RV Solar Power 101

To the RV owner who cherishes independence from RV Parks and campgrounds and who hates the noise and smell of running a generator, solar power is, without a doubt, one the greatest technological advancements to hit the RV industry in the last 25 years. What makes solar power so great for RVs? What are the benefits? First, the system requires practically no maintenance, as there are no moving parts. It’s also lightweight, mounts primarily in out-of-the-way locations on the roof, and doesn’t take up valuable storage space. Solar power also adopts well to the electrical systems found in RVs and is relatively easy to add. Perhaps more importantly, the solar power system doesn’t require fuel like a generator, is clean and quiet, and utilizes a free and renewable source of energy that’s good for the environment. The purpose of this RV Solar Power 101 article, is to present some basic concepts and equipment related to solar power for RVs, a beginner’s guide on solar power if you will, on how to get started.

An RV solar power system isn’t that complex either. You don’t have to be an electrical engineer to design a system or even install it. The solar power system consists of one or more solar panels to generate a charging current, a charge controller to regulate that charging current, and one or more deep cycle batteries to store that current. It also consists of ancillary components such as a roof top combiner box for connecting multiple solar panels to the system, mounting brackets for attaching the solar panels to the roof, wiring to connect everything together, fuses to protect your system, and an optional battery monitoring system. Now let’s take a look at each:

1. Solar Panels

The key component that makes solar power possible, the solar panel is rated in watts and is constructed of numerous photo-voltaic (PV) silicon cells that convert the sun’s energy into Direct Current (DC). Each individual cell consists of a positive and negative side and a middle area called the P-N junction. When the sun strikes the surface of the cell the electrons in the P-N area become excited and generate a voltage and current. Each cell generates about .5 volts, so you’ll need two cells to generate a single volt of DC.

The 12 volt solar panels used in the majority of RVs are designed to produce a maximum voltage anywhere between 17 and 21 volts. The difference may seem strange, but when you consider the environmental factors at work to reduce this output, and that you need at least 13.8 volts to provide an effective charge for a 12 volt battery, it makes sense. Any excess voltage is regulated by a charge controller, anyhow, so no damage will occur to your battery. There are three types of solar panels available in the market today:

Monocrystalline: These photo-voltaic cells are simply the best that money can buy. The manufacturing process is the most expensive and employs the use of very pure silicon and a complex crystal growth process that involves a single block of crystals. Black in color, monocrystalline solar panels are more expensive, but they offer the highest efficiency, at 15 to 19 percent, and last the longest of the three. If you’re interested in buying one or more monocrystalline solar panels, Renogy produces a 100 watt solar panel lauded as one of the best with a maximum efficiency of 18.5 percent. And at only $114, is a real bargain.

Polycrystalline: These are an excellent alternative to monocrystalline cells. The difference with polycrystalline cells is that they are grown from a large block of crystals rather than a single block. This produces the distinctive shattered glass appearance that is characteristic of polycrystalline solar panels. Dark blue in color, polycrystalline solar panels are only slightly less efficient, at 14 to 16 percent, but cost less than its more expensive “mono” cousin.

Amorphous: These solar cells are created not by using crystals or wafers, but by depositing a thin layer of silicon over a base material such as glass, metal, or plastic. Flexible solar panels are an example of this technology. Amorphous solar panels are cheaper, react better to diffuse and fluorescent light, and work better at higher temperatures, but they are also the least efficient, at 6 to 8 percent, and need more physical space than regular crystalline solar panels.

As mentioned, several environmental factors affect the efficiency or electrical output of a solar panel. These factors include shading, overcast skies, and temperature. How shading and overcast skies effect sunlight is self-explanatory, but temperature isn’t. As temperature increases the voltage of the panel is reduced, this is especially true when temperatures hit the triple-digits, like here in the Southwest. The negative impact of high temperatures can be alleviated, somewhat, by mounting the panel an inch or two from the roof to allow cooling air to circulate underneath. Since heat reduces the efficiency of each panel, it follows that the panel will perform better during winter months. Unfortunately, cooler weather also means that the sun will be lower in the sky, thus reducing the angle and the amount of sunlight the panel receives. Tilting mounts, however, can be used to mitigate this low angle and improve the system’s performance during the winter.

