Friday, September 17, 2021
Friday, September 17, 2021

Part 2: What’s the best solar panel for your RV? How many do you need?

If you missed yesterday’s installment, part one, read it here before continuing.

By Dave Solberg
Deciding how many solar panels you need and how big you need them to be depends on a lot of variables. We covered some of those variables in yesterday’s segment, but here are a few more:


One other variable is your battery condition and an issue called sulfation, which is more prevalent in lead-acid and some in AGM batteries. As you drain power from your battery, sulfur collects on the plates. If you do not have a multistage charger or some type of battery conditioning system, your batteries will not be able to store a charge as designed. You can have the best solar panel system for your rig, but if your batteries are compromised, you will not be happy. Check to see if your house charging system has a multistage charger or battery conditioning mode such as the Charge Wizard from Progressive Dynamics, or if your solar panel has a multi-stage controller. Otherwise, it is a good idea to get some type of conditioner such as BatteryMINDer or the Charge Wizard.

One of the most popular questions I get in my boondocking seminars is, “How many batteries do I need?” This is a difficult question to answer as it depends on the amp draw calculation we discussed earlier according to usage. Batteries are rated in “group” sizes such as 24, 27, 31, etc. These are rated as to the amp hours available and the lowest group 24 has 80-100 amp hours and they go up from there. As we stated earlier, the lead-acid and AGM batteries can only be drawn down to 50 percent. We will cover batteries in a future article.

Rooftop vs. portable solar panels

For years, solar panels have been very large and very heavy so it made sense to mount them permanently on the roof rather than trying to find a place to store them and haul them in and out every day. Modern technology has provided lightweight versions of both, and portables have become a popular option. They are lightweight and fold into a convenient carrying case that can be stored easily. The advantage of the portable unit is you can park your rig under the shade and cool the interior down by more than 30%. This makes sense since you are probably dry camping and don’t want to run the generator to run the roof AC!

However, if you are under the shade of the tree, you don’t have much line of sight to the sun and therefore not getting much use out of your panels. With a portable system, you can park in the shade and put the panels out in the sun and get the best of both worlds.

You can aim the portable solar panel at the sun

Another advantage of the portable is the ability to place it at a direct angle to the sun. For example, in the morning, the sun rises in the east and rooftop panels may not get charging ability until the sun rises to a certain angle to the panel. Same with the evening when the panels are facing west. This could mean you only get a few hours for optimum exposure. However, with a portable panel you can place the unit facing east and move it west to get more charge time.

Some roof mount enthusiasts state they like to charge the batteries while driving down the road, but most RVs have a battery isolation manager solenoid (BIM) that provides a charge from the engine alternator to the house batteries while driving. Trailers with a 7-pin connection also provide a charge from the tow vehicle alternator.

One of the disadvantages of a portable unit is if you need a large charging solar system like 570 watts, you need several portable panels strung across the yard, and need to find a space to store them, and the task of setting them up and taking them down.


For the past several years, most RV manufacturers have been prewiring their coaches for solar applications. Roof-mounted panels typically have the correct gauge wire embedded in the ceiling and a sticker on the roof as a locator. Some are even mounting a plug that you can attach three panels to. This makes it easy to roof mount as you just need to attach the legs to the panel, apply a little butyl tape under the legs, and screw the feet to the roof.  Make sure you verify what power cords and cables might be close to the area you are placing the panel. If your unit is not prewired, you will need to find a way to route the correct gauge wire through the roof and to the house batteries with a charge controller in-line.

We installed a panel on a 2003 Winnebago Brave and ran the wire under the refrigerator vent cover and back behind the refrigerator. Notice we took the “batwing” antenna off as they had previously installed a King Controls permanent mount model. We then applied a generous amount of self-leveling sealant, NuFlex 311RV, recommended by Winnebago for fiberglass material. Check with your manufacturer for the recommended sealant for other materials such as EPDM, TPO, and Alpha roof material.

The converter was located in the cabinet next to the refrigerator so we were able to mount the charge controller underneath and connect it to the battery cables on the converter.  That way we did not have to find a path to the house batteries, which were located in a compartment behind the steps. On a travel trailer or 5th wheel, the house batteries are typically up front on the tongue or inside the front compartment, so finding a path from the roof to these is a challenge. That’s why it’s easier in most cases to use a portable and install the port up front.

Do I even need solar panels?

I get this question quite often at shows and rallies. The basic question you need to ask yourself is how often and how long will you be camping without being able to plug in? If it’s just overnight for a few times a year, the house batteries will be able to provide enough power to withstand that… if they are conditioned as we discussed earlier. In this situation, the investment in a solar system is a waste of money.

The need for a solar system is directly related to the amount of time you will be away from charging power either from a campground or shoreline source or generator power.

