Quote:
Originally Posted by 1der
Greg  perhaps you can shed some light on the combining of panels of different wattage. I have read what I consider very informed information about the inefficiencies of combining panels of differing wattage. Say a 100 watt additional panel is added to a my 300 watt panel. I am of the understanding that the 300 watt panel will be restricted by the 100 watt panel and thus the system will be 200 watts not 400. Is this correct? Does it matter if they are hooked up in parallel or series?
The other question is the practice of adding a portable panel to an all ready installed fixed system This is where the panel leads from the portable panel are added to the PVIn terminals on the existing solar controller. What happens here and is it really different from the above? Thanks!

Ray,
Sure, mixing panels of different specifications can lead to issues, and worse it can be easy to do because we tend to focus on the wattage rating of the panel. The wattage rating is really made up of two major specifications of the panels Vmpp x Impp = Wattage.
I previously had an Kyocera Panel 140 watt panel it had Vmpp of 17.7 volts and Impp of 7.91 amps to get 140 watts (17.7 X 7.91= 140.007). I was looking to upgrade to a second panel and looked at Grape Solae 180 watt panel. This panel had a Vmpp of 19.67V and Impp of 9.71 Amps.
The simple math would sat that I have 180 + 140 = 320 watts. But lets dig deeper, since I have an MPPT controller I could have put the new panel in series or in parallel, either one of these could give me different results. If I chose to put them in series I would added the voltage 17.7 + 19.67 = 37.37 Volts. The current in series would be restricted to the lowest current panel, in this case 7.91 amps from the Kyocera Panel. If I do the math 37.27 X 7.91 = 295.6 watts. That is a Far cry from the 320 watts expected.
If I chose to go in parallel I would now add the currents, 7.91 + 9.71 = 17.62 Amps. For the voltage we would take the lesser of the two panels, in this case 17.7 volts. We do the math, 17.7 X 17.62 and we get 311.87 watts. While again this is not the 320 watts that we expected it is better than 295, we would have produced in series. In short, I decided to sell the Kyocera and purchase two new GS– 180 panels, so that they were matched.
Now if you were to take some panels that were mismatched even worse, the problem gets even worse. Take a LG 300 watt panel Vmpp = 32 volts x 9.4 Amps & a Renogy 100 watt panel Vmpp of 18.9 X Impp of 5.29 amps. The specs are much farther apart, so let’s do the math again.
First in Series: 32 + 18.9 = 50.9 X 5.29 Amps = 269.261 Watts.
In Parallel: 9.4 + 5.29 = 14.69 X 18.9 Volts = 277.641Watts.
In both cases the combined output of the mismatched panels is worse than original 300 watt panel.
Generally, if you are hooking up a portable panel it is done in Parallel to the hardwired panel and it is done at the controller. Will this be a perfect solution, a lot depends on the specs of your panels, generally if they were close it won’t be so bad.
Hope this helps