Solar Panels: Series Vs Parallel Array

[MandN]

Due to a mistake, pointed out to me by Airmiles in a previous post: (https://www.airforums.com/forums/f44...tv-205468.html), I have a perfect situation for a head to head comparison of two solar panels wired in series vs two solar panels wired in parallel. I conducted several tests below to decide whether I should re-wire so that all panels will be wired in parallel or alternatively in a series-parallel configuration. (series-parallel means two panels wired in a series to each other AND in parallel to the other two solar panels which are also wired in a series to each other).

As many of you know, wiring solar panels in a series increases total voltage and hence allows you to run thinner wires as the voltage drop becomes less significant, they will also 'kick-on' sooner in the AM or under lower light conditions. However solar panels wired in a series are more susceptible to partial shading (so if one of the solar panels has a little bit of shade on it will actually decrease total yield from both panels). People have compared series vs series-parallel arrays between RVs but here was a perfect opportunity to test them against each other on one roof, one system.

Set up:
Airstream is aligned north to south. It is late February so sun is still in the souther sky through out the day. 2 panels in the front are installed in a series and 2 panels in the back are wired in parallel. Also notice that due to the airstream curvature, I did not perfectly level the panels (due to the brackets that I had and perfect level panels (aligned front to back) on an airstream detract from the aesthetic (in my opinion). Readings from Victron MPPT. Tests as follows: plug in one set at a time wait 2 min for power to stabilize then take reading, swap and repeat. Images included below as PDF attachment.

Test 1:
Midday sun overhead but in souther sky, slight haze. Getting 152 Watts from series connection pair and 151 Watts from the parallel connection pair.

Test 2:
Partial shade (tree branch shadow simulation) laid across one panel of the pair. (see pics). 82Watts on the series and 92Watts on the parallel array.

Test 3:
Second half of the day. Sun in the western sky (pic) which creates relative partial shade for the east side panels of both the series and parallel arrays. 89 Watts parallel and 79 watts Series. Repeated the same test in the AM with sun in the eastern sky, and got the same 10 watt difference.

Conclusion:
I will be wiring all my 4 panels in parallel rather than in a series-parallel config. Yes I do get that extra watt per hour throughout the day (likely secondary to gaining about 1% from reduction in voltage drop), but the shading issues with my set up causes significantly more loss. If you decide to wire in series-parallel configuration, make sure that your panels are perfectly flat on the airstream roof to avoid the daily relative partial shade situation (see test 3 above) and make sure you are not getting partial shade from roof objects (most important) and/or stray singular branches (having a uniform light leaf canopy above your airstream probably will not make a difference).

[AirMiles]

Thanks for sharing your test. Looks pretty realistic to me. I don't think 10W per 200W is going to make the difference of whether your batteries get charged or don't get charged in a day. When I've needed to use my generator for charging is when I get 100Wh from a full day of charging from 400W of panels due to extremely dark conditions from either a full thick canopy of tree shade or extremely dark rainy days. On typical solar days, my batteries are fully charged before the late-day sun affects the output. Actually, my batteries are frequently fully charged before peak sun each day.

With 400W of solar, you can configure either in parallel or series-parallel on the factory prewire and you'll get fully charged batteries either way. With 600W, you must configure series-parallel because the amps running over the prewire would be too high.

The bottom line is to configure in the manner you feel will work best for up to 400W and configure in series-parallel for 600W on the factory prewire. I would choose parallel for 200W and series-parallel for 400W and 600W.

[pteck]

MandN

Thank you for taking the time to perform this objective test. It demonstratively shows the factors at play and reinforces what has been recommended by professional installers, engineers, and notable experiments on the web for awhile now.

For a static installation (i.e. house) where shade conditions are measurably accounted for and well controlled, one can optimize for transmission of power (distinct from production) with a series installation. The start earlier assumption is a false premise that's continually propagated on these boards.

On an RV, where shade, conditions, and other factors are completely variable, one should always optimize for production with a parallel setup as you have demonstrated. If one wants to also optimize for transmission too, it's really not too bad to run heavier gauge wire. Best of both worlds.

Great point on the daily relative partial shade due to angles of the various panels. As well as roof mounted gear. Both sources of shade that again are typical and unique to an RV installation vs a static structure.

[Crackerman]

Most mppt controller need to see higher than battery voltage to get the charge started. This makes room for the argument of series panels, that higher voltage buys you more time in a charging state throughout the day.

I do prefer a parallel configuration if all 4 panels are the same. But, if another pair is v retrofitted, and voltage differences are present, I will do a series/parallel with 1 old, 1 new.

[pteck]

Originally Posted by Crackerman

Most mppt controller need to see higher than battery voltage to get the charge started. This makes room for the argument of series panels, that higher voltage buys you more time in a charging state throughout the day.

That's a false premise.

By design, every panel has a nominal voltage some 30% higher than the system voltage.

