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Old 03-17-2022, 07:15 PM   #1
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Flexible Solar efficiency

I am concerned that when a flex panel is placed on the roof, it's never really pointing directly at the sun. Does anyone have any actual installed data on power output, eg, a 100w panel placed crosswise.

How about summer versus winter data?

My "never done it but have some thoughts" rule of thumb would be that for a 100w panel in summer I'd expect to get 50% from a flex panel on the roof, averaged over 10 hours (so about 500 Wh or 50 Amp hours) and in dead of winter I'd derate that another 50% over 8 hours, or about 20 Amp hours.

So in winter I'd expect to need 4 100w panels to keep one 100 Ah batter reasonably charged. How far off am I?
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Old 03-18-2022, 09:06 AM   #2
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Not sure about efficiency of flexible panels. Just wondering what you mean by this.

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Originally Posted by Zeppelinium View Post
So in winter I'd expect to need 4 100w panels to keep one 100 Ah batter reasonably charged. How far off am I?
Are you talking about while the trailer is just in storage? If so 1 100w panel should be more then enough, self-discharge should be less then 20 Ah a month and parasitic draw shouldn't be much more then 20 Ah a week (if it is you need to try to reduce it).

Or do you mean using the trailer? Then it would depend on how much power you use.
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Old 03-18-2022, 09:22 AM   #3
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Flexible Solar efficiency

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Originally Posted by Zeppelinium View Post
I am concerned that when a flex panel is placed on the roof, it's never really pointing directly at the sun. Does anyone have any actual installed data on power output, eg, a 100w panel placed crosswise.



How about summer versus winter data?



My "never done it but have some thoughts" rule of thumb would be that for a 100w panel in summer I'd expect to get 50% from a flex panel on the roof, averaged over 10 hours (so about 500 Wh or 50 Amp hours) and in dead of winter I'd derate that another 50% over 8 hours, or about 20 Amp hours.



So in winter I'd expect to need 4 100w panels to keep one 100 Ah batter reasonably charged. How far off am I?

TLDR; itís not that simple, but your winter number seems low and your summer number is probably reasonable, including for rigid panels.

Long version: Iím not sure a flex panel makes any real difference. Rigid panels are also installed at an angle on Airstream roofs. Iíve never seen a rigid panel installed horizontally level on an airstream. Flex panels hug the curvature of the roof and rigid panels average the curvature. I donít think thereís a significant difference between them as far as their angle to the sun.
I have six 100W flex panels and I will exceed 600W output occasionally, even though they canít all simultaneously ďpointĒ directly at the sun. Mine have a dimpled texture on the surface, so perhaps that helps gather light.
No panel is going to be anywhere close to putting out 100W for 10 hours per day though. Thereís a short window around noon when sunlight is at maximum and it is downhill from there. The 100W rating of any panel assumes ideal conditions at solar noon. At 4 pm the sun is shining through dramatically more atmosphere than it was at noon, regardless of the angle of your panel. Your location makes a massive difference too. The northern US is nothing like the southern US and neither is anything like Alaska. Then thereís weather, which can be an ever bigger factor.
Thereís no real answer to your question. When itís sunny my batteries are usually full long before sunlight runs out, so I canít say how many Wh they would have made since the panels spend much of the day idle. 500 Wh seems reasonable though for a safe estimate. On occasion I produce more than that, likely when coming off of several days of bad weather and the batteries are run down so they can take the extra power.
Generally if my batteries arenít filled and the panels idled by the end of a day, itís because of some combination of weather, latitude, and season, which thereís no way to compare to someone elseís circumstances unless theyíre parked next to you in that exact circumstance.
How much power you need to keep your battery charged is also entirely dependent on how much power youíre using. Whether you have one battery or ten doesnít really matter. If your trailer is stored, a single panel would be plenty to keep your battery charged. If youíre trying to run the air conditioner all day, four panels obviously wouldnít have a dream of keeping your battery charged.
Looking at my history it seems that we generally use about 2 kWh of power per day, which our 6 panels easily provide, even in winter. That would be over 300 Wh/panel, so I think your winter number is low. Again, that is how much we use, not how much the panel is capable of making.
While power is never an issue for us when the weather is good, in the PNW in winter when the days are short, the sun is low, and you get a stretch of dark rainy days, we canít keep up and will have to run the generator every few days to recharge. Thatís a worst case when season, latitude, and weather are all working against you.
My advice would be to put as many panels as you can fit. Panels are cheap. Having too many never hurt anybody and it helps you when thereís poor conditions. I prefer flex panels because they weigh nothing, are aerodynamic, and donít affect the look of the camper. Some people like rigid because they think theyíll last longer. I wouldnít pick rigid because you think youíll get a lot higher power output from them though. Theyíre both rated 100 W, which already takes any panel differences into account. As I said, I think the mounting angle difference is negligible. Some people do mount their rigid panels with a manual pivoting mechanism to be able to tilt them up, but I suspect the few that have that rarely if ever utilize it. The benefit of rooftop panels is being able to forget about them. If you want to optimize angle to sun, portable panels would be best.
This is way too long and rambling, but Iím just going to post it anyway. Good luck with whatever you go with!
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Old 03-18-2022, 10:07 AM   #4
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Thanks for your detailed answer.

