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Old 08-20-2012, 10:44 AM   #1
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Solar - help with figuring out panel voltage

I've been talking with the support team at Grape Solar - going solar seems more and more like a no brainer upgrade at this point.

I found them at Costco - $199 shipped for 100 watt panel. This led me to find that Home Depot carries their whole line of products for the same pricing - including $399 shipped for a 250 watt panel. Amazon has the 250 watt panel for $399 with free shipping and no sales tax to California, but does not carry the 100 watt panel.

I did some emailing with Grape Solar, and found that they recommend AM Solar as their RV partner. AM Solar sells the panels for about $10 more shipped than do the bigger folks above - and they also make RV mounts and installation kits for the panels.

My thinking is to start with 200-250 watts, but design a system that will scale to 500 watts or more.

So I have a question for you solar gurus - I'm trying to figure out what input voltage to design for given the output voltage is fixed at 12'ish volts. The 100 watt panels are speced out at 17.7 volts, and the 250 watt panel at 31 volts.

I know that I could wire two 100-watt panels together in parallel to get more amps at 17.7 volts, or in series to get 35 volts. I know I can use thinner gauge wire and have less current drop at higher voltage for the same amount of power - so just looking at transmission losses and cost of wire it would be preferable to design the system to run at higher input voltage.

So my question for you gurus is about how charge controllers work vis a vis input voltage vs. the fixed 12V'ish out put voltage - I'm not finding it easy to figure out of they will run efficiently at higher input voltages. Does anyone know - will they effectively convert the higher DC voltages and send more amps to the 12V battery system, or do they clip at some level of input voltage? I see Max voltages listed in the specs, but no discussion about the optimal input voltage ranges. I called Xantrex tech support, (their C series line seems like a good fit) but gave up after an hour on hold.

If I can optimize around a single 250 watt panel at higher voltage (at least to get started) this would seem to be the way to go in terms of price / performance - but I should not go this direction if it makes the charge controller less effective than two 100 watt panels in parallel. That's what I am trying to sort out.

Note - I invited the grape solar guys to join the Airforums and recommended they put together a special package for Airstream owners. They seemed to be interested; we'll see if they do it.

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Old 08-20-2012, 11:54 PM   #2
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A little more Internet research and I was able to answer my own question.

You can use a higher voltage panel(s) with a 12V airstream system - but to actually benefit from the higher voltage except for reduced transmission losses you need an MPPT controller. An MPPT controller is basically a smart DC to DC converter - it converts higher voltage lower amperage current from the panels into higher amperage lower voltage current.

A normal solar charge controller (non-MPPT) will apparently clip the voltage of higher voltage panels - potentially wasting a large amount of power.

So with a normal charge controller, you want 18 volt'ish panels - and if you have more than one, wire then in parallel to get more amps not more volts. With an MPPT charge controller, you can either have higher voltage panels or wire multiple panels in series, to reduce transmission loss and avoid having to install very heavy gauge wire.

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Old 08-21-2012, 12:25 AM   #3
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did you install the 250? how did it work out? I was thinking about that size panel for my 31'.
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Old 08-21-2012, 12:46 AM   #4
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Yes, you are correct about the higher voltage panels requiring a different type of charge controller, the MPPT type. The conventional 12 volt systems and charge controllers use panels in the range of 17 to 18 volts nominal.

Further, if you use a higher voltage panel and connect it to a conventional charge controller, you will probably toast it.

As you may have found, the higher wattage panels are generally only available in the high output voltage configuration, so finding a 200 watt + panel with an output voltage of 17 to 18 volts is hard to do. While 12 volt systems are still used some, most larger systems have gone to 48 volts when used with batteries, and to several hundred volts when used with grid tied, no battery systems.

So, a warning to all considering solar systems, be careful about the panel voltage. It is not just all about panel watts anymore.
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Old 08-21-2012, 10:24 AM   #5
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That would make a lot of sense - as it becomes more cost effective to deploy more wattage, you'll have to go to higher panel voltages or the cabling requirements will get ridiculous.

I'm guessing that solar system economics for an RV are a bit out of whack right now with panel costs low and the MPPT controllers demanding a premium - I expected the panels to dominate the price of the system, but for a system in the 250 watt range a quality MPPT controller with some headroom is just as expensive as a 250 watt panel.

I'm currently thinking about a system that would start out with

1 - Grape Solar GS-S-250-Fab5 250-Watt Panels (8 amps @ 31 volts) - $400
1 - Morningstar TS-MPPT-45 Tristar 45 Amp MPPT Charge Controller - $380
1 - Morningstar TS-RM-2 remote meter - $80

This setup could be easily expanded to include as many as three of the 250 watt panels, wired in series, providing roughly 8 amps @ 90 volts into the charge controller and delivering approximately 45 amps @ 12 volts to the batteries.

What is compelling to me about a series wired setup with an MPPT controller is that the wiring doesn't need to be upgraded when adding more panels - and 8 amps does not require crazy fat cables between the panels and the controller.

This setup should also have the lowest price per watt of all the configurations I looked at - adding in some more dollars for mounting hardware and cabling, it comes out to about $4 per watt for a 250 watt system, and $2.50 per watt when upgraded 750 watts - all before tax credits.

Next steps for me are to look a the roof of my AS to see if these particular panels could really fit up there, and then calling in some professionals to verify that the thinking above is correct and for bids.
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Old 01-07-2013, 07:50 PM   #6
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Sorry to bump an old thread but just looked up these grape solar panels and thought it should be good to add that they are on the very large side - about 40x65 inches. Seems like it would be hard to get more than one on the roof and then you'd need one of the larger AS, correct? Or are people commonly using panels this large?
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Old 01-07-2013, 08:46 PM   #7
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i've been watching the prices for a few years and the big dip has been in the 200-250 watt sizes, sometimes as low as $1.50 a watt...BUT...shipping can also approach $1 per watt for that size panel, then you toss in a MPPT controller and there is another $1/watt...

I've decided to build some panels, a 60 and a 120 W @18 volts and use a $65 controller. I won't save a bunch of money but it's gonna be FUN and I can size them so they wont be so wide
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Old 01-10-2013, 12:13 AM   #8
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Do yourself a favor and make a cardboard mock-up of your intended panels for test fit. I'm finding it difficult to fit panels on my 26' with all the vents etc. I shopped panels and really wanted a 140 w kyocera but even in the middle of the roof it would stick out and look odd, so I settled on the 100w sizes. I know the price per watt goes up the smaller the panel so this is a conflict, I contemplated making my own for the best fit and most watts , but effective glazing, sealing/soldering and the associated time combined with lack of warranty were all factors I couldn't realistically overcome, the education is something to consider though, another skill to add to your repertoire ! Just get on a ladder with a cardboard template before you build anything. I was surprised how broken up the available open spaces on my roof were, and how many panel sizes wouldn't fit anywhere.
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Old 01-10-2013, 02:50 PM   #9
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It's amazing how much I've learned since I started this thread. Thanks so much to all the wizards on the forum. I totally agree with aluma's advice - lots of thinking needs to go into what will fit up there.

I ended up going with flexible panels - about half as efficient per square foot, but really easy to peel and stick onto the roof.
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Old 01-10-2013, 04:46 PM   #10
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You may want to check the adds in classifieds. There are several solar panels for sale. I have a complete system for sale in the classifieds. It includes everything you need.

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