solar panels VMP
Submitted: Wednesday, Feb 05, 2014 at 15:40
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Member - daz (SA)
This seems to confuse me especially when I am looking at buying another portable solar panel. My current 120 watt solar panel made by rich solar & purchased off EBay (Bit Deals) is specified as 17.8 maximum power voltage with a maximum power current of 6.74 amps.
I have been to
Home of 12 volt in SA who tell me their 120 watt Blue apple panels are A grade panels & I will admit are in a better frame & a much better leg support. But the VMP on the blue apple is 19.5 volts so therefore the mpc is only 6.44amps I know this is negligible but I would like to know why & how there is a variation and two is a 19.5 volt panel better than a 17.8 volt panel. I do know that some cheap panels are sold that are 14 volt mpv and understand that they will struggle to fully charge a battery.
As an aside are these cheaper panels on EBay a second grade panel or are they A grade as the sellers state.??
Thanks for your time to ponder, consider & reply
Daz
Reply By: Brian 01 - Wednesday, Feb 05, 2014 at 17:46
Wednesday, Feb 05, 2014 at 17:46
There is really very little difference between those two (about 5 watts), and it could depend on the actual test method used and the temperature of the panel when tested (Vmp falls as temperature rises, STC tests require the cell temp to be held at 25C and an air mass of 1.5, alter either of those downward and Vmp rises)
19.5 is a bit higher than the norm, and at that current would actually make that panel 125 watts output, so I would doubt the figures based on that alone (why would they advertise it as only 120w when it is 125w?).
Assuming the figures are correct, if you're using an MPPT controller, then the 19.5v one will theoretically perform marginally better.
Some Ebay sellers are providing very good panels at good prices and with excellent service.
One of those is Bit Deals in
Melbourne.
Some others grossly over-rate their panels, and a check of the relative dimensions versus claimed output will often disclose the anomalies.
Others offer 12v panels with extraordinary Imp, but a check of other specs will show a Vmp that is too low to be of any benefit to charging a 12v battery, don't touch anything under about 16Vmp especially if heading North in warmer weather.
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