One other variable to consider is that more campgrounds are charging for electrical usage with a meter, so more RVers are using propane and inverters to power the TV and even the refrigerator. That means draining power from your house battery, which can be charged by free solar power. However, nothing is “free” as there is an initial investment in the solar system.



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Ron Cadenhead
1 month ago

I have 2 panels on the roof of our trailer for 200 watts, going into a 40 amp solar charge controller, and then into my 4 flooded lead acid batteries of 6v each wired for 12 volts. I love the system. My only problem is it will not charge the batteries over 14.2 v and therefore I never get into equalization. I am usually at 13.5v before noon. Panels are flat on the roof. No shade. Checking the charts after being parked for a week, the voltage will not have gone over 14.2v with the batteries switched off on the connection from the batteries to the chassis. The batteries have 425 amps each and the wired for 12v with each pair parallel and then wired in series. I may have the parallel and series backwards in my description, but it is wired correctly.
Do I need to add two more 100 watt panels?

Beverly Cleaveland
1 month ago

I was hoping to see a little more talk on the battery subject. Since we do not boondock, but we want to go to a12v 8 cu ft fridge and only need to use it during travel to our destination, which is no more than 5 hrs away, seems like we could add a 2nd battery (paralleled) to our 24M Marine Battery (75 amp hours). Is there documentation somewhere that helps us decide if this can work for us? I did do the form in part 1 and according to this we’d only be using 29 amps if we traveled 8 hrs with only the fridge and propane alarm drawing. Did I just answer my own question? 🙂

Last edited 1 month ago by Beverly Cleaveland
1 month ago

“As we stated earlier, the lead-acid and AGM batteries can only be drawn down to 50 percent.”

Once again this internet myth is rearing its ugly head. If lead acid batteries can only be drawn down to 50% SOC then why do battery manufacturers show a life cycle chart at 80% discharge that is very similar to a 50% discharge chart? You still get about the same number of amps out of the battery over its life you just do it quicker at 80% DOD. Lead acid batteries do not suddenly die a horrible death if discharged below 50% SOC.

1 month ago
Reply to  Don

It’s possible the voltage dips so much at 50% inverters start failing.

1 month ago

We bought a used MH with a GoPower system including two solar panels on the roof (not sure their size/capacity). These are great most of time, but for times when we camp under trees, we would like to have additional portable panels to add to this. How complicated would that process be? Or is it even feasible?

1 month ago
Reply to  MsSing

Not complicated.. if you know a few things first. If you’re using just one controller, make sure the added amperage can be handled. And make sure you match the voltage of the other panels.

1 month ago
Reply to  MsSing

The process of adding a portable solar panel is usually not difficult. Most of the portable panels I have seen have their own charge controller and have alligator clips on the end of the wires. With this setup, you would simply connect the wires to the battery. Many newer RV’s are prewired for portable solar panels, if this is the case for your RV, you would need to make sure that the panel has the appropriate plug on the wires.

Carl Jones
1 month ago

I’ve never heard a discussion on hard vs. flexible panels glued directly to the roof. Has anyone ever used flexible panels and with what results?

1 month ago
Reply to  Carl Jones

I had a hard panel fail, so I left it in place and glued a flex panel over the top. I do not want to put a flex panel directly on the roof. They get very hot.

Larry Lee
1 month ago

Solar panels are much quieter than any generator. Of course, that doesn’t help when everyone around you is running theirs!

1 month ago
Reply to  Larry Lee

Running a generator to watch TV is a real peeve of mine.

Sink Jaxon
1 month ago
Reply to  chris

ohh yeahhh…

Larry Lee
1 month ago

Once we passed the 1000 hour mark on our Onan generator I calculated the operational cost per hour. It surprised me to be $1 per hour including fuel, routine maintenance which I did (oil & filters) and the 1000 hour in the shop maintenance (belts, hoses, coolant flush etc.)
At that point I decided to install solar panels!
So I built a frame extending between the two A/C covers so the solar panels are just above the tops of the A/C’s to avoid A/C shadow on the panels. I started with 2 panels for 320 watts total. After operating for awhile I see that I need two more which will max out my 45 amp controller and should supply everything except the A/C’s.
Will still have generator for backup and engine alternator when driving.

1 month ago

The battery sizes you mention are BCI group sizes which are the actual dimensions of the battery. They don’t relate directly to storage capacity. A group 27 is physically larger than a Group 24, but does necessarily have more storage. The group number relates more to the hole that the manufacturer provided to store the battery.

Tommy Molnar
1 month ago

To my way of thinking, you can’t have too much solar power – if you boondock with regularity.

James Shoe
1 month ago

One of the issues I have with portable panels is theft. It would be a pain to have to pack up the panels everytime I want to leave my RV.

Ron Cadenhead
1 month ago
Reply to  James Shoe

I agree with you James. We have more than enough to pack up already when we leave the RV, and even more when we arrive or leave a spot.

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