Shading obviously reduces the amount of sunlight that strikes the solar panel, so care is needed in choosing the best location on the roof of your RV to mount it. Avoid placing the panel near air conditioning shrouds, satellite domes, storage pods, and collapsible television antennas. The same applies where you park your RV when you camp. Avoid camping next to trees or other terrain that can shade your RV and solar panels from the sun’s rays.

2. Charge Controllers

Simply put, the solar charge controller acts as the “brain” for the solar power system. It harvests and regulates the voltage from the solar panels to prevent your batteries from being overcharged, and in the case of wet cell batteries, boiled dry (note: charge controllers aren’t needed for small 1 to 5 watt trickle charge panels). The solar charge controller provides effective three-stage or four-stage battery charging just like the 110 volt AC battery chargers found in your RVs. Unlike solar panels, which are sized by watt, solar charge controllers are sized by amps with higher amperage models generally costing more. While charge controller features vary by make and manufacturer, there are two primary types of charge controllers used in the industry today: Pulse Width Modulation and Maximum Point Power Tracking. Let’s take a closer look at each.

Pulse Width Modulation (PWM):

Strengths of the PWM charge controller design include the fact that it is built on a time-tested and proven technology, is inexpensive–a single unit capable of handling 25 amps can be purchased for less than $100–and is durable. The PWM charge controller comes in various sizes up to 60 amps and can be used in all but the largest systems found in an RV (to give you an idea how large a 60 amp system is, a single 120 watt solar panel generates about 6.5 amps). Most PWM controllers being built today provide three-stage battery charging, though some companies, like Zamp’s ZS-30A, provide five-stages.

While the PWM design is simple and rugged, there are some inherent flaws with the design. For one, the PWM charge controller is only about 80 percent efficient since the controller connects the solar panel to the battery directly. This reduces the voltage developed by the solar panel from the nominal 17 volt output to the battery’s voltage, which lowers the power available from the solar panel. Another flaw with the design is that the pulses generated by the device can create interference in radios and TVs. This is due to the lower frequencies typically used in the PWM charge controller compared to the higher frequencies used in the MPPT controller.

Maximum Power Point Tracking (MPPT):

The newest technology and the latest rage, the MPPT charge controller combines the most effective features of a PWM controller with additional functionality–a PWM controller on steroids, if you will. Instead of connecting the solar panel directly to the battery like a PWM controller, it uses a DC to DC “buck converter” stage before the PWM charging stage. The buck converter ingests the solar panel voltage and transforms it to the optimum battery voltage or “Maximum Power Point.” Secondly, the MPPT charge controller uses an embedded microprocessor driven algorithm to scan or track the solar panel for where the voltage and current are optimized between the solar panel and the battery (the Tracking portion of Maximum Power Point).

The pros of the MPPT charge controller are pretty significant. It harvests at a high-efficiency rate, about 92 percent, and can save considerable money on larger systems since they can provide up to 20 percent more power in winter and up to 10 percent more in summer. Moreover, the MPPT charge controller is totally compatible with solar panels of various voltages, such as 24 volt and 36 volt models (just make sure all the solar panel sizes and voltages are the same in order for the tracking function to properly work). These higher voltage solar panels can be wired in series or parallel and are generally less expensive per watt than 12 volt models, thus giving you greater flexibility in the panels you can buy. The MPPT charge controller can also be sized up to 80 amps and beyond, providing greater flexibility for system growth. Most MPPT controllers available in today’s market provide four-stage battery charging.

Cons of the MPPT controller include greater cost, two to four times more to a comparably sized PMW controller, and greater physical size. However, the pros associated with the unit’s efficiency and features far outweigh the cons. If you use solar power a significant amount of time throughout the year, and size and cost isn’t an issue, then the MPPT controller is the way to go. If your system is generally small and is used perhaps six to eight times a year on weekend trips, then a PWM controller will more than suffice and will give you years of excellent service.

