Posted by *Eeyore* on October 20, 2008, 4:19 pm

Bruce Harvey wrote:

*> The OP just wanted to know how to do it. I just gave an example of how he*

*> could. I suspect that it is possible that he may have a potentiometer that*

*> would handle the job.*

How exactly would a potentiometer help ?

Graham

Posted by *daestrom* on October 20, 2008, 11:16 pm

Bruce Harvey wrote:

*>> On Mon, 20 Oct 2008 03:49:05 GMT, "Bruce Harvey"*

*>>*

*>>> I have few home made solar panels. I would like to ask you experts*

*>>> here in this forum about accurately determine rated output power of*

*>>> the panels. Can I just simply multiply open circuit voltage by*

*>>> short circuit current? My open-circuit-voltage is about 18V at*

*>>> full sun and short circuit current*

*>>> is 3.2A. Does that mean the rated power (P=VxI) is about 57*

*>>> watts? If it does not, how I am going about determine the rated power?*

*>>>*

*>>> Thanks in advance.*

*>>>*

*>>>*

*>>> JIMMY*

*>>>*

*>>> Jimmy,*

*>>> This is not correct. The Voltage and Current must be measured at*

*>>> the same point to get the Maximum Power Point of your panel. I will*

*>>> use your example,*

*>>> of 18Voc and 3.2A Isc.*

*>>> As you will realise, when the voltage open circuit (Voc), i.e. no*

*>>> load, the*

*>>> current (Ioc) is 0.0A, and when the current (Amps) is measured in*

*>>> short circuit (Isc) the voltage short circuit (Vsc) should be 0.0*

*>>> V Therefore: 18 Voc x 0.0 Ioc = 0.0Watts i.e. there is no usable*

*>>> power produced by the panel at this voltage.*

*>>> 0.0 Vsc x 3.2A Isc = 0.0Watts i.e. there is no usable power*

*>>> produced by the*

*>>> panel at this Current.*

*>>> What you need to do is connect a measurable load across the*

*>>> terminals to graph the output of the panel at different loads. For*

*>>> example using a load of some kind big enough to handle the panel,*

*>>> you need to measure the current*

*>>> in amps at different voltages and work out by multiplying the*

*>>> figures together what the maximum power output is.*

*>>> For example, you measure these at 1V intervals from say 6V to 15V*

*>>> and come up with a table of outputs similar to the following table:*

*>>> 06V 1.0A = 6W*

*>>> 07V 1.1A = 7.7W*

*>>> 08V 1.2A = 9.6W*

*>>> 09V 1.4A = 12.6W*

*>>> 10V 1.7A = 17.0W*

*>>> etc., etc.*

*>>> 15V 1.0A = 15W*

*>>> From this you should be able to get a pretty good idea of the*

*>>> maximum output*

*>>> of the panel. Keep in mind that other factors will effect the*

*>>> results like the insolation (sun power) during the test and the*

*>>> temperature of the panel*

*>>> among others, but this basic test will give you some idea of the*

*>>> actual output of your panel.*

*>>> Hope this helps.*

*>>>*

*>> The concept is correct, but the example is not correct.*

*> Please show me where the error is.*

In your example you show current at 1.0A at 6V and it rises to 1.7A at 10V.

What Bill is trying to tell you is that a solar panel won't have a rising

current like that.

The *maximum* current point is at 0V (Isc). As resistance in the circuit is

increased, the voltage will rise, and the current stays constant for a

while. In most silicon panels, the current is pretty constant from Isc up

to a point while voltage rises (as the external circuit's resistance is

increased from 0 ohms), then you reach the 'knee' of the V-I curve and a

further increase in voltage causes the current output to drop sharply toward

zero. The maximum power is right at the bend of the 'knee'.

The only trick is, the 'knee' shifts with changes in panel temperature.

