable choice?

https://www.digikey.com/en/products/detail/anysolar-ltd/SM531K08L/9990469

I was hoping for battery charging current of about 120mA, and about 40mA for the Arduino and other stuff.

Renate:
I think that you have to really check your load current, your battery capacity and the worst conditions.
Will you want this to run 24/7 in the wintertime where you only have 8 hours of sunshine and a couple of overcast days?

Are you limited for space? Does this have to be installed on the device/package?

Spending $18 for a Watt doesn't seem like good value.

Here's 15 Watt for $30: https://www.digikey.com/en/products/detail/tycon-systems-inc/TPS-12-15W/14296570
Note also that this gets you a real outside unit with glass and frame, not just a slice of silicon.

With solar there is always some time when your capacity is overkill and some time when it's insufficient.

(I run 2 x 300W and in the worst winter weather usage is restricted, in summer the electrons fly out my ears.)
300 Watt < $1/Watt

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