San Diego California Campground Sunset

RV Solar Series: Portable Solar Panels

Portable solar panels are very popular with RVers. Do they work? How well do they work? When to use them and how best to use them?

Did I need a portable solar panel?

That is an interesting question. I have friends with portable panels and my brother uses a portable every time he camps.  I have also said a 300-watt portable array that was adjusted with the sun path would create more energy than my 700-watt, flat panels produce, especially in the winter.

David's ground-mounted solar
David’s Portable Solar Panels

When I first designed my system, I did not completely understand the full implications of having a lead-acid battery. This included how resistant to charging a lead-acid battery is and how a lead-acid battery likes to (demands to) be treated.

Not Optimum

I also designed my panels to not be optimum to the sun angles because I didn’t want to get up on the roof to tilt them. All my solar panels on one side of the roof so that I could walk on the other side.  I tilted my solar panels only enough to help let the rain runoff. I made allocations for these less than optimum conditions by increasing the size of the panels.  Portable solar panels that are in the RV when you are on shore power only take up valuable storage space. Solar panels on the roof when you are on shore power are not noticed and they shade the roof.

As a side note, panels alone don’t alone make a portable solar battery charger. The other critical item is the solar controller (and the wire).  Even though some manufacturers hold that having multiple charge controllers on one battery causes evil interactions between charge controllers, I can’t find any real data to support these claims. After talking to a big charge controller manufacturer and he claimed his controllers would not play well with each other – so I picked a different controller company.  Several manufacturers endorse parallel controllers and I have seen tests that say it is fine as long as the charging parameters are the same for both controllers. 

Huge Advantages

Portable solar panels have two huge advantages over fixed panel systems, especially one that is fixed flat to a roof.   The first advantage is obvious; you can put the remote panel in the sun, even if the RV is in the shade.  This would not have worked at last year off the coast of Oregon where most of our camping locations were in the shade, deep in the shade. If you looked in the right area, through the trees, sometimes you could see a speck of blue sky.  If you camp exclusively in such an area, don’t waste your money on a solar battery charging system.  

The second advantage is also obvious and relates directly to the first, but the reason may not be as obvious. The best orientation in the Northern Hemisphere between 30 degrees and 60 degrees north, is a tilt angle of 60 degrees. Sorry, I was wrong on the tilt angle, 45 degrees is not the best. I was also wrong on the North-South orientation facing East – South – East is best. Of course, if you track your panels point them at the sun.

Photo portable folding solar panel stock
Portable Solar Panel

The best-fixed-tilt is 60 degrees and the best-fixed orientation is East – South-East

The above recommendation assumes that your goal is to charge a lead-acid battery and is based on how a lead-acid battery wants to be treated.  The lower the charge state of a lead-acid battery, the first thing in the morning is where a portable panel kicks butt versus my system. It is because a portable, 60-degree panel pointed east as the sun rises, will deliver the maximum current, exactly when the battery needs the maximum current.  Vertical orientation, on the side of the RV, is in some locations, is fine.

For Lithium batteries orientation towards the east is not important. Tilt is also not as important and since pointing east isn’t important, the angle can be relaxed and more oriented south at an angle similar to the latitude.


Last year I watched as a guy move his solar panels late in the day to catch the setting sun. At the time, not knowing how solar panels and batteries interacted, I assumed he was correct in trying to get the most from his solar panels.  I had the wrong assumptions and he didn’t understand battery charging.

Late in the day, your charger and battery is in float mode, your charge controller is throwing away most of the energy hitting your solar panel, totally wasting it, ignoring it, and any orientation of the solar panel would have produced the same effect on the battery. The battery internal resistance (to accepting a charge) was at it’s daily high and you can’t force-feed a lead-acid battery. Pointing the panel to the empty sky would have produced all the energy the battery could have accepted. The only way of tracking the late day sun would help is if the solar panel was undersized and the battery desperate.

Early in the morning, however, the guy tracking the portable panel would have been one hundred percent correct. In the morning, the battery is happy to accept whatever you can give it. My panels are flat, I can’t give the battery enough early in the morning, and the battery could take more than I can deliver. This problem can be corrected by having better batteries.

Not like Buckets

Batteries are not like buckets where you can poor an amount of energy into them and fill them up.  Batteries are more like compressed air tanks. When empty, the compressed air tank can easily take all the air from the compressor, without offering any resistance. As the pressure in the tank rises, more resistance is given to the incoming air, which has to be at a higher and higher pressure, to bring the tank up to maximum.  Don’t take the analogy to far. Batteries are not just like compressed air tanks but they are more like compressed air tanks than they are like buckets.


My flat-mounted system handicap is the biggest reason I put on more panels.  More panels make the playing field more level. Every day my system starts recharge when at poor (even shaded) sun angles, just when the batteries are most willing to accept, my panels are unable to supply.  Remember the three stages of charging, bulk, absorb, and float.  I spend longer in the bulk stage than a correctly aligned panel would. I also spend more time in absorb than a correctly aligned panel would and my panels are just as capable at float – as any panel is because my battery is almost full. My charge controller knowing the battery state is nearly full is just filling the battery slowly.

Cloudy Results

Only one day in the last year the panels didn’t work the way you should now expect.  It was cloudy and rainy the entire day.  My total solar gain on my battery was 900 kWh and I was consuming more from the battery than the solar was able to deliver. I spent the entire day, sunrise to sunset in bulk mode. Nothing was wasted. Since any solar panel would have been pointed at a cloud all day, my 700 watts of solar with poor orientation produced roughly twice the kWh of a panel that was only 300 watts and pointed at the clouds. As they said pertaining to cars, when I was growing up, a bigger engine is better, (then they had a gasoline crisis).

What if I have a portable solar panel charging a lithium battery?

This battery is more like a bucket than any lead-acid battery could be. The advantage of a portable panel is minimized a little, but not erased. Forty to Sixty degrees or less when you are further south pointed south is fine. More south than east still applies but isn’t as critical as the sun rises. Tracking is great (assuming the battery is not full). Lithium batteries don’t resist charging and are much more willing to accept a charge, than lead-acid batteries.

Higher Amperage Charge

Lithium batteries are also willing to accept a higher amperage charge, over a longer time and really only two charge modes, bulk and float. In fact, the float mode isn’t really an issue with a lithium battery for two reasons. #1 lithium batteries don’t self drain like lead-acid and lithium batteries don’t care if they are full. The life span of all lead-acid batteries, suffer anytime they are not kept at 100%. The life span of lithium increases when not fully charged. Slower charging rates over longer periods also increase the life span of lithium, which is already five times longer than the best lead-acid.

A Smaller Panel can effectively charge a lithium battery,

The advantage of lithium batteries as associated with portable solar panels is that a smaller panel can effectively charge a lithium battery, even if it is a smaller panel than would normally be desired. If it were tracked, then much more of the available energy would end up in the battery and far less energy would be wasted.

Besides storing the portable panel and the associated moving to track the sun, I would also mention that wind has a negative effect on portable panels if they blow over, and portable panels do create a theft possibility. I have not heard of any being stolen, but I know a campground in California where multiple generators walked away in the middle of one night.

We created this site to help our friends with their RV questions. Feel free to ask questions or comment and we will address them to our best abilities.


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