I agree with your daily output comment. I just figure that in a long summer day of 14 hours, planning on getting 50% average over 10 hours is safe, which is 500 Wh per 100W panel. (I prefer to think in terms of Amp-hours, since battery capacity is how I plan my days. I know how many amps the fans, pumps, and lights take, so it's easy to estimate available battery charge.)

On my 23' Safari there is obvious room for two panels, maybe a place for a third one. Three years ago I got stuck up in the San Juan Islands for a week of gray sky. I had been using portable panels in parallel with a PWM controller and wasn't getting squat out of them, which made me switch to series and an MPPT controller. It will take up to 100v input, so a max of 5 panels (all in series).

The Renogy panels are all 7 amps, so it's easy to put flex and rigid together in series. But planning for the portable panels to be "optional" makes the wiring slightly more complicated, requiring one manual switch. Connecting them in parallel as two sets in series is easy, but since the rigid and flex aren't exactly matched, that would introduce a small loss, but I think that's what I'm going to try as a starting design. I can always change it, since connections will be accessible.



Quote:
Originally Posted by AKNate View Post
TLDR; itís not that simple, but your winter number seems low and your summer number is probably reasonable, including for rigid panels.
...
No panel is going to be anywhere close to putting out 100W for 10 hours per day though.
...
Looking at my history it seems that we generally use about 2 kWh of power per day, which our 6 panels easily provide, even in winter. That would be over 300 Wh/panel, so I think your winter number is low.
...
While power is never an issue for us when the weather is good, in the PNW in winter when the days are short, the sun is low, and you get a stretch of dark rainy days, we canít keep up and will have to run the generator every few days to recharge.
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Old 03-19-2022, 10:44 PM   #5
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Flexible Solar efficiency

I wired my panels in series-parallel, with three parallel sets in series (I have the luxury of a big roof on my 30í). I did it largely because of the size/current capacity of the factory pre-wire I wanted to use, but I am happy with how it performs and might do the same even if I had the larger pre-wire newer trailers come with. The three series gets me up to 60V, so they turn on and are making power at very low light levels. Full parallel may give you more power in direct sunlight when your AC casts a shadow on a panel, but Iím not really worried about power when it is bright and sunny. The days of grey like you mention are when you want to eke out as much power as possible and for that I think series wins easily. The sunlight is diffuse so thereís no shadows, so parallel loses itís only advantage. Series turns on sooner and has the efficiency of higher voltage, so I think itís ideal for fringe conditions. Even a few watts on a dark day add up.
One thing you might look into is separate charge controllers for the rooftop and portable panels. I believe Victronís controllers (and I assume other brands too) can sync to each other and make it easy to charge one battery bank using two separate solar arrays. I would think that would end up being more efficient since the two different panel types sitting in two different places would probably have different MPPs.
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Old 03-21-2022, 03:44 AM   #6
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Totally agree with series/parallel. I am wiring the system so that I can operate with only the two roof panels in series. If I add the portable panels, they will also be in series, but can be added in parallel with the roof or series with the roof, with only the flip of one SPDT switch. That gives me the best option for either gray days or bright days.

I'll post data when I get it (most of the MPPT controllers are fine with multiple controllers for different strings, but I don't think I'll go there), but that may be a few months. I'm just at the point of reinstalling the interior skin...

