RV Solar Part I – The Discovery Process
It’s a cold winter day here in S.FL we’re generating power from the sun. As a self-confessed geek I find this outstandingly cool and for the past 5 days we’ve been glued like little kids in front of the solar charger display, oohing and aahing over the power coming in. We’re now ready to hit the road and live “off the grid” in the best of ways, but it took us a fair few months to get to this point. So, these next few days I’m going to take you aspiring solar RVers on our journey so that you too may one day enjoy the fruits of the sun. We’ll start with the discovery process, then the equipment and end in grand style with the installation. So, here we go…..
Why Go Solar?
First things first, solar (for most people) is not exactly a cost-saver. Everyone likes to think it is, but given the price tag (anywhere from $1500 to $6000, depending on number of panels, equipment and installation) the return can take a good few years to make sense financially. If you use mainly private campgrounds or just do the rare day of dry-camping, the good ‘ol generator will do just fine. If, however, you like to go “off the beaten track” on a regular basis solar can be a definite bonus. We like a lot of spots that don’t offer electricity, and we love the idea of being self-sufficient and not having to run the generator. And, of course…it’s just so very cool….
How Many Panels Do I Need?
The first question most people start with, is how many panels do I need? Panels are expensive and you can’t expect to generate enough solar power cover everything (e.g. aircon’s are too big a power draw for regular solar use). The best way to estimate how many to get is to calculate your average usage (in amp hours) versus the average power you can expect to get from your panels (in amp hours). A very approximate rule of thumb is to target ~1oo Watts of solar for every 100 amp hours of battery capacity, but it’s best to do an individual assesment. Most RVers want to be able to use their computers, lights, appliances and maybe TV/satellite, but individual needs vary (by alot). Also, panels are usually rated for sun hitting directly on the panel at 25˚C, so actual power output will almost always be lower than their “optimum” rating. . I won’t go through the individual calculations, but I’ll let you know (later) what we decided to do and I can direct you to Jack Mayer’s page for good examples on how to calculate your own needs.
The Basics of Solar = Minimizing Loss
Solar is basically about trying to convert sun power into real power. All this sounds good and dandy, but very basic problem with this whole idea is that it’s an inefficient process….in fact a really inefficient process….and everything you do along the way (adding wires, routing through machines etc.) causes you to lose some of the original power you generated. Just to give you an example, solar panels themselves are only about 15% efficient (so, of the power coming in from the sun, only ~15% gets converted, even less if the sun isn’t directly on the panels or it gets too hot or bits of shade are present etc. etc.). Then you add a wire onto that panel and you lose a bit more (wires have resistance and the longer and thinner the wire the more resistance it has == more power loss). Then you put that wire into a charger (== more loss) , then more wires, then finally into your battery. So, every step of the way you’re losing power, kind of like trying to put water through a hose with a bunch of holes in it. It’s a fact of life with solar so in order to do it right you’ve got to do everything you can to **minimize** loss. If you start off by understanding that basic concept you’ll save yourself a lot of tears and head-banging later on.
So, How Do I Minimize Loss?
Once you start reading up about solar you’ll find a lot of info (much of it conflicting) on how to do it. I’m not going to go through all the details, but what I am going to do is give you a few key basics and route you to a couple of sites that will teach you the rest. Here’s the top tips we gleamed from all our research:
- Buy Good Quality Panels & Go Higher Voltage if You Can – You want quality panels that have a long-term warranty, and if you can go with a higher voltage. The reason is to minimize loss. You can run 24V two times longer than 12V on the same wire for the same loss. It’s the very reason industrial power-lines run at such high voltages (often 110kV or above) = higher voltage means less loss over distance.
- Beware of Shade – You might not think you need to worry about a little bit of shade on your panels. After all, the rest of it is getting sun, right? As it turns out even a teeny bit of shade can cut power output by huge amounts, and putting one square of your panel in shade can sometimes lock out power altogether. So, beware of any shade from roof elements (aircon, antennas etc.) when you install.
- Use Thick Wires – One of the most important decisions you can make in your solar installation is using the right size wires. Wires are the pipes that route whatever power you get from your panels to your batteries. They’re often the single biggest loss point in any solar system and they’re often the single biggest mistake people make. Thicker wires will carry more power for longer distance with less loss, while thin wires can cause you to lose so much voltage that your batteries never get fully charged. The “standard” size wire (usually #10) that you get with a 12V solar system will typically be too small. You want to minimize loss to 2% or ideally 1% which means thicker wires (e.g. #6 or #4), especially for the parts of your system carrying the largest current. Higher voltage panels give you extra leeway, but you still need to think about proper size. Use tables or calculators or graphs to help calculate what size wire you need.
- Get a Good Controller – The controller’s job is to convert whatever power you’re getting from your panels to usable power for your batteries. Typical deep-cycle RV batteries have 3-stage charging profiles, and they are sensitive to temperature. So, you want a controller that allows adjustable multi-stage, temperature-controlled charging. If you’re using higher-voltage panels you will also need a controller that can handle the higher incoming voltage.
- Locate the Controller Close to the Batteries – The further your charger is away from your batteries, the more voltage you loose. You want it as close as possible (but not in the same bin) as your batteries.
- Use the Right Charging Profile for your Batteries – When setting your controller, look at the battery manufacturer’s specs to determine the right charging profile for your batteries. Many flooded cell batteries require a higher charging voltage (typically 14.8V) than what the controller manufacturer recommends (often 14.4V). Go with the battery specs.
- Consider AGM Batteries – If you can afford the switch AGM batteries have a much lower internal resistence than flooded cell batteries which means less loss getting power into the cells and a much faster charge time.
Those are the big basics. There is a lot more, as well as details pertaining to mounts, wire connections and such, but for those I’ll direct you to the experts:
- Jack Mayers Site – Lots of good, detailed info here on Solar including using higher-voltage panels.
- Handybob’s Site – He’s very opinionated (no doubt), but there’s lots of good info hiding here. Take time to look through the text and pictures.
- AltE Site – This is a commercial site, but they have plenty of detailed articles written in plain English explaining how solar power works.
- AM Solar – Another commercial site with some easy education articles.
Next, we’ll reveal our choice in equipment….stay tuned….