# Solar panel power calculation

Posted by JS on October 20, 2008, 1:42 am

: quoted-printable

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?

JIMMY

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<DIV><FONT face=Arial size=2>I have few home made solar panels. I would like to

<DIV><FONT face=Arial size=2>in this forum about accurately determine rated
output power of the panels.</FONT></DIV>

<DIV><FONT face=Arial size=2>Can I just simply multiply open circuit voltage by
short circuit current?</FONT></DIV>

<DIV><FONT face=Arial size=2>My open-circuit-voltage is about 18V&nbsp; at full
sun and short circuit current</FONT></DIV>

<DIV><FONT face=Arial size=2>is 3.2A.&nbsp;&nbsp;&nbsp;Does that mean the rated
power (P=VxI) is about&nbsp; 57 watts?</FONT></DIV>

<DIV><FONT face=Arial size=2>If it does not, how I am going about determine the
rated power?</FONT></DIV>

<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>JIMMY</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV></BODY></HTML>

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Posted by Martin Riddle on October 20, 2008, 2:46 am

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?

JIMMY

No its not Isc * Voc.
You  need to charaterize the panel, by creating a I V curve.
V.
ex:
I V I*V
0 18 0
0.5 17 8.5
1 16 16
1.5 15 22.5
2 14 28
2.5 13 32.5
3 12 36    Max power
3.2 11 35.2

Or you can just measure I and V under the load you run.

Cheers

Posted by Bruce Harvey on October 20, 2008, 3:49 am
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?

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
Hope this helps.

Posted by Bill Kaszeta / Photovoltaic Re on October 20, 2008, 4:34 am The concept is correct, but the example is not correct.

Maximun current  is always at zero volts (Isc).
Current decreases as voltage increases, the maximum voltage is at zero current
(Voc).
At both of these points the power (product of volts x amps) is zero.

It is not likely that you have a variable load that you can attach so that you
can
measure the current at other voltages.  Try a 12-volt battery, it will start at
12.5 volts and increase to about 14 volts.  14 volts will be about the maximum
power voltage for a silicon solar cell panel with a Voc of 17 volts.  If you have
rather good solar cells, and no weak ones in the circuit, you could get a
maximum power of about 40 watts (clear day, noon, pointed at the sun).

Bill Kaszeta
Photovoltaic Resources Int'l
Tempe  Arizona  USA
bill@pvri-removethis.biz

Posted by Bruce Harvey on October 20, 2008, 2:21 pm Please show me where the error is. That is what my example says. 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. •
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• Date   Re: Solar panel power calculation Bill Kaszeta / ... 10-20-2008    Re: Solar panel power calculation Bruce Harvey 10-20-2008     Re: Solar panel power calculation Eeyore 10-20-2008     Re: Solar panel power calculation daestrom 10-20-2008   Re: Solar panel power calculation Bruce Harvey 10-21-2008   Re: Solar panel power calculation Mike 10-23-2008  Re: Solar panel power calculation Eeyore 10-20-2008