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.
