Solar panel current measure help
Submitted: Sunday, Jun 19, 2011 at 07:46
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SmokeyD
Hi,
Just wondering if anyone can confirm how to accurately measure the current from a Solar panel.
I have a tripple panel (150Watt) monocrystaline solar setup, and have measured the volts, but want to know if the advertised 150w is accurate. The volt reading was 22volts - what should I expect in the amp measurement, and should I use a multimeter - short circuit current?
Paul.
Reply By: Member - John and Val - Sunday, Jun 19, 2011 at 07:59
Sunday, Jun 19, 2011 at 07:59
Paul,
To measure the short circuit current delivered by a panel, you MUST disconnect all loads and batteries, then simply connect a suitable current meter directly across the output. This will give you a measure of current that is close to the maximum available from that panel. The maximum power point, where the panel is spec'd, occurs at about 17 to 18 volts. So - multiply your current reading by say 18 to get the panel's power rating.
It is vital that there be no batteries connected while measuring current as the battery will supply lots of current and let all the smoke out of your meter! For this reason, if using a multimeter, it is good practice to always return it to voltage measuring mode after you've made a current measurement.
Cheers
John | J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
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Follow Up By: SmokeyD - Sunday, Jun 19, 2011 at 09:56
Sunday, Jun 19, 2011 at 09:56
Hi
John,
Thanks for your reply!!
So the power output would be read in watts, and the reading would depend on the sun's output to some degree?
Thanks for your info - I appreciate your wisdom :)
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Follow Up By: Battery Value Pty Ltd - Sunday, Jun 19, 2011 at 10:08
Sunday, Jun 19, 2011 at 10:08
Good advice
John,
I might add that multiplying the short circuit current with 16.5V will be closer to the mark because Isc is around 5% higher than Imaxpower.
Of course, the sun needs to be perpendicular to the panel's surface for the measurement to be meaningful.
And in real life, the max wattage one can expect from the panel is at least 10~15% lower because of cell heating. And this is with a 'true' MPPT solar regulator.
If not 'true' MPPT, the max useable wattage will drop by another 10~15%.
Have a great weekend.
cheers, Peter
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Follow Up By: Zebra400 - Sunday, Jun 19, 2011 at 10:36
Sunday, Jun 19, 2011 at 10:36
Hi Paul
We have a Tvan with a solar panel. We have a meter to read input, and so far the one panel on the roof of the Tvan has only generated 1 Amp/hr of power. It is not angled to the sun as it is flat on the roof. So far, we have been unimpressed with its capability.
Laurie
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Follow Up By: CSeaJay - Sunday, Jun 19, 2011 at 10:45
Sunday, Jun 19, 2011 at 10:45
Laurie
The TVan has a reputable brand solar panel.
But it is small - only 50 W. Good enough to top the batteries not much more.
Plus one also try to set up in the shade which makes it even less useful. Which is why a portanble set connected parallel is a good option.
CJ
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Follow Up By: Member - John and Val - Sunday, Jun 19, 2011 at 12:43
Sunday, Jun 19, 2011 at 12:43
Paul,
Power = voltage multiplied by current. The voltage is not that which you measure with no load on the panels, but as we are interested in power, is the voltage at which maximum power occurs. That's about 17 - 18 volts, though as Peter (a Queenslander!) says it drops with increasing temperature, so for him, 16.5 is maybe better.
There are a lot factors in getting an absolutely right figure. The panel should be perpendicular to a line drawn to the sun. (A good way of establishing the correct angle is to simply aim the panel so as to cast the biggest possible shadow.) Panels are spec'd at a radiation level of 1kW per square metre; in southern Australia at this time of year you won't achieve that radiation level from direct sunlight, even at noon. Any cloud has an interesting effect - scattered cloud may increase total radiation by reflection, or reduce it if interposed between panel and sun.
Using the approach suggested you'll get near enough for your purposes, but I don't claim absolute accuracy!
Cheers
John | J and V
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