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Choosing a small solar panel
Old 03-24-2012, 11:42 AM   #1
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Choosing a small solar panel

I'm looking at purchasing a small (50 to 100 watt) solar panel for my camper. Prices are all over the map.

What key characteristics should I look for?

Are the Chinese panels sold on eBay adequate or will I regret a cheaper panel a few years down the road?
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Old 03-24-2012, 11:55 AM   #2
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decreasing cost of solar panels rv - Google Search
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Old 03-24-2012, 12:04 PM   #3
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AM Solar have some good information on their site - look under the RV Solar Education tab. Although I haven't done a great deal of research, they manufacture their panels to fit into spaces that work well for many campers and RV's, and are happy to answer questions on the phone too.
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Old 03-24-2012, 05:22 PM   #4
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I am not really an expert on solar panels for RV, or grid-tie systems for that matter, but will share my experience so far.

"Turn-key" systems or kits would get me going fast, but I always like to tinker. Hey, what's an ESR guy to do to spend his days?

So, I wanted to buy piecemeal. The first part to get was the panel. Just now looking at the panels on eBay, and saw that the price has come down a bit more. The price used to be around $2.5/watt, but that's for a large panel of 200W+. Wholesale price has dropped to something like $1.5/Watt, I have read, but that's for a large volume, and does not include shipping.

Smaller panels would have to cost more per watt. I wanted a big panel, but the cost of shipping was fairly high. The panels might have been made in China (most of them are!), but most of the eBay sellers I looked at were American importers or resellers.

I also did some research on solar panel failure modes. How can a panel fail? There's no moving part there, although as the panels age, their output tend to drop. Most makers guarantee something like x% output after y years. To my surprise, solar panels could have catastrophic failures, meaning severe or total loss of output power. Most of the failures were mechanical. No, it's not about glass breakage (tempered glass could stand up to some hail). Rather, the causes were such things as the thermal expansion and contraction of the panels that could cause breakage of the flat ribbons that connect the individual cells in series. As the back of the panel is protected by a insulation sheet, it is not feasible to peel that back to resolder any wire that breaks.

I also discovered that a name-brand Japanese solar maker suffered a very high failure rate of a batch of its panels. It appeared that they had a boo-boo in the manufacturing process. However, they honored the warranty and replaced them all at a very high cost.

The above was all about panels for residential installation or grid-tie systems, but the difference in grid-tie and 12V-charging applications is only in how the cells are wired together (series/parallel) for different voltage outputs. Grid-tie panels have higher voltage outputs, so that the current would be lower. Panels for RV tend to have a Vmp (max power) of around 17V, and a Voc (open circuit) of 20-21V. This allows for a drop of a few volts through the wiring and the charge controller, before reaching the battery which could get up to 15V under charge.

While pondering where and how to get a panel, I happened to spot on craigslist someone selling used 215W panels for a very good price of $1/Watt. As the panel was made by an established and well-known maker, I jumped at the chance to buy in-town and not having to pay shipping for something that big.

I tested the panel before buying with a DVM. With the panel squarely facing the sun, I measured its open-circuit voltage, and then its short-circuit current, and compared the values to the spec. The spec was on a sticker on the back of the panel, or I could have looked it up on the maker's Web site too.

The panel I bought was meant for grid-tie, and has a Vmp of 40V. This means that I cannot use a simple charge controller, which could be bought fairly cheaply. Mismatching the voltage would mean a big power loss. To avoid that, I would need an MPPT charge controller which can also handle the large voltage difference between input and output. I found only one maker who had a suitable controller, and he wanted $200+. Now, that gives me more of an excuse to design and build my own, which I am doing right now.

All this RV work play keeps me self-entertained cheaply. I love it! But I need to finish this before hitting the road soon. Summer is already around the corner.
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Old 03-24-2012, 07:33 PM   #5
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I have this installed in my horse barn.
Solar Panel Kit - Save on this 45 Watt Solar Panel Kit
It was inexpensive and easy to install. You have to buy a battery to store the juice for later use. I purchased a deep cycle marine battery. Works like a charm.

I think this set up would be easy to use on an RV. You could mount the panels in some kind of temporary arrangement and maybe even use a lazy susan type device to so you can rotate the panels for southern exposure.
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Old 03-24-2012, 08:04 PM   #6
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Thanks for the input so far.

I am looking at something like this 60 watt panel for $120 including shipping.

New 60w watt Sun Solar Panel PV Poly-crystalline 25 Years Warranty MC4 Connector | eBay

My concern with the Harbor Freight solar kit is that it uses amorphous panels, is relatively heavy and it seems overpriced compared to just buying a separate panel and controller.

Part of my constraint is that my camper roof is only 10 feet long with an opening vent in the middle that splits that space. I think a 200 watt panel would give me enough lift at 60 mph to clear the tires from the pavement.
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Old 03-24-2012, 09:11 PM   #7
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Looks like a winner! At $2/watt and free shipping, that's hard to beat. Wow, prices are dropping more than I thought. The specs look right for 12V battery charging. Looking good!

Just add a pair of MC4 connectors, a charge controller for less than $20, and you are ready to roll.

PS. This panel size is 31"x27" = 837 sq.in. = 5.8 sq.ft. => 10W/sq.ft.

The 215W panel I have is 61.4"x31.4" = 1927 sq.in. = 13.4 sq.ft. => 16W/sq.ft.

Mine is a monocrystalline panel (German made), hence is more efficient. I am happy to get it for a good price, but it is going to take me a bit of work before it's done. The charge controller's brain is a microcontroller, whose firmware will also include the battery monitor function like the popular Xantrex LinkPRO.

So, that's $500 that I am going to save (MPPT charger+Batt Monitor), but by spending a few hundred hours of labor for hardware design+firmware development!