daestrom

Posted by *Bruce Harvey* on October 21, 2008, 4:51 am

*> Bruce Harvey wrote:*

*>>> On Mon, 20 Oct 2008 03:49:05 GMT, "Bruce Harvey"*

*>>>*

*>>>> I have few home made solar panels. I would like to ask you experts*

*>>>> here in this forum about accurately determine rated output power of*

*>>>> the panels. Can I just simply multiply open circuit voltage by*

*>>>> short circuit current? My open-circuit-voltage is about 18V at*

*>>>> full sun and short circuit current*

*>>>> is 3.2A. Does that mean the rated power (P=VxI) is about 57*

*>>>> watts? If it does not, how I am going about determine the rated power?*

*>>>>*

*>>>> Thanks in advance.*

*>>>>*

*>>>>*

*>>>> JIMMY*

*>>>>*

*>>>> Jimmy,*

*>>>> This is not correct. The Voltage and Current must be measured at*

*>>>> the same point to get the Maximum Power Point of your panel. I will*

*>>>> use your example,*

*>>>> of 18Voc and 3.2A Isc.*

*>>>> As you will realise, when the voltage open circuit (Voc), i.e. no*

*>>>> load, the*

*>>>> current (Ioc) is 0.0A, and when the current (Amps) is measured in*

*>>>> short circuit (Isc) the voltage short circuit (Vsc) should be 0.0*

*>>>> V Therefore: 18 Voc x 0.0 Ioc = 0.0Watts i.e. there is no usable*

*>>>> power produced by the panel at this voltage.*

*>>>> 0.0 Vsc x 3.2A Isc = 0.0Watts i.e. there is no usable power*

*>>>> produced by the*

*>>>> panel at this Current.*

*>>>> What you need to do is connect a measurable load across the*

*>>>> terminals to graph the output of the panel at different loads. For*

*>>>> example using a load of some kind big enough to handle the panel,*

*>>>> you need to measure the current*

*>>>> in amps at different voltages and work out by multiplying the*

*>>>> figures together what the maximum power output is.*

*>>>> For example, you measure these at 1V intervals from say 6V to 15V*

*>>>> and come up with a table of outputs similar to the following table:*

*>>>> 06V 1.0A = 6W*

*>>>> 07V 1.1A = 7.7W*

*>>>> 08V 1.2A = 9.6W*

*>>>> 09V 1.4A = 12.6W*

*>>>> 10V 1.7A = 17.0W*

*>>>> etc., etc.*

*>>>> 15V 1.0A = 15W*

*>>>> From this you should be able to get a pretty good idea of the*

*>>>> maximum output*

*>>>> of the panel. Keep in mind that other factors will effect the*

*>>>> results like the insolation (sun power) during the test and the*

*>>>> temperature of the panel*

*>>>> among others, but this basic test will give you some idea of the*

*>>>> actual output of your panel.*

*>>>> Hope this helps.*

*>>>>*

*>>> The concept is correct, but the example is not correct.*

*>>*

*>> Please show me where the error is.*

*>>*

*> In your example you show current at 1.0A at 6V and it rises to 1.7A at *

*> 10V. What Bill is trying to tell you is that a solar panel won't have a *

*> rising current like that.*

*> The *maximum* current point is at 0V (Isc). As resistance in the circuit *

*> is increased, the voltage will rise, and the current stays constant for a *

*> while. In most silicon panels, the current is pretty constant from Isc up *

*> to a point while voltage rises (as the external circuit's resistance is *

*> increased from 0 ohms), then you reach the 'knee' of the V-I curve and a *

*> further increase in voltage causes the current output to drop sharply *

*> toward zero. The maximum power is right at the bend of the 'knee'.*

The current does not stay constant for a while as you state but changes at a

lower but increasing rate. The 'knee' of the curve you describe is the

maximum power point as you stated and therefore the most efficient point to

produce power for that panel at that temperature with a given amount of

sunlight.

*> The only trick is, the 'knee' shifts with changes in panel temperature.*

I did mention in my original reply that temperature and insolation are

factors to be taken into account but the OP may or may not need to know this

much detail.

*> daestrom*

OK point taken. This is an example I made up. It was a tool to describe what

kind of numbers the OP MIGHT get. Not an example of an actual panel test.

Bruce

Posted by *Mike* on October 23, 2008, 10:03 pm

On Mon, 20 Oct 2008 19:16:18 -0400, "daestrom"

*>The *maximum* current point is at 0V (Isc). As resistance in the circuit is *

*>increased, the voltage will rise, and the current stays constant for a *

*>while. In most silicon panels, the current is pretty constant from Isc up *

*>to a point while voltage rises (as the external circuit's resistance is *

*>increased from 0 ohms), then you reach the 'knee' of the V-I curve and a *

*>further increase in voltage causes the current output to drop sharply toward *

*>zero. The maximum power is right at the bend of the 'knee'.*

*>The only trick is, the 'knee' shifts with changes in panel temperature.*

A picture being worth a thousand words....

http://img171.imageshack.us/my.php?image=kyocerakd205gh2pengmarcqj7.jpg

--

Posted by *Eeyore* on October 20, 2008, 4:17 pm

JS wrote:

*> I have few home made solar panels. I would like to ask you experts*

*> herein this forum about accurately determine rated output power of the*

*> panels.Can I just simply multiply open circuit voltage by short*

*> circuit current?*

*> NO*

*> My open-circuit-voltage is about 18V at full sun and short circuit*

*> currentis 3.2A. Does that mean the rated power (P=VxI) is about 57*

*> watts?If it does not, how I am going about determine the rated power?*

*> You measure at the peak power point ideally. A course in basic*

*> electricity would assist.*

*> AND DON'T POST IN DAMN HTML !*

*> Graham*

> The OP just wanted to know how to do it. I just gave an example of how he> could. I suspect that it is possible that he may have a potentiometer that> would handle the job.