Adding solar is a complicated thing to do, but I have given it a lot of thought. My trailer has a 210 watt 12 volt panel on the roof, and two auxiliary 100 watt 12 volt panels that can be deployed on the ground. The aux panels are in series, forming a 200 watt, 24 volt system. Both the roof and aux systems have their own Victron 75/15 charge controller. These controllers are maxed out at 220 watts of 12 volt panels or 440 watts of 24 volt. They also can only do 15 amps charging at their max, which would be achieved at around 220 watts. Having additional solar capacity wouldn’t increase the max output of these controllers, but it would make it so they produce 15 amps for a longer period during the day. The 200 or 210 watt max was achieved by my aux panels around noon with no clouds, with the panels pointed in the right direction. Most of the day I seemed to get around 150 watts from them, and about 120 watts from the roof (which was never pointed in the right direction). These numbers are rough estimates and they changed all the time depending on clouds, sun angle, and shade from trees.
There are two options for adding more panels. I have ruled out adding to the aux system because I would need another two matching suitcase solar panels to make two pairs of panels in series that are in parallel with each other to stay at 24 volts. Four 100 watt panels would be under the 440 watt limit but would exceed the 75 volts open circuit voltage that my controller can handle, so I would also need a bigger controller. Victron has a 100/20 controller that would work within my physical space constraints, but now I am buying two panels and a controller for an aux system that is not always used. It would also be twice the work to set out and move 4 panels during the day instead of 2. And two panels are also pretty efficient, even making the 15 amp max in perfect conditions.
The second better option I think is to add to the roof. I have a 210 watt panel up there but hardly ever see that much because the panel doesn’t get moved during the day to track the sun. That being the case, there is nearly 100 watts of additional capacity for my 75/15 controller that is going unutilized most of the time. Adding panels up on the roof would get me the most bang for my buck, I think. I don’t have room up there for a second 210 watt panel, but I do have room for two 100 watt panels, on either side of the fan cover.
Adding more panels of different sizes is tricky though, because the whole system can be dragged down by the weakest panel. The best case would be to have matching amps and voltage in all panels so there is no weak link. Finding panels with matching stats is not as easy as it sounds.
My 210 watt panel is a Hightec Solar unit, very high quality. It has a Vmp of 22.4 volts and an Imp of 9.3 amps. It is still considered to be a “12 volt panel” because it doesn’t get above 24 volts. To match this, I will need to find two 100 watt panels that are about 22 volts Vmp and 4.75 amps Imp. The two 12 volt panels can be paralleled to make an equivalent 200 watt 12 volt panel that can be put in series with the existing 210 watt panel. Thus, I would be making a system that is about 400 watts at 24 volts, which my 75/15 controller could handle. The open circuit voltage would be under 75 with only three panels, so it should work. The goal would be to produce close to 15 amps of power (from about 220 usable watts) for most of the day. If I find that a lot of good watts above 220 are being wasted, then I could upgrade the controller to the 100/20 model, which can convert a max of 290 watts of sun to 20 amps of charging. I doubt I would need a controller capable of using all 400 watts at once because the angle on roof panels will rarely be optimal.
The best match for 100 watt panels that I have found so far is by Renogy. I have Renogy briefcase panels for my aux system and have been very happy with them, so I wouldn’t mind adding some of theirs to the roof. The Renogy panels have Vmp of 20.4 volts and Imp of 4.91 amps. In parallel, two of these would produce 9.82 amps, and this could then be put in series with the Hightec panel. Here is where the math gets tricky and I could use some help.
Two of the Renogy 100 watt panels in parallel is 20.4 Vmp, 9.82 A Imp
That in series with the 210 watt Hightec panel which is 22.4 Vmp, 9.3 A Imp
20.4x9.3 + 20.4x9.82
189.72 + 200.328 = 390.048 watts for the whole array (95% of 410 watts) at 40.8 volts
OR do I do it this way?
22.4x9.3 + 20.4x9.3 = 398.04 watts (97% of 410 watts) at 42.8 volts
Either way (using the lowest volts or lowest amps) would be great efficiency for mixing different panels in the same array. Which math (if any) is correct for mixing these panels?
Thanks in advance!
