Harnessing the sun’s rays for usable RV electric sounds like a good idea, but is it for you?
By Mark Quasius, F333630
For motorhome owners who camp off the grid, maintaining adequate electrical power can be a challenge. The batteries can power the coach’s 12-volt-DC systems for a limited time before they need to be recharged. If an inverter is used to transform 12-volt-DC battery power to 120-volt-AC power to run appliances, the batteries are asked to do even more, so it becomes necessary to operate your generator to recharge the batteries or operate the devices.
The downside is that your batteries may require recharging when it’s not convenient for you. Plus, there’s the cost of fuel to run the generator, as well as a limited fuel tank capacity.
To offset these drawbacks, some motorhome owners look to solar power as an alternative power source. Solar power is a green energy source.
Other than the initial cost and resources used to build a system, there is no cost to using the sun’s energy, no pollutants are emitted, and valuable natural resources are saved. It’s an excellent environmentally friendly solution.
Solar power is widely misunderstood. Most RV owners want to know what they can run using the solar panels. The truth of the matter is that solar panels don’t “run” anything. Batteries provide power to the motorhome’s appliances. Solar panels are more like a trickle charger in that they add a steady stream of current whenever the sun is up to help keep the batteries charged.
In theory, if your motorhome’s battery bank were large enough, you could charge it up before you left home; power all of your electrical devices via the batteries when camping; and then recharge the bank once you returned home. But in the real world that’s not possible, because the battery bank would need to be huge, and the time spent recharging between trips would be extensive. So, we outfit our motorhomes with a reasonably sized battery bank and recharge it as frequently as necessary.
Unless you are a real miser when it comes to power usage, odds are you will not be able to install a solar array large enough to recharge the battery bank without some assistance. Eventually you will need to run the generator to bring the batteries up to a full state of charge. But by installing a solar panel array that is closely matched to your style of camping, you can extend the interval between generator recharge cycles.
Solar power isn’t for everyone. A decent-size system will require a substantial financial outlay. If you spend the majority of your time connected to shore power at RV parks, it would be difficult to justify the expense of a solar system. Instead, you could extend your dry camping run time simply by adding a couple of extra batteries. Much will depend on your lifestyle. If you dry camp in parking lots overnight, then spend the next day driving, your batteries will recharge while you drive. If your battery bank is of sufficient size, you’ll be fine for an overnighter. If not, adding a couple of batteries should make the difference for you.
Before running out and buying a bunch of solar panels, you must first determine your electrical needs. An accompanying chart shows some “Typical Motorhome Electrical Loads.” In the case of the AC loads, I’ve converted the current draw to the battery load on the inverter, assuming a 90 percent efficiency rating on the inverter.
The easiest way to calculate battery draw is to convert watts to amps by dividing the wattage by the voltage to derive the amperage. In order to determine how much battery power will be used on a daily basis, you need to determine how long various electrical devices will operate. In the “Daily Electric Requirements” chart, load times are multiplied by the actual hours of use to determine the total daily load on the battery bank.
As you can see from the “Daily Electric Requirements” chart, the appliances will draw approximately 180 amp-hours from the battery bank per day. The popular 6-volt-DC deep-cycle batteries used in most motorhomes are rated at 220 amp-hours at 6 volts. A typical bank of four 6-volt batteries will have a total capacity of 440 amp-hours at 12 volts. Because flooded deep-cycle batteries should not be drawn down below 50 percent state of charge, we can safely use 220 amp-hours of battery power before recharging is required.
Based on our 180-amp-hour sample load, we can run these loads for one day without a recharge cycle, but only 40 amp-hours will remain for the next day’s use. This will require daily recharge cycles with the generator to prepare the batteries for the next day’s power needs.
If we add a pair of 140-watt solar panels to the motorhome and assume that we will have seven hours of sunlight, we can gain an additional 160 amp-hours of battery power. This would allow us to extend the battery recharge cycle to every other day.
