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RV Solar Series: How much solar did we need?

RV Solar Series: How much solar did we need? My answer was 600-700 watts of solar installed flat, (700 installed) to recharge our battery and live daily. Given six hours of sunshine, (winter) assuming the batteries were not overly discharged we should be good.

Part of the assumption was that you can’t always count on the sun, and sometimes it gets cloudy for part of the day. Another item of note is that our panels are only tilted enough to make the water run-off, about 5 degrees from flat. We don’t tilt the panels nor do we use any portable panels that could be pointed perpendicular to the sun. 

Tiffin RV 34TGA Solar Panels 700 watt
Tiffin RV 34TGA Solar Panels 700 watt

Most of the solar energy will be wasted

Now something we didn’t understand fully at the beginning, is that most of the time, most of the potential energy that the panel could produce will not be used. That statement applies to daily use and annual use. Most of the capability of the 700 watts is wasted – never used. Initially, I was heartbroken but it is the nature of the beast. Here is how that works.

The battery is like a stomach

The problem is not with the panels, the problem lies in how a battery likes to be treated. If I were to draw an analogy to the human body I would describe it like this. The battery is the stomach. A plate of food is the panels. It doesn’t matter how big the plate of food is, the stomach can only take so much at a time. To further the analogy when you are really hungry you can eat food faster, but after a short while, you need to slow down and consume at a more reasonable rate. The battery operates almost exactly like this. Feed it fast early in the morning then taper off as the day goes on.

Non-tracking

The way a non-tracking solar panel system works is that they are most capable at noon.  Unfortunately, the battery wants to be fed the fastest early in the day. A solar panel that you can point at the sun, early in the day is much more effective than my flat-mounted panel. This is one of the reasons I needed a larger than necessary solar panel system is to charge the battery early during the day – and another reason why so much potential energy is wasted.

How to squeeze enough solar out of a right-sized system without too much waste.

The key item in the system was a good charge controller.  Among other things, it feeds the battery the right amount at the right time.

Early in the charge cycle, it transfers most all available energy into the battery.  This is called the bulk charge portion of the cycle. Unfortunately, unless the panels are aligned with the sun, they aren’t ready to put out maximum, thus the bulk portion of the charge takes longer than it would have had the panels been aligned correctly. As you can see, before 8 am, bulk does not mean the same thing as mega. Just that all the available energy is transferred to the battery.

Screenshot Victron Solar Controller
Screenshot Victron Solar Controller

My panels and my charge controller could potentially kick out 50 amps in one hour. Reality not being the same thing as theory, it will never do so because of #1 the orientation of the panels and #2 the needs of the battery.

Not tilted?

In the calculations in sizing my array is that I didn’t want panels that tracked or even tilt, so they were installed almost flat on the roof.  A panel that was less than half as big, if it was pointed at the sun and could track the sun has the potential to put out an equal or greater charge as my flat panel array. The battery is most discharged first thing in the morning, thus the ideal would be to have the panels facing east as the sunrises.  By the time my panels are ready to kick out the amps, the battery is nearing full. 

Not South?

Typical advice would be that panels should be pointed south, not east, and this would have been my advice, but it is not true. East, first thing in the morning is best for the battery. Tracking is great, but not really necessary. Tracking doesn’t matter to maximize the output of the panel — if the battery is nearly full.  Screen capture at one pm. Ideal sun angle, battery almost full.

Screenshot Victron Solar Controller
Screenshot Victron Solar Controller

All lead-acid batteries love to be full. The opposite is also true; all lead-acid batteries hate being emptied, even a little bit. All lead-acid batteries are damaged by emptying and leaving them empty for any periods, longer than a day.    This is a problem for solar on an RV because all lead-acid batteries are very resistant to getting full. Charge profiles meant to prolong the life of lead-acid batteries, reduce the amperage at the end of the charge cycle. The last ten percent of a charge takes as long as the first forty percent. (my guess based on my observations)

Design Goal

My design goal was to reduce my interaction with the system down to watching it work. This is why my panels were mounted flat on the roof. This should explain partially my previous statement that most of the potential energy that could be produced is wasted.

RV Solar Series: How much solar did we need? I hope I got the answer right one of my design goals was that if 700 watts of solar installed flat, (700 installed) isn’t enough, then we can expand easily to recharge our battery and live daily.

Update: July 21, 2020, Our needs changed drastically with our new RV this post is an update on how we are charging our new battery system without solar. Electricity the Hard Way

Update: August 29, 2020, All summer we proved we could camp without electrical hookups. Here is that story. Boondocking Without Solar

Update: September 15, 2020, Why just design for minimum needs. Could I run the air conditioner from the battery and replace the energy used by the Air Conditioner using a big solar array. Here is an analysis and test about that subject. Air Conditioning on Batteries and Solar

Update: January 23, 2021, Our long desired solar array is being installed. Here is the story. Zamp Obsidian Solar Install

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