Panels are a power source. Not voltage source. If panels cannot maintain higher voltage than system voltage to actually deliver power output, two in series won't magically create any meaningful power either.

[JGF79]

My 2018 Int 27FB has the 160w factory panel options with a third connection point on the roof....is it wired series from the factory or parallel?

[AKNate]

Originally Posted by JGF79

My 2018 Int 27FB has the 160w factory panel options with a third connection point on the roof....is it wired series from the factory or parallel?

They should all be in parallel. If they weren’t your two ports wouldn’t work until you connected a panel to the third or jumpered it out.

[Crackerman]

Ptek


In term of wattage, yes. However, say most panels in the 100w range are 15to 17 ish volts. Low light situation, you MAY see 12-13.... Maybe.

In series you would see 24-26. More than enough to get the controller turned on and generating SOME power.

This would be especially true on ones running a 24v system.


In reality though, through some personal testing, and other's experiences, parallel offers better production throughout the entire day than a series or series/parallel setup would.

[AKNate]

A problem with the test as performed is that the series panels were arranged on opposite sides of the camper. With the curvature of an Airstream, one panel is always pointing away from the sun (except at exactly noon) and limiting the output of the other. I would never recommend doing series that way. Nearly everyone who uses series does so in a series/parallel arrangement. You want to make sure panels on opposite sides are paralleled and then you series the sets together. This gives you the desired voltage advantages without much of the series shading issues.

[pcskier]

Originally Posted by AKNate

A problem with the test as performed is that the series panels were arranged on opposite sides of the camper. With the curvature of an Airstream, one panel is always pointing away from the sun (except at exactly noon) and limiting the output of the other. I would never recommend doing series that way. Nearly everyone who uses series does so in a series/parallel arrangement. You want to make sure panels on opposite sides are paralleled and then you series the sets together. This gives you the desired voltage advantages without much of the series shading issues.

With the trailer aligned north/south, both sets of panels had equal angles/sun at any given time that should be valid.

Like Pteck alluded to, I have found, with parallel panels, that the assumption that series turn on the controller earlier with their higher voltage and therefore charge longer is not a practical consideration. It *was* a concern of mine, as my setup calls for a 5 volts over battery voltage in order to turn the controller on. The reality is, though it's low amperage, my voltage kicks on the controller virtually within moments of dawn, with the sun not yet over the horizon. It might only be 10 watts, but the controller is 'on' and ready to start cranking as the sun appears.

I can't imagine the voltage from a series configuration triggering the controller any sooner that would make an appreciable difference to charging effectiveness. Even if triggered a few minutes earlier, the wattage would be so low, it would not make any difference to charging. In either set up, by the time there was enough light to create any significant charging wattage, either configuration would have been on for nearly an equal amount of time.

[AKNate]

Originally Posted by pcskier

With the trailer aligned north/south, both sets of panels had equal angles/sun at any given time that should be valid.

I’m confused. If the trailer is aligned north/south, which I understand to mean the front and back are pointing north and south and the sides are east/west, then as the sun rises one side of the camper will have the panels angled toward the rising sun and the opposite side will be angled away from the sun. That would greatly hurt a side/opposite-side panel series (but not parallel) since the series would be limited by the west facing panel’s output through the morning. The same but opposite would be true in the afternoon.
If however you parallel the opposite sides of the camper but series along the length, then it doesn’t really matter anymore since every panel along the length has the exact same angle to the sun and will have the same output anyway. There’s no longer a detriment to series-ing and you get the voltage advantage.

[pteck]

Originally Posted by AKNate

I’m confused. If the trailer is aligned north/south, which I understand to mean the front and back are pointing north and south and the sides are east/west, then as the sun rises one side of the camper will have the panels angled toward the rising sun and the opposite side will be angled away from the sun. That would greatly hurt a side/opposite-side panel series (but not parallel) since the series would be limited by the west facing panel’s output through the morning. The same but opposite would be true in the afternoon.
If however you parallel the opposite sides of the camper but series along the length, then it doesn’t really matter anymore since every panel along the length has the exact same angle to the sun and will have the same output anyway. There’s no longer a detriment to series-ing and you get the voltage advantage.

The challenge here is trying to cherry pick an alignment and sun angle. This is useful with a statically installed array on a house for example.

The reality with an RV is planning for mis-alignment and shade. The sun is always moving through the day and mis-alignment is the norm.

Parallel installations will consistently bring in more power through the day.

[AKNate]

Originally Posted by pteck

The challenge here is trying to cherry pick an alignment and sun angle. This is useful with a statically installed array on a house for example.

The reality with an RV is planning for mis-alignment and shade. The sun is always moving through the day and mis-alignment is the norm.

Parallel installations will consistently bring in more power through the day.