Solar panels can be either monocrystalline or multicrystalline. The difference is in the way the crystals are grown. For practical purposes, the output won’t vary significantly, but multicrystalline panels generally are less expensive than monocrystalline panels.
Calculating how much output you will derive from a solar panel is iffy at best. A solar panel’s maximum power output occurs only when the sun is hitting the panel at a right angle. Throw in a little cloud cover or factor in the low angle of the sun early or late in the day, and less than optimum performance results. Some panels perform better than others in low-light conditions, but actual performance will vary according to your latitude and the weather. You’ll get great performance in sunny southern Arizona but less power in the rainy coastal area of northern Washington. A solar panel’s performance will range between 25 percent in low-light conditions to nearly 100 percent in peak sunshine. Assuming 60 percent as an average output is a safe bet. You then have to factor in how many hours of daylight you’ll have each day and multiply that by the rated panel wattage times 60 percent to determine the output of a panel on a typical day.
Any solar array should be equipped with a charge controller. The charge controller connects the solar panels to the battery bank and is mounted near the battery bank, typically in a basement storage compartment. Charge controllers vary in cost and performance according to size and quality. A solar panel will send current to a battery during the day as long as the output voltage of the panel is higher than that in the battery. Once the sun goes down, the battery will have more voltage than the panel, and the battery could actually drain back into the solar panel. This is called dark current. A charge controller is equipped with diodes to prevent any dark current from draining the battery. In addition, the charge controller will regulate the solar panel’s output voltage to prevent it from overcharging the battery and boiling the electrolyte.
A quality charge controller should have three-stage charging, beginning with bulk charge, tapering to absorption, and then going into float mode to prevent battery overcharging. An ideal solution is to opt for a mean power point transfer (MPPT) charger.
MPPT technology allows you to exchange volts for amps. All a battery needs to see when being charged is amperage. As long as the charging voltage is higher than the present battery voltage, current will flow into the battery and not out of it. Typical solar panels have a peak output of 15 volts. Because amps times volts equals watts, a 140-watt solar panel will be able to dish out 9.33 amps at full sunlight. When the sun’s angle or intensity is reduced, the output voltage will drop. As the voltage drops, the amperage will increase. A charge controller with MPPT technology will monitor the incoming voltage and trade off voltage for amps by reducing the voltage minimally required to charge the batteries and increasing the amperage accordingly, resulting in some extra battery charging amps not available with a non-MPPT controller.
For example, if you have a non-MPPT controller receiving 15 volts from a 140-watt panel, 9.33 amps will be sent to the battery, regardless of how low the battery’s state of charge is, even if your battery is at only 10.8 volts. An MPPT charger will change that by reducing the output voltage to 11.0 volts, which is still enough to allow battery charging. As the battery’s state of charge increases, the MPPT controller will raise the charge output voltage accordingly. But at 10.8 volts, the MPPT charge controller will trade off the extra voltage from the solar panel for amperage, allowing a full 12.73 amps to pass through to the battery. An MPPT charge controller typically offers a 30 percent improvement in output, so it’s the best controller to select for your motorhome’s solar array.
During times when the sun is low in the sky or shrouded by cloud cover, the voltage from the solar panel is reduced. If that voltage falls below the battery voltage, you will no longer be able to charge your batteries. Some charge controllers are designed to take a higher-voltage solar panel array and transform it down to 12 volts for your battery bank. This has two benefits. First, it can reduce the wire gauge size required to pass current from a multiple-panel array to your charge controller. A typical 140-watt panel will output an average of 10 amps of current. If you add a second panel in parallel, you’ll still have a 12-volt array, but now you’ll get 20 amps. Going to a four-panel array increases this to 40 amps at 12 volts. In order to pass 40 amps of current through to the charge controller, you are going to need heavier, #8 gauge wire, which is more expensive and isn’t as easy to handle. If you connect those same four panels in series, you would then have 10 amps at 48 volts. This would require less expensive, lighter-gauge wiring, which is much easier to install.
