Technical review of 20 Watt Solar Panel Model DJ-20D
Customer Rating: 
Don't expect 20 Watts from this product. This is the second purchase of the Extreme Power Corp's 20W Solar Panel. The first unit did not meet the specifications defined on the label attached to the rear of the panel so I purchased this second unit to determine if the first unit was faulty. The second unit performed the same way. Here are the details:
I tested the two units at 1000W/Sq. Meter in the sun (Calibration by Davis Weather station solar sensor) at 25 degC and 55 degC into a resistive 16 ohm load (VPmax/IPmax). Here are the results, spec. values in parenthesis:
Open Circuit Voltage 19.4 volts (21.18V)
P Max at 25deg C - 16.5 Watts (20 Watts)
P Max at 55deg C - 12.8 Watts (Not specified but usual operating temp)
Voltage at Pmax - 14.45 at 55 deg C (17.5 at 25 deg C)
Short Circuit Current at 55 deg C 1.0 Amps (1.33 Amps at 25 deg C)
I reported these resulte to Extreme Power Corp but there has been no comment. Two points to bear in mind. 1. - There is a tendancy to overspecify solar panel performance and 2. - Specifications are for performance at 25 deg C and no solar panel operates at this temperature.
The maximum power output under any load conditions that I can get from these panels when cool (immediately after exposing to the sun) and at 1000 W/Sq. is 16.6 Watts.
In operation do not expect more than about 14 Watts from these panels - if that is ok then the product, although misprepresented gets a pass.
John Beukers
2009.05.19
Power point is a little wierd...
Customer Rating: The panel took about a week to arrive; the packing was "wierd" (bubble wrap wrapped
around the cardboard! ) but it was intact.
The frame is aluminium, however the backing is plastic and the front side is glass
(not Tefzel). The whole thing is sealed with copious amounts of silicone rubber.
The wierd part of this panel is that it's _not_ exactly a 12-volt panel. In
full noonday sun, it puts out 20 volts open-circuit, at 1.03 amperes. This is
somewhat higher in voltage and lower in amperage than many would prefer; you will
need a charge controller at the very minimum as this _will_ cook a 12-volt
battery if used for a long time. Be sure your charge controller can handle
it- most charge controllers are rated for 16 to 18 volts input and this panel
puts out 20 volts or a bit more.
The panel comes with a pair of 6-foot wires (with something written on them
in Chinese), one red, one black, and permanently attached to a box centered in
the back of the panel. The other end of the wires go to a pair of alligator
clips, but as warned above, don't just clip these onto a lead-acid battery
for long term use unless either the battery is really big or you have a
charge controller.
A downside: lead-acid batteries typically charge at 14 volts; any additional
voltage is wasted. So, this panel loses about 30% of the energy produced
if the charge controller is not an active (buck-boost) or maximum power point
tracking (MPPT) controller.
On the up side, the 20 volts open circuit stayed pretty solid even when
the sun went behind clouds; what dropped was the amperage (down to 0.2 amps
when the array was shadowed, down to 0.1 amp when the sun was _behind_ clouds.
But the voltage stayed up, which means that even on a cloudy day your panel
will get something into the battery, maybe only 10% of a good bright day but at
least something. I think (but am not sure) that this is because it's a
monocrystalline panel.