It’s not cherry picking. The geometry of an Airstream applies in all conditions. It’s a simple fact that Airstreams are curved across the width of the roof. A panel on the curb side will never under any condition point the same way and receive the same light as a panel on the road side of the trailer. It would be utterly absurd to series two panels that point different directions. Nobody does that.
All the panels on the same side however point in exactly the same direction and will always receive exactly the same amount of light so there is no detriment at all to series them in normal conditions. The only time there is a detriment is with a stray shadow. If you have a telephone pole or something casting a shadow over one panel it becomes as if the same shadow was cast on every panel. That’s a bit of a fringe case IMO though since Airstreams aren’t very long and most of the time all the panels see the same light conditions. If you want to design for that case though, then yes, parallel is the way to go. Keep in mind though, the rest of the time when you don’t have that stray shadow, you’re increasing your resistive losses by 3x (depending on the array).

[Kalashnikov]

We have our 600W system in series-parallel:
https://www.airforums.com/forums/f44...28-204851.html


So far our camping experience has been this:
1) On completely overcast days the system doesn't charge sufficiently to maintain the batteries. We're forced to use a generator.
2) On sunny days even a 50% shaded array is more than enough to charge 100% by around 11am. (That's shaded front-to-back, ie. our system should barely be charging)


We tend to always be shaded when camping. A true parallel setup might be better on paper but in real life I don't think it matters. Cloud cover is the biggest factor.

[MandN]

I think AKNate has a good point. If you're going to do a series-parallel set-up , the series part should be on the same side (eg driver-side, curb-side) with driver-side and curb-side parallel to each other. The reason I did not do it and went was because where we were parking the trailer at home the rear of the the trailer is completely shaded under the overhang, unfortunately we could not plug in, and I did not think about the consequences for the road. Of course I rewired to parallel config shortly after we left. Now it is 3 years later and we have been on the road full time, spending 75% time boondocking. Kalashnikov made a very good practical point we might be splitting hairs here: the biggest issues are heavy shade (eg camping in a forest with a thick canopy) and strings of HEAVY cloud cover days. Light cloud cover is actually ok, unless you are partially shaded from trees etc.. Our energy usage is actually on the heavier side because I telework (but only about 10 hours per week) plus starlink, Apple TV, watching movies on the TV, listening to music on airstream speakers etc... And yes it can be feast or famine. On sunny days especially during summer months, we are full by noon or earlier and I switch the fridge to AC to not waste all that extra solar. (we have standard airstream 2 way propane/AC fridge, but I rewired its circuit to run off the inverter which also electrified the non inverter ac plugs in the trailer).

Like Kalashnikov we also have a 600Watt solar array but we don't own a generator. We made it work by splitting the solar-lithium install between truck (200 watts, 200amphours) and airstream (400watts, 200 amp hours). The truck can be moved into a sunny spot, the panel on the truck tilts using a linear actuator (recent upgrade) for the winter months, and the truck lithium bank is also charged when running the truck (as we run errands or drive to trailheads etc). We have several posts describing the install and recent upgrade on Airstream forums if interested. Thanks for your feedback.

[pteck]

Originally Posted by AKNate

It’s not cherry picking. The geometry of an Airstream applies in all conditions. It’s a simple fact that Airstreams are curved across the width of the roof. A panel on the curb side will never under any condition point the same way and receive the same light as a panel on the road side of the trailer. It would be utterly absurd to series two panels that point different directions. Nobody does that.
All the panels on the same side however point in exactly the same direction and will always receive exactly the same amount of light so there is no detriment at all to series them in normal conditions. The only time there is a detriment is with a stray shadow. If you have a telephone pole or something casting a shadow over one panel it becomes as if the same shadow was cast on every panel. That’s a bit of a fringe case IMO though since Airstreams aren’t very long and most of the time all the panels see the same light conditions. If you want to design for that case though, then yes, parallel is the way to go. Keep in mind though, the rest of the time when you don’t have that stray shadow, you’re increasing your resistive losses by 3x (depending on the array).

I see what you're saying. It's a reasonable point.

I'm with kalashinova as sun angle is not the only concern. Shade of all types whether cloud, tree, etc. is the norm for me. So many times one end of my trailer is parked under tree cover where only the other end is sticking out.

Those are the times where solar generation is compromised and I need the remaining panels to perform and a series-parallel wiring would really compromise production.

To your point - gotta wonder how many people are series wiring panels not facing the same way. Or can't get good production when partially under shade.

TL;DR - Always parallel for an RV

[MandN]

And for the record I’m with Pteck, and would recommend parallel wiring as first choice, followed by same side series as a distant second. As a current example, we are currently in NOLA and and the front side of the trailer is getting a lot more sun then the back due to trees so same side series would yield less than parallel. Given that’s it’s winter with the sun low in the southern sky, it actually matters.

[Nishad]

Excellent thread! Thank you all. I wish I was more patient in my search before starting a new thread.

https://www.airforums.com/forums/f44...ng-255614.html