One valuable charge controller feature is the so-called battery temperature sensor. This requires the use of a special wire lead that attaches to both the battery and the controller. This feature automatically adjusts or compensates the charge voltage based upon the temperature of the battery. A cooler battery requires a higher charge voltage whereas a warmer battery requires a lower charge voltage. Applying the proper charge voltage improves battery performance and life, and minimizes battery maintenance. Obviously, this feature is particularly valuable for RVs with outside battery compartments and not so much for those compartments that are found inside.

3. Batteries

Storing all of that energy being generated by the solar panel is the function of the 12 volt battery. First and foremost, the 12 volt battery you get for your RV should be a true deep cycle battery. Avoid buying an automotive starting battery or a RV/Marine battery (a hybrid of the deep cycle and starting battery) as neither are designed to withstand severe discharges. Indeed, deep cycle batteries are designed to be discharged up to 80 percent or more numerous times, and still provide amperage at its rated capacity. When it comes to the battery’s ratings, amp hours are the key and you want more of them. That means buying the largest battery or batteries that will fit in your battery compartment. The typical group 27 deep cycle battery provides about 100 amp hours of service.

In RV applications there are basically three types of deep cycle batteries from which to choose: Flooded, lead-acid (or wet cell for short), Absorbed Glass Mat (AGM), and Lithium Iron Phosphate (LiFePo4). Yes, Gel-Cell batteries are technically a fourth alternative, but in my opinion these shouldn’t be considered as they don’t work well in RV deep cycle environments. Let’s take a closer look at each:

Wet Cell Battery: These are the cheapest and most common batteries found in the market today, and are available in numerous sizes. You can’t go wrong with wet cell batteries, but the big negative with them is that they need periodic maintenance and proper charging. Overcharging can boil out the electrolyte in them and warp the plates, while under charging will leave sulfate on the plates which reduces storage capacity. The maintenance consists of periodic gravity checks using an hydrometer, periodic equalization charges to remove sulfate from the individual battery plates, and careful monitoring of electrolyte levels to make sure the plates are covered (make sure to use distilled water only when maintaining electrolyte levels). All things considered, wet cell batteries work great and are well worth the cost. With proper care and maintenance, they will provide many years of reliable service.

AGM Battery: In sharp contrast with wet cells, AGMs batteries, like those made by Lifeline, require no maintenance or watering as they are sealed. AGM batteries are still technically lead-acid batteries, but the electrolyte in them is encased in a fibrous glass mat that can’t be spilled. Since AGMs contain no liquid they are practically impervious to freeze damage and can be mounted on their side, an important benefit for some battery and storage compartments. Most AGMs, like those made by Lifeline, also have very thick positive plates and can suffer more discharge cycles. More importantly, they charge up to five times faster and have a slower self-discharge rate, about 1 to 3 percent a month. Unfortunately, AGM batteries cost two to three times more than a regular wet cell battery, but the benefits to your solar power system make them well worth the cost.

Lithium Iron Phosphate (LiFePO4) Battery: The best battery you can buy for your RV, period. Unlike the standard lead-acid battery, which offers only 50 percent usable capacity, the lithium battery offers an impressive 90 percent. This means a 240 amp hour lithium battery bank will provide the owner with 216 amp hours of usable capacity instead of only 120 amp hours for a regular lead-acid system. But that’s not all. The lithium battery is lighter, provides a higher current output, charges faster because they can be bulked charged to nearly 100 percent, and suffer from zero voltage sag. The sealed battery also doesn’t outgas, lasts longer than the standard lead-acid battery (6,000 full cycles—six times more than a lead-acid battery—is the norm), doesn’t require regular full charges, and doesn’t require equalizing. Aside from the high cost, the only real negative with the lithium battery is that it can be damaged if it’s charged below freezing, but when it comes to the pros for RVs, nothing beats a lithium battery. Nothing.