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...
The days of grey like you mention are when you want to eke out as much power as possible and for that I think series wins easily. The sunlight is diffuse so thereís no shadows, so parallel loses itís only advantage. Series turns on sooner and has the efficiency of higher voltage, so I think itís ideal for fringe conditions. Even a few watts on a dark day add up.
...
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Old 04-27-2022, 09:25 PM   #7
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AKNate, which panels are you running? Zep, Which panels are you thinking of using? Thinking about Flex panels on my 66 TW.
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Old 04-28-2022, 06:32 AM   #8
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I will check my trailer today. I am in Toronto Canada. There are 4 68watt flexible panels on my roof. It is sunny today.
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Old 04-28-2022, 06:36 AM   #9
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Yesterday with heavy cloud cover I got a high of 105watts. Average was probably 80 watts. That seems good to me.
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Old 04-28-2022, 10:33 AM   #10
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AKNate, which panels are you running? Zep, Which panels are you thinking of using? Thinking about Flex panels on my 66 TW.
I was all for flex panels, then started looking at the power output of panels when they aren't oriented (mostly) towards the sun. The difference was dramatic, like 50 Watts vs 85 Watts. This was for rigid panels lying flat on the pavement vs being tilted up about 30 degrees. It would be worse for flex panels that on one side of the roof would be curved away from the sun.

So I bought the new Renogy 100W rigid panels (they are narrow, so fit nicely on either side in front of the a/c). I am going to use an adjustable mount like Craig Lathem (see him on Facebook) uses, so I can tilt the outer edge of the panel up. This requires a small additional investment in a telescoping ladder so you can reach the roof. My panels will be mounted lengthways with the length of the Airstream, so they will already have a 15 degree angle already. This means I'll only tilt the panel that's tilted away from the sun.

BTW, if you have Fantastic fans installed, the typical Airstream has too many vents in the roof. If you remove and skin over one vent (I always remove the vent over the bed, since the one thing I don't want to get wet if I screw up is the bed*), you make plenty of room for three PV panels, pretty much conforming to the roof line and not looking too bad.

*another unasked for opinion: Always get the most simple Fantastic fan. When you think about it, the reverse switch is useless--the fans should always be in "blow out" mode. Also, the rain sensor works so great that if there is water within walking distance the damn things close and you can't get them open. I'm sure this is a problem on my end, since I typically don't read the user manual.
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Old 04-28-2022, 09:18 PM   #11
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Zep, I already installed two new Max fans, front and rear. I got rid of the middle one for the exact purpose of wanting more room for the solar. I also got rid of the roof top air and am going with the mini split. I did go for the simple max fan, at least it had no remote and I'm pretty sure it was the basic model. I must say when I first started reading some of these builds, I read a lot of your and others posts from back a ways. It's great to see you and others posting again! Thanks for all your contributions!
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Old 04-29-2022, 07:04 AM   #12
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AKNate, which panels are you running?

Iím using the newest Renogy flex panels. Unlike many other flex panels they are made from ETFE, which has a long track record in outdoor applications. Iíve had them three years now and they still look like new.
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Old 04-30-2022, 07:06 AM   #13
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Question, is the AS being used during the winter?

Ours BB's have been isolated in the AS since last October.

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Old 06-23-2022, 07:14 AM   #14
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I am concerned that when a flex panel is placed on the roof, it's never really pointing directly at the sun. Does anyone have any actual installed data on power output, eg, a 100w panel placed crosswise.



How about summer versus winter data?



My "never done it but have some thoughts" rule of thumb would be that for a 100w panel in summer I'd expect to get 50% from a flex panel on the roof, averaged over 10 hours (so about 500 Wh or 50 Amp hours) and in dead of winter I'd derate that another 50% over 8 hours, or about 20 Amp hours.



So in winter I'd expect to need 4 100w panels to keep one 100 Ah batter reasonably charged. How far off am I?


Both flex and rigid panels will vary output depending upon angle relative to the sun. Buy a little suitcase panel and have some fun by playing with its position as you monitor output. There are iphone apps which will give you the optimum positioning at your location and time of year. Both flex and rigid panels also vary widely by model in open circuit voltage and current as well as shade tolerance and efficiency. Solar installation design is detailed work. Layout on any Airstream (any RV) roof is a challenge due to aircons, vents etc., and you will never approach 100% system output due to the variables involved, including how you position the trailer in a camp site. That said, go for it and put up more than you expect to need, if not as much as you can fit.
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