But is it not what retirees do, spending a lot of time for activities that have insignificant economic value?

Need to log off now to get back to the schematic. Doggone it, the weather is getting warm here today. My time is running out before I need to hit the road.
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Old 03-25-2012, 09:26 AM   #8
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You may want to check out this blog. This couple has been living off grid for quite a while.
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Old 03-25-2012, 09:38 AM   #9
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You may want to check out this blog. This couple has been living off grid for quite a while.
Thanks for the link - it has good, specific recommendations.
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Old 03-25-2012, 10:53 AM   #10
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The above "technomadia" couple's blog is among the list that I often read. I forgot that they lived on only 200W of panel, the same as what I am installing. They have recently upgraded to a converted bus, and have put in a compact, wonderful, and expensive Lithium-Iron-Phosphate (LiFePO4) battery bank (>$3000). It made me want the batteries in my RV to die quickly, so that I can get that new battery type for myself. No, not a stupendous $3000 battery, but a smaller $620 one for me.

Talk about RV blogs, I just now remember this one by HandyBob. He rants about mis-installation of solar systems, such as undersize wiring, incorrectly set charge voltage, and loss of power by minute partial shading of the panels. He said many people just listened to installers or vendors to pile on more and more panels, while never got 100% juice that they should get out of the existing ones.

Even as an EE, I learned from his blog things I did not know or did not appreciate. Highly recommended.
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Old 03-25-2012, 11:06 AM   #11
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.........Talk about RV blogs, I just now remember this one by HandyBob. ..........
I've read this blog. Lots of good information, but he sure is a pistol.
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Old 03-25-2012, 11:54 AM   #12
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A very timely thread thanks to all comments.

Just begun considering electrifying my camp/mancave. Have two generators, one 8 KW and one 800 watt. Had them for years. Hauled them over to to the camp. Depending a on load needs is what is running, or none when prefer just quiet time.

Utility line is out of the question at minimum $ 25.000 for running the power lines from several miles away.

Putting in a modest solar power set would allow for more quiet time. And reduce the gasoline hauling factor.

On to reading more and getting edumacated.
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Old 03-25-2012, 02:27 PM   #13
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A problem I see with solar panels in a remote cabin is that you may not be able to bolt them down enough for protection from thieves. Other than that, a moderate investment of perhaps $1K-$2K should give a lot more power than one needs, given that solar panels are so inexpensive now.

You have a TT, I believe. How about installing panels on the TT, then plug your cabin to the TT when you are there?
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Old 03-25-2012, 04:50 PM   #14
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I bought the eBay 60 watt panel (after being nudged by NWB ).

Now I need to decide on a controller. I'm thinking maybe a 10 amp Morningstar to allow for a second panel, if I decide to go that way.
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Old 03-25-2012, 06:06 PM   #15
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I was about to say that after receiving your panel, you might have to wait a while for your MC4 connectors to arrive from the Orient. But, then I remember where you are, it's probably still cold. You still have plenty of time before you can go camping. Not where I am. It's 85F today.
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Old 03-25-2012, 06:10 PM   #16
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I was about to say that after receiving your panel, you might have to wait a while for your MC4 connectors to arrive from the Orient. But, then I remember where you are, it's probably still cold. You still have plenty of time before you can go camping. Not where I am. It's 85F today.
Ironically, it has been in the 80's here in SE Michigan. We are cooling off for more normal temperatures this week.
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Old 03-25-2012, 06:21 PM   #17
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Talk about camping in cold weather, my RV does not have enclosed holding tanks that are heated. I have been wondering if I should pay more attention to the weather if I go up "North" in early summer.

Having the holding tanks cracked or the drain pipes broken because of freezing is a nightmare I want to avoid. So far, I have not been caught in freezing weather. Knock on wood!
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Old 03-25-2012, 06:47 PM   #18
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My experience is you are fine with exposed tanks if overnight temps don't dip below 25 or so and it warms up well above freezing during the day. But if it stays below 32 for more than 24 hours, you may have problems.
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Old 03-25-2012, 08:14 PM   #19
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A problem I see with solar panels in a remote cabin is that you may not be able to bolt them down enough for protection from thieves. Other than that, a moderate investment of perhaps $1K-$2K should give a lot more power than one needs, given that solar panels are so inexpensive now.

You have a TT, I believe. How about installing panels on the TT, then plug your cabin to the TT when you are there?

Thievery could be a problem, I'll need to work on some form disguise. Just want to have enough power for a few LED arrays for light.

I have sold my TT, got enough $ out of it to buy a 1998 GMC 1/2 ton 4x4. Access requires 4x4 or trail bike.

I have been to all of the states that are accessible via RV. DW does not care for camping at all nowadays. In fact it was her idea to get the camp, even though her idea of camping is a Hilton on the beach with balcony. Hmmm hint in there somewhere. Hence the mancave.

I bought this camp real cheap (less than 2k/acre) from a motorcycle gang. Needs some cleanup of beer cans and booze bottles etc., but plenty of elbow room on 14 acres. Doubt many are willing to go there given the reputation, which I am not about undo.

The real challenge will be to make the building at least 80% solar heated. Ie. very unlike a solar heated tent wich fully tracks outside temperature swings. At minimum I'll work on making it so it stays about 20F above freezing if no sun for say 6 days at all. A fun engineering and technical challenge.
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Old 03-25-2012, 08:20 PM   #20
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My experience is you are fine with exposed tanks if overnight temps don't dip below 25 or so and it warms up well above freezing during the day. But if it stays below 32 for more than 24 hours, you may have problems.
Ditto.

At one time my motorhome's wheels were frozen in about 6" of ice for over a week just outside of Eugene Oregon. It had exposed tanks. No harm was done to them, luckily.
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