Part 6 - The Panels

How Many?

The first thing you need to decide about solar panels is how many you need.  First of all, how much physical space do you have on the roof of your rig for solar panels?  This could be a limiting factor. 

Assuming there is enough space to meet your needs, how do you decide how many watts to put up there?  One rule of thumb is to have enough wattage on the roof to replace your average daily amperage use in half a solar day in the location where you will usually be boondocking.  By "solar day" I mean the number of hours in a day you can expect peak solar performance depending on your location.  A map which shows this kind of information is called an insolation map.  One such map for the U.S. is provided by Wholesale Solar here.

Why so much wattage?  Because, practically speaking, you never get as much wattage out of a solar panel as they are rated, for various reasons.  For one, clouds often interfere.  For another, to get the most amount of watts a panel is capable of producing, it must be aimed directly at the sun 100% of the time, and that is just not practical without installing an automatic dual axis solar tracking system at great expense.  Ambient temperature affects a panel's performance (they are less efficient when it is hot out, for example) as well as the panel's age.  The reality is, you will never experience your panels' full rated output all the time because of these shortfalls of the technology.  It's just the nature of the beast.  To compensate for that, you need more wattage on the roof. 

80 Watt Solar Panel

So let's take a look at this example. You think you'll use 50 amps of power on average from sunset until sunrise the next day, in a location that has a 5 hour solar day.  So, using the rule of thumb mentioned above, you want enough watts on the roof to return the battery bank to its former state of charge in 2.5 hours.  An unpleasant fact of life is when you use an amp hour from your battery bank, you have to replace more than an amp hour to get the battery back to its former state of charge.  This is due to inefficiencies in the system and in the batteries themselves.  How much more is debatable, but for the sake of this example let's say 10 percent. 

So for this example you would need to put 50 + (50 x .1) = 55 amps back into the system in 2.5 hours, or 22 amps per hour.  Since watts = volts x amps, and if your panels produce 14.4v, that means you need 14.4v x 22a = 317 watts (roughly) on the roof.  This is not exactly correct, since the voltage and amperage will vary during the charging process and your panels are not going to be producing maximum wattage all the time.  So this will not guarantee that your system will actually return the batteries to their former state of charge in 2.5 hours due to the inefficiencies mentioned above.  But it will make it much more likely that they will do so in a solar day, day after day, than if you designed the system based on taking a full solar day to recharge.  This is not an exact science because there are so many variables that will affect your system.

Maybe the rule of thumb mentioned above is too severe for you.  It's certainly not written in stone, and different expectations will certainly change your needs for solar recharging.  If you only want to prolong the time you're able to camp with no expectation of being fully recharged every day, then maybe planning on taking a day or more to recharge fully would be sufficient for you.  Or maybe you are OK with supplementing the solar with a generator from time to time.  Or maybe you simply don't have the roof space for so many panels.  Or any number of other factors.  The decision is yours. 

Reducing Electricity Usage

Another way to reduce the amount of wattage needed on the roof is to reduce your average daily electricity usage.  One way to do that is to replace traditional incandescent light bulbs with LED lighting which uses a fraction of the electricity needed by those traditional bulbs.  Avoiding use of high-draw appliances such as a microwave or electric coffee pot is another way to reduce usage.  Appliances with that much of an electrical draw are going to require a much more substantial electrical and solar recharging system than would otherwise be needed.  If you've got a generator you could just plan to use those higher-draw appliances while the generator is running, or just do without those items.  This would allow you to build a lower capacity system than otherwise would be needed.  For me, it's not an issue.  I boil water on the stove for coffee and use a thermos to keep it hot, and don't have a microwave in the rig. 

Of course if you've got the roof space and battery bank to handle it, you can certainly design a system that will handle your higher-load appliances, with the exception of air conditioning.  As far as I know there is nobody out there in an RV routinely running an a/c system on battery power.  It's just not practical due to the high power demand and the huge battery bank that would be required.  There are some whose systems are big enough to start the a/c and run it for a short while, but keeping the rig cool throughout the day with a/c is just not in the cards if all your power is coming from batteries.  Those who run a/c while RVing are plugged into the grid or are running one or more generators that are big enough to handle the load.   Again, none of this is an issue for me.  I don't have a/c in my truck camper, and don't normally camp where it's hot enough to need it anyway.