Charge controllers that can take a higher voltage input and transform it to a lower voltage output are necessary to take advantage of this. A second benefit is that you’ll have higher voltage for a longer time. When the sun starts to go down, a parallel array may drop to 10 volts, which isn’t enough to charge the batteries. But a series-connected array of four panels will still be outputting 40 volts, so you can continue to charge your battery bank during the fringe hours of sunlight, resulting in more amp-hours per day than a controller that does not have that ability.
Installing Your Own RV Solar System
Assuming that you have done your research and decide that a solar system is a good choice for your motorhome, the next step is to plan your installation. First, you’ll need to see whether you have enough room on the roof for your solar panels. Solar panels up through 140 watts measure approximately 26 inches wide by 58 inches long. The next level gets you into 240-watt panels, but they are around 40 inches wide, which is generally too wide for most motorhome installations. Be sure you have adequate roof space to mount your panels while still allowing access to other rooftop components that require service and maintenance.
The solar panels reside on the roof, but, as previously noted, the charge controller needs to be mounted in your motorhome’s basement or an area close to the batteries. This means you’ll have to find a place on your roof to pass the wiring through. That route should be in an area that has a vertical cavity, such as a hollow wall or cabinet. One common installation is to pass the wire down through a refrigerator vent and behind the refrigerator to access the floor. If your motorhome has a residential refrigerator or a refrigerator located in a slideout, another route will be needed. Depending on your solar panel location, it may be possible to pass the wires through a rear cap and engine area.
The proper wire gauge is critical for solar panel operation. Running current in excess of a wire’s capacity will result in loss of performance and potentially create a safety hazard. If possible, choose a charge controller that will allow you to connect your solar panels in series rather than in parallel. That will increase the voltage on the array and minimize the current demands placed on the wiring. Once the electricity exits the charge controller, the output will be around 12 volts, so the wiring to the batteries will need to be upsized accordingly. Try to place the charge controller as close to the batteries as possible to keep the wire run short and minimize voltage drop.
The charge controller most likely will come with a remote display to mount on an interior wall of the motorhome in any convenient location. The monitor usually is connected to the charge controller via basic cat5 or phone cable, so cable length is not a big concern.
Solar panels can become hot, so they need to be mounted on raised rails to allow air to circulate beneath them. Panels can be flat-mounted to the roof or installed on adjustable mounts that can articulated to provide a better angle to the sun. Most motorhomes will benefit by flat-mount panels. Adjustable mounts require owners to climb on the roof to change the angle and, in reality, are beneficial only if your motorhome is parked in an ideal spot for a long time.
Rivets are the best fastener for mounting solar panels. Threaded screws can strip out in thin fiberglass roof materials, but rivets will expand on the underside of the fiberglass and offer greater holding power with less chance for pullout. Be sure to use plenty of sealant beneath the mounting brackets as well as over the top of the fasteners to prevent any water penetration.
Solar panels can be equipped with junction boxes or MC4 cabling. Weatherproof junction boxes are located on the back side of each panel, and regular shielded cable is run to connect the panels to the motorhome. Most panels now come with the modular MC4 wiring connections. These connections press together in a true plug-and-play configuration and ease the wiring task. MC4 wire gauges are limited so they work best in a series-connected array. Adding panels in a parallel configuration increases the current flow and larger-gauge wire may be required. If you plan your array with the idea of possibly adding panels later, be sure to size your wiring accordingly.
As noted, solar power isn’t for everyone. Adding a single panel won’t provide much benefit. If you’re serious, you need to dive in and create the proper size solar panel array with a good charge controller in order to make the system effective.
If you spend most of your time at RV parks with hookups, it will be hard to justify the expense. But for those who spend the majority of their time dry camping, a well-designed solar array can be a worthwhile investment. Operating expenses are virtually zero. The solar panel output will help to minimize the need to run the generator to recharge the batteries. This will result in less fuel usage and extend the times between recharge cycles so that you can run the generator at your convenience rather than out of necessity.
With no exhaust emissions or depletion of fossil fuels, solar power is also environmentally friendly. And many RVers opt for such a system on the basis of considering it a “green” alternative rather than the economics.