RV Solar - Frequently Asked Questions

How can I figure out how many solar panels I need?

Numerous charts and lists have been published with long lists of appliances and how much power they draw. You are then supposed to add up all the watts, then find out how many hours of sun you get, etc etc. You then usually end up with a figure that says you need 44 panels, 24 batteries, and a 5000 watt inverter.

The worlds First Solar Powered RV

The worlds First Solar Powered RV

It ain't gonna happen. These charts are OK for rough planning in a larger system, such as a remote home, but RV's are much more limited. Regardless of what the chart formula says, if you don't have room for all of it, it is not much help.

We have found that the best rule of thumb is to make a wild educated guess. Generally, figure about 75 to 130 watts of panel for every 100 AH of house battery, or about one watt per AH that you have. If you have a pair of 220 AH golf cart batteries, or a single 4D or 8D 12-volt battery, you should be looking for something in the 85 to 250 watt range. This of course can vary considerably, depending on many factors, such as how much and for how long you dry camp.

Few RV systems use more than 500 watts of panel, and typical is in the 75 to 400 watt range. Often the best thing to do is just buy a one or two panel system and see if it gives you enough. It is very easy to add on more later if you need them - and if you don't need them, you've saved a few (OK, a lot of) bucks. If you take that route, make sure you get a charge controller that will handle any extra amps that you might add on later - other wise you will have to buy an additional controller later.

Will the small solar panel that came with my RV charge my batteries?

Eventually, yes. But it might take several days or weeks. Nearly all of the small panels that are supplied as original equipment on RV's are intended only as trickle chargers. These are intended to keep the battery from going dead while sitting for long periods of time, and were never intended for actual day to day recharging (regardless of what the salesman told you). Most of these are in the 1.5 to 5 watt range, and put out from 50 to 300 milliamps. Also, if you expand your system, the wires from those small panels to the battery are not large enough to handle the higher current of the large panels - often the prewire is only #16, and we have seen as small as #18.

Are higher voltage panels better?

That depends on what you mean by "higher voltage". Nearly all "12 volt" panels are actually around 17 volts when working. But lately we have seen a couple of places touting their "high voltage" (on the order of 21 to 23 volts) panels on the premise that you get better low light and hot weather performance.

Unfortunately, it simply is not true. The simple fact is, panel voltage drops very little in low light (the amps are what drops), and as for high temperatures, a modern standard panel would have to reach over 175 degrees F to drop below 15 volts, which is still more than enough to charge a battery.

One of those same places claims that their 100 watt panel produces more power than a Kyocera 130 watt panel - neglecting to mention that they used an MPPT controller on their panel and a standard Morningstar controller on the Kyocera for their "test". That is like saying that our pump is better because it pumps faster into a 2-inch line than our competitors does into a 1/4" line. Some of the claims border on the unethical, but we leave that for you to decide. At the very least, it is hype - not fact.

Typical tilting RV mount

Typical tilting RV mount

Are tilting or adjustable solar panel mounts worth the effort?

In general - no, but it depends. While you will get more power if you tilt your panels to catch the sun better, the amount of improvement is not as much as most people would think. For example, if the sun is 30 degrees off from how your panel is set, you will lose around 13.4%. While a significant amount, you have to consider the extra expense of the tilting mount, the additional labor to install, the extra wind drag on the RV, and the hassle of having to climb up on the roof at each stop to adjust them.

If you park for longer periods of time in the same place or are a "hardcore" dry-camper, then they might be worth it.

What charger or controller setting should I use for AGM batteries?

If using Concorde batteries (Sun-Xtender, Lifeline, Chairman) you should use the AGM setting. Use the FLOODED setting only if the charger or controller does NOT have an AGM setting. AGM are NOT gelled batteries, and take a higher voltage setting. Most of the newer chargers and controllers have an AGM setting - in most, the only major difference between the flooded and the AGM setting is that AGM's do not generally require equalization. It won't hurt them unless you overdo it, but they don't need it like flooded batteries do (the main purpose of equalizing is to stir up the liquid, and AGM's have no liquid to stir up). For more information on charge controllers, and how they work, see our Charge Controller Page.

Are 6 volt batteries better than 12 volt?

Nope. There is no inherent reason why a pair of 6 volt batteries (usually the popular 180 to 220 AH golf car batteries) is any better than a 12 volt battery. The problem is that it is often difficult to find a true deep cycle 12 volt battery in your usual battery store. Some of the "Marine" batteries are no more deep cycle than your car battery. If it does not specifically state "deep cycle" on the battery, it probably is not. Also, many of the 12 volt true deep cycle batteries are quite large and heavy, and may be much harder to handle than a pair of smaller 6 volt batteries. There ARE a variety of 12 volt deep cycle batteries available, but many of them are the more expensive AGM type and are often not available from your local discount battery place. These include the Concorde. These AGM batteries have many advantages over flooded (and gelled), including longer life. However, the cost is typically 2 to 3 times the price of a pair of golf car batteries from a discount house (about the same as gelled).

Are some panels more shade tolerant?

Yes, but... The output of ANY panel will be reduced or cut off if shaded, but some are better than others, up to a point. If a single cell is heavily shaded, that cell is cut off. Most modern panels have "bypass diodes", which send the output from the remaining cells around that dead cell. However, you have lost the output of that cell, so the total panel voltage will drop by about .48 volts per cell. Most panels are in the 16 to 18 volt range, and most batteries need at least 13.5 volts to charge completely. So if 2 or more cells are shaded, the output voltage of the panel may drop too low to charge the battery. The Unisolar panels are somewhat more shade tolerant than crystalline panels because each cell cuts out only when totally shaded.

Dirt, dust, leaves, bird "stuff", and shadows from trees, vent pipes, etc can all cause reduced output of any panel due to shading. It's a good idea to wash your panels down once in a while, especially if they are laying flat on the roof like most RV installations do. All you need is a quick "slosh" with a soapy brush or mop and a rinse most of the time. "Tree droppings" such as pitch should be cleaned off with a solvent such as turpentine. The pitch itself won't reduce the output much, but being sticky it can collect leaves and trash.

Can I run the output of my converter through the solar regulator?

Not a good idea. In fact, usually a really bad idea. Because many of the converters that come as original equipment are, quite frankly, junk. We get a lot of inquiries about running the output of the converter through the solar charge regulator for better regulation. Their are two major problems with that - the first is that the output of the converter is not usually regulated, and is in fact "pulsating" DC with no filtering - it is basically 120 Hz pulsating DC. This drives most controllers nuts trying to figure out how to regulate this constantly changing current and voltage. The second, and more serious, is that most converters are not "current limited" like PV panels are. (This means that if you short a PV panel, it will only put out about 10% more amps than it's rated current this is known as Isc, or short circuit current). Because the converter is not limited, it is quite possible for them to put out more into a discharged battery than the charge controller can handle. You run a real risk of smoking the controller.


Source: Northern Arizona Wind & Sun

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