Tilting

Wander out to Slab City, California, in the winter and you will see scores of rigs whose owners understand the importance of tilting panels when the sun's angle stays low.

If you camp in a location or time of year where the sun will be at a relatively low angle, having the ability to tilt your panels (even just on one axis) will improve their performance immensely. My panels tilt upward on a single axis, and in winter I usually leave them tilted at about a 45 degree angle all day while pointed at solar south.  "Solar South" is the position the sun is in at solar noon (the time of day that is exactly half way between sunrise and sunset).  In summer I don't bother tilting them since the solar day is so much longer and the sun usually passes almost directly overhead in my location.

I once did an impromptu test while camped in the Southern California desert in January by watching my array's output while tilted, as compared to when it was laying flat.  While flat it produced just 59% of what it did while tilted.  If you camp in winter when the sun's angle is low, single-axis tilting can be a big benefit. 

Dual-axis automatic tracking is probably prohibitively expensive, however.  In most cases it would probably make more financial sense to add more panels, assuming you have space for them, than to have an automatic dual-axis tracking system installed.

What Type?

Solar panels come in a variety of types and sizes.  I have three 50 watt panels and one 40 watt panel in my array that put out about 21.8 volts Voc (Open Circuit Voltage).  These, wired in parallel, work well in a 12v system.  There are panels available that put out much more, for use in 24v or 48v systems, for example.  Higher voltage panels can be used in 12v systems if the charge controller is capable of moderating the voltage.  The advantage of this is smaller wires can be used between the solar panel array and the controller without as much voltage drop as would occur in lower-voltage panels wired in parallel using the same wire size.  In the world of solar charging, Public Enemy #1 is shade, and Public Enemy #2 is voltage drop.

The same "higher voltage" effect can be achieved by wiring panels together in series, which adds their respective voltages cumulatively.  But again, this can only be done with a charge controller that has the capability of bringing the voltage back down for use in a 12v battery bank. This requires a certain type of Maximum Power Point Tracking (MPPT) controller.  More about MPPT later.

Solar panels also come in a wide variety of wattage capabilities.  When deciding which ones best fit your needs, you'll need to consider their physical size and how they will fit on your roof, if roof mounted.  If you want 300 watts on the roof, what's better for your situation - three 100-watt panels or a 200 and a 100?  Maybe a 230 and a 60 will get you close enough.  There are a wide variety of combinations and you'll need to make your decision based on physical size as well as price.  

To avoid complications and building inefficiencies into the system, buy panels with matching voltage ratings.  Mixing panels with different voltage ratings can be done, but the voltage of the array as a whole will be affected by that.  And if you're planning on using an MPPT controller, definitely avoid mixing voltages because this can adversely affect the MPPT controller's ability to calculate the maximum power point correctly.

Solar panels also come manufactured using different technologies.  Some of the different types are monocrystalline silicone, polycrystalline silicone, thin film and building integrated photovoltaics (BIPV).  They have different levels of efficiency, which directly translates to the amount of surface area a panel takes up to produce a given amount of electricity.  Obviously, there is not a lot of room on an RV roof, and even if you plan on setting your panels up on the ground away from your rig, you will likely want to minimize the amount of surface area your panels' take up.  So you probably want the most efficient technology you can get.  At the present time it appears that is monocrystalline silicone, but many use polycrystalline as well.

Where to buy? 

There are many outlets for solar equipment.  I've listed a few of them on the bottom of Part 9, Installation and Monitoring, but you can find many more by just doing an online search.

 Warranties

Finally, different manufacturers may have different warranties.  A rather standard warranty is a guarantee that in 25 years the panels are still capable of producing 80% of their rated watts.  You can find name-brand panels as well as inexpensive made-in-China panels that provide that same warranty.  

What's Next?

So, now that you've got the panels figured out, what about the charge controller?  That's covered next in Part 7, The Charge Controller.