4. Ancillary Components

Wiring: Choosing the right size wire for your solar power system is critical. Undersized wiring will reduce the efficiency of your system, and with environmental factors already working against your system, you obviously want to avoid this. Many professional installers use 10/2 wire (the number “10” refers to the gauge of the wire and “2” refers to the number of wires) to connect the solar panels to the combiner box, and 8/2 wire to connect the combiner box to the charge controller, and the charge controller to the battery. This approach will work for most solar power systems installed in all but the largest RVs with the largest systems, but you should still size the wiring in your system by using a tool like Blue Sea’s excellent Circuit Wizard.

Fuses/Circuit Breakers: In-line fuses or circuit breakers are important to protect the wiring and components in your system from shorts and other catastrophic failures. Place one on the positive wire within a foot of your battery and another in between the combiner box and the charge controller. The size of the fuse or breaker depends upon the size of wire used in your system. Place no larger than a 30 amp fuse or circuit breaker for 10 gauge wire; no larger than a 48 amp fuse for 8 gauge; no larger than a 74 amp fuse for 6 gauge; and no larger than a 120 amp fuse for 4 gauge wire.

Combiner Box: Some kind of system wire interface or combiner box will be needed on the roof to connect the solar panel to the solar power system. Most RVs with factory installed solar power systems use a simple two-pronged plug as a roof top interface. This kind of interface works fine, but can accommodate just one panel, so if you want more than one solar panel in your system, you’ll need a combiner box. The combiner box offered by AM Solar can connect up to seven solar panels to your system.

Mounting Brackets: There’s not a lot to say here, except that you’ll need a way to mount your solar panel to your roof. There are two basic types of brackets from which to choose: simple aluminum Z-brackets, which are fixed and immovable, and aluminum tilting brackets, which allow you to raise and tilt the panel to provide a better angle to the sun. Z-brackets are extremely cheap and a set of four can be purchased for around $8. Tilting brackets cost much more and come in various styles and provide the added benefit of allowing you to raise the panel to perform maintenance underneath.

5. Sizing Your Solar Power System

At this point you may be wondering how many solar panels you’ll need for your RV. Figuring this out isn’t too difficult. You can either calculate the amp ratings of the devices you use and how often you use them during a typical day, or you can conduct an experiment and boondock using all the 12 volt lights and appliances you normally use when you camp and see how long it takes to discharge your battery. If it takes, say, three days to fully discharge your battery bank to 50 percent, and you have two group-24 batteries rated at 160 amp hours total, that means you used about 27 amp hours per day (only 50 percent of a battery’s capacity should ever be used to prevent damage to the battery).

Now that you’ve determined your daily amp hour usage, you’ll need to determine how many solar panels you’ll need to replenish the 40 amp hours consumed in your average day camping. A 120 watt solar panel can produce up to 6.8 amps and during spring and summer this usually means you’ll get four to five hours of peak performance. So a 120 watt solar panel will produce at least 34 amp hours on a clear day. Two 120 watt solar panels will double that, providing nearly 70 amp hours a day. As a general rule, I like to oversize the system to provide more amperage in case it’s ever needed like on extra cold days when you need to run the furnace a lot during the night.

Another basic sizing rule used by some is to simply match each 100 amp hour battery in your system with a 100 watt solar panel. So if you have four group 27, 12 volt batteries (4 x 100 amp hours), you’ll need four 100 watt solar panels. This surprisingly effective rule assumes you’ll use an average amount of amp hours a day, but it works. It’s the rule I use.

6. Portable Solar Panels

For those who want more wattage for their solar power system, but lack the requisite roof space, the portable solar panel is an excellent alternative. Most portable solar panels or “solar suitcases,” as they’re popularly called, consist of two small solar panels wired in series (for example, the Renogy 100 watt moncrystalline solar suitcase consists of two 50 watt panels). Portable solar panels can be purchased with or without a charge controller, meaning it can tie-in with your existing rooftop solar power system or operate in parallel with it. A big advantage of these portable solar panels is that they can be tilted and aimed toward the sun to increase the panel’s output even more. The only negative with these portable units is that they can be stolen if you’re not careful. We highly recommend the Jackery SolarSaga 100. It’s a terrific portable solar panel. It comes with a MPPT charge controller and weighs only 9 pounds.