@garfield28 It is very important when connecting a solar system to connect batteries to the controller before the panels or you can burnout your controller, when setting up a system you must decide whats the most important part of the system and this is where you put the most $value, for me this was the controller and batteries as if these fail nothing else will work very well, my main battery is a lifepo4 (lithium) that I made myself from cells imported from China, My main controller is a Victron 100/50 (100Volts and 50 Amps) my second (backup) is a Renogy PWM 30Amp, the performance difference between the 2 controllers is negligible but the Victron is top end hardware while the Renogy while a reasonable brand is 1/10 the price, but a backup.
On another forum it was stated that places like Ebay, Aliexpress, Wish and so on are just disposal sites for rejected products that didn't pass qc inspections an alike.
Now we know this is not entirely true but it is also not far from the truth either, I'm not saying don't buy from these places, but it is buyer beware!
the best way to know what output to expect from a solar panel before you buy is read the description, the size of the panel should be worked out for square meters then times it by 17, for example a panel that measures 1200 x 800mm = 1.2x0.8=0.96^2m =0.96x170=163W, so the output should be rated at 160 Watts there abouts, this is only a generalization but a good guide when buying, next is the sticker on the back that gives the Vmp (Optimum Power voltage), Imp (Optimum operating current), Voc (Open circuit voltage) and Isc (Short circuit current). when selecting panels you need to be mindful of how you are going to install them (in series or parallel) in series you add the voltages and the output amps is limited to the lowest rated panel, and when connected in parallel the amps are added and voltage output is limited to the voltage of the lowest panel.
Example if you have a 140w and 160w panel of different voltages but same amps (Imp) then there will be no losses connected in series, but if they have the same voltages and different amps then the output amps will be the lower of the 2 if connected in series. so if connected in parallel the amps are added but the output voltage is limited to the lower output panel.
I have two 145W and one 100W panels also a 200W blanket (backup) I know the 145W and the 100W panels outputs are correct but I know the blankets output is going to be approximately 50% of stated if I'm lucky, now when I hook these up it goes something like this, I connect the two 145W panels in series this gives me 2xVmp which is stated at 18V and 8.06A (Imp) so 18x2x8.06=290W so i have 36V at 8.08A going into the MPPT controller and coming out is upto 290W/13.2V nominal which is around 22A if the battery will accept that amount of charge, then there is the 100W panel well this only come out if the conditions aren't the best and I'm not getting the input required to fill my battery/s and the blanket is the same and they get connected via there own controllers just to keep it simple.
As others have stated the only real way to test a panel without solar test equipment is to test the Voc and Isc using a multimeter, or a clamp meter when connected to a discharged battery via the controller, either of these tools are considered a must have when dealing with solar, I have both.
