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).
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