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Old 03-08-2020, 05:08 PM   #1
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2018 25' Flying Cloud
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Solar Panels: Series Vs Parallel Array

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).
Attached Files
File Type: pdf Series Parallel Test.pdf (301.6 KB, 4 views)
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Old 03-08-2020, 05:57 PM   #2
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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.
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Old 03-08-2020, 07:00 PM   #3
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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.
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Old 03-08-2020, 09:55 PM   #4
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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.
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Old 03-08-2020, 10:41 PM   #5
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Quote:
Originally Posted by Crackerman View Post
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.
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