oh great... (solar panel watts)

It's "music" peak power ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

So, er, short-circuiting the panel would provide 100W of electrical heat, but for only a short period of time...? ;-)

Off by 50%... geez.

So we're looking at more like $10.50/watt if going with the Harbor Freight panel, eh?

The average place selling solar panels tends to not have a good grasp of how the things actually behave. For example, they tend to write the open circuit voltage and short-circuit current, then assume they'll get both at the same time. They multiply both to get a false max power value.

In practice, you get about 70% of that as realistic peak power in full direct summer sunlight, and you don't even get that in average days.

I'm guessing the manufacturer stated the oc voltage and sc current, and supplier multiplied those to get 100W. The manufacturer probably quoted 5.6A at 12V as a realistic max power point on the VI curve.

See:

for an interesting article using solar cells.

is worth a look too.

Speaking as someone who lived on solar power for some years...

Its normal to quote both open circuit voltage and short circuit current, these numbers are stamped on the back of most panels. They are good for comparing panels. You cannot multiply them to get the wattage.

My rule of thumb in buying solar (assuming a standard 36 cell panel) is not to pay much over $100 an Amp. Easy to remember. I still own 5 each 5 amp panels purchased for about $500 each.

Also look for panels that are certified to run 'partially shaded'. These have bypass diodes on every cell insuring that shading one of them will not kill the entire output.

Luhan "the desert rat"

I expect they are rating them at Open Circuit voltage X short circuit current at full sunshine. I expect the panel has 36 cells each with an open circuit voltage of about 0.5V to give an open circuit voltage of

18V. At 5.6A this gives 100.8 Watts.

A solar panel has an almost constant current output at a given illumination so the current will be maintained until about 16V. If you are using it to charge 12V lead acid batteries you will need about that as the voltage of a charged 12V battery is going to be close to 15V plus you need voltage for a diode and charge controller.

kevin

Not if you do it with relays...

Luhan

You can download the product manual from the site. All two pages of it which looks like it is for a 50W unit.

They specify and measure open circuit voltage, short circuit current and what they call "rated peak power" which isn't the product of the first two. It looks like they must have a variable load and do measure the maximum actual power generated. They don't specify at what voltage or under what illumination so it isn't very helpful.

It looks like the 50W (well the one with 50 in it's part number) unit is specified at not less than 45W peak power.

No, a real short wouldn't dissipate any power at all. P = E * I, and if E == 0, then P == 0. :-)

Cheers! Rich

Ha!

Thanks, guys.

Michael

That's 100 mW, where 'mW' stands for 'marketing Watts'.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Matter cannot be created or destroyed, nor can it be returned without a
receipt.

Yes. You get the 100W with a peak-power-follower circuit, which adjusts the voltage draw (using a buck-boost type circuit, which we all know about) so as to extract the optimum amount of power, which in this case is 17.8V at 5.6A. If the panel were rated at a nominal 12v output, that would be a "usable under many lighting conditions" rating, not a peak rating.

However, what you need to read is that it's FOR charging 12v batteries. It doesn't PRODUCE 12v. 17+ volts is about right for charging a 12v battery, which is normally 13.8v when fully charged. You need to have a positive delta voltage to force charge into a battery.

And the 5.6A seems to be pure fibbing !

Graham

You come from England- and I don't envy you. If you compare the radiation data for your country with those from here, we really have more than double the yearly kilowatthours. And the max midday radiation is over 1000W/m2 during 3 hours. This is what will output typically 5.6A with that panel at

17.1V. But this is only at 25°C, more realistic would be 15.5V at 50°C, nevertheless that current *is* reached here, for England it has to be derated appropriately. :-( Of course that would make this panel only a 95W model, unlike Siemens which rather specs conventional. But these figures are good for comparison and all PV panels are specd like this, similar to the Pdissipation of a transistor, which we know how to handle. A 200W Fet needs its case at 25°c for that. So you only show your ignorance in this metier, sorry to say.

--
ciao Ban
Apricale, Italy

Andy writes:

Harbor Freight is NOT a technically accurate supplier. Most products are overstated a bit --- not enough to be out and out fraud, but enough for a buyer who is REALLY interested in specs to look deeper than the catalog statement....

To accomplish this , they use "metric" watts. To convert "metric" watts to "american" watts, you have to multiply their number by XM, which is a general purpose variable constant.....

The VALUE of XM differs with each application. It can be determined by dividing the REAL ANSWER by the HARBOR FREIGHT ANSWER.....

Then, to find the REAL ANSWER, on has only to multiply XM times the HARBOR FREIGHT ANSWER......

Such is the glory of XM..... I used it a lot back in college when I copied the answer off someone's paper and had no earthly idea how they got it..... ...... XM rules..... especially when you had a prof that didn't really give a damn and only looked at the number you put in the answer box......

Andy in Eureka, Texas

Ah, there *was* something obvious I was missing! I wasn't used to downloading .pdfs from Harbor Freight for anything. ;)

Thanks...

Michael

Really ?

I saw no mention of 5.6A on the datasheet.

Graham