Solar on Roof -- How You Park-Series vs Parallel ? UPDATE

Update 8 March
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Got my second 255w on the roof and went camping where I could check it all out. Solar here in January with such low sun angle is not worth much, but I was able to get some (IMO) interesting measurements.

I have a 255w at the back sloped down a bit and another up front flat. Between I have the three 100s flat. (see photo in profile--wood shows where three 100s are in the middle. New 255 in front of that, old 255 in back by cargo rails driver's side)

The back 255 is on an Eco-Worthy MPPT 20 amper and the front 255 is on a Tracer MPPT 20 amper where the controllers are in parallel on the battery bank. The middle array of three 100s in parallel, is on a PWM Solar30 amper also in parallel on the bank. ( I will add a photo of the roof if that works out)

No solar in the woods at the campground, but went to the library in town. I was parked nose down a bit on the street. Sun was toward the back, favouring the back 255.

I got, back to front:
3.43a, 3.8a, 2.9a. ( total = 10.13) Trimetric said batteries were getting 8.4 amps so load was 10.13-8.4 = 1.19a

I wanted to see what happened if parked more level, so I moved the C to where it might be better. Sun a bit more to the side too. (Also note it was closer to high noon by then, so amps better) This time I got, back to front:
3.86, 4.4a, 5.2a (total = 13.46)

So now the front 255 was doing better. Hmmm, what would all this be like if I had the panels in series all on one giant controller, so amps would depend on the lowest amps panel? Oops?

If I understand it, in the second example, if I only had the two 255s, I would get two 3.86 = 7.72a instead of 3.86 + 5.2= 9.06a as I got with them in parallel.

I then went to a place where it turned out one of the 255s was shaded and I got nothing from it, but got 4.56 and 4.4 from the other two arrays. If they were the usual set-up all in series, and that end of the roof were shaded, what would I have got?

So all in all, I know about the wiring to be saved and don't park in the shade in the first place, but still---I think I am better off as is.

These low amps don't mean much for differences between them, but in summer at full amps that could mean significant amps.

I think maybe there is more to the story about series vs parallel. A typical RV roof is not flat, but part of it slopes down at the back, so it can depend on how you are parked wrt to the sun, which is the "weakest" panel that will set the pace if they are in series.

Never mind the ugly mess, I don't care about it. It works!

BTW a previous owner tossed the air conditioner and put in a fantastic fan, so that is what you see. I don't have rain cover for it, but could if wanted( not needed) Panels cover two skylights (don't care) and part of the fridge vent cover, no problem. Roof is a mess of Eternabond bandages I did, plus Dicor goops as required. :) It is how it is!

twodownzero
Explorer
Jan 16, 2019
I'm running 400 watts with an MPPT charge controller. All of my panels are in series now, but I've wondered if I'd be better off with them in 2x2, series/parallel. I believe I would, it'd just take more wiring.
185EZ
Explorer
Jan 16, 2019
That's hideous
just start the generator
BFL13
Explorer II
Jan 16, 2019
185EZ wrote:
That's hideous
just start the generator

Lucky thing is you can't see it all from ground level. :)

No built in gen anyway. I do carry a portable gen for when there is not enough solar to get by. That works too. Takes two hours for a 50-80 with my 75 amp converter/charger on the 450AH of AGMs.
azrving
Explorer
Jan 16, 2019
185EZ wrote:
That's hideous
just start the generator

They say there is one at every party. :)
BFL13
Explorer II
Jan 16, 2019
twodownzero wrote:
I'm running 400 watts with an MPPT charge controller. All of my panels are in series now, but I've wondered if I'd be better off with them in 2x2, series/parallel. I believe I would, it'd just take more wiring.

There is no way to know unless you can measure each panel's output in different configurations. It doesn't matter if all it means is your batteries are full by 3PM instead of by 2:30. In other scenarios maybe it would matter.

From what I got, where each 255w panel can be measured, it seems to vary quite a bit depending on which way you park even with them out of the shade. Unless you have a flat roof end to end and park level, series might not be optimum. Then you have camber with some RV roofs with panels tilted to opposite sides. If you follow installation instructions, panels should be tilted slightly to allow rain water to run off, so the RV roof can be ideal for that.

IMO it all leads to where one same wattage panel will do less output than another, but not the same panel each time. With series, that means it sets the total output pace.

Once you have your solar all done with only one controller, you can't see what is really happening, and would think that what you are getting is "what you get".

The point of the OP is to show the measurements I got, which is an aspect to the series vs parallel discussion I have not seen before.
2oldman
Explorer II
Jan 16, 2019
BFL13 wrote:
I then went to a place where it turned out one of the 255s was shaded and I got nothing from it, but got 4.56 and 4.4 from the other two arrays. If they were the usual set-up all in series, and that end of the roof were shaded, what would I have got?
Maybe the same thing if the shaded panel has good diodes. If no diodes, then zero...well, ok not zero but not much.
BFL13
Explorer II
Jan 16, 2019
Thanks on the shading effect.

The usual story with series is with no shading, you can calculate that each panel will do the same amps (same watts/panel), but that was not so with mine as described in the OP.

I am thinking that makes parallel the better choice, so you can just add the individual amps and get the most for a total.

In previous set-ups, I was unable to realize any measurable advantage you read about where higher voltage does better in low light overcast so there is still some voltage above battery voltage. So IMO that would not counter the loss in series where it uses the weakest panel's amps. ( I know you would have to do a bunch of proper comparison testing to prove that, but I have done some, and that's what I found. )
DarkSkySeeker
Explorer
Jan 16, 2019
How are you measuring the current? Does your charge controller display that? For each batt?
pianotuna
Nomad III
Jan 16, 2019
The "perfect" panel for RV'ing in the shade would have bypass diodes between each and every cell. Unfortunately the only maker I am aware of that did do that is UniSolar and they have been out of business for a dozen years now.

Since I happen to have them, my system 'punches' above its weight as far as output goes with 256 watts from 4 panels wired in series/parallel for an input of 33 volts. I know BFL13 wants to know my amps at 12 volts--it works out to 17, post controller, if the battery bank is hungry. (voltage circa 15).
BFL13
Explorer II
Jan 16, 2019
DarkSkySeeker wrote:
How are you measuring the current? Does your charge controller display that? For each batt?

I have three controllers, one for each array as in the OP. Each controller has a display showing the amps it is getting in. The Trimetric monitor shows what the battery bank is getting/accepting out of that total.

The 450AH bank is not split, so all three controllers are in parallel to the one bank. The three are set to the same voltage, so they add their amps.

However, the Tracer does have a two hour time after the batts reach the set voltage, while the others do not have that time limit.

In a long day of solar, after the bank reaches the set absorption voltage and then two more hours, the Tracer will drop to 13.6, so no amps anymore from its 255w panel. By then, the total amps from the other two controllers will likely be more than the bank will accept anyway, so no matter. (Where we use solar, it is ok to stay at Vabs till sundown--no need to reduce to Float during daylight time)

I have this arrangement from using my collection of solar stuff from previous set-ups in previous RVs. If starting from scratch of course you would not do it this way. However, it is from doing it this way that I discovered this OP aspect to the series vs parallel discussion.

Normally, you can't tell how much each panel in an array is providing towards the total array output. After this, I now picture all you folks up there with clamp on ammeters poking at your combiner box inputs to find out! Too bad for those with panels in series and the junction boxes out of reach under the panels :)