The Admin's Blog

By Pianotuna

Our solar system is going to look something like this:

panels–>charge controller–>battery bank.

There are several panel types. Some are more efficient than others. In the main, what is most likely to be used are either monocrystalline or polycrystalline.

 

Mono appear to put out slightly more current under perfect conditions, but real life testing may give poly panels the edge. A good system can be designed with either.

 

Shade of any kind on panels may cause output to drop to nearly zero. It becomes important to avoid items on the roof such as the air conditioner shroud that might cause shading.

 

If I upgrade my system, I will be using poly panels.

 

Panels come in various voltages. Most RV’s run on a nominally 12 volt system, but in order for the batteries to be charged fully they many need to “see” up to 14.8 volts. So most “12 volt” panels in fact output 17 or more volts. Each cell on a panel may put out about 0.5 volts so they are wired in series. There are often more than one row of cells, so just like a battery the panels are wired in series/parallel.

 

Depending on the type of charge controller, it may be possible to use even higher voltage panels.

 

When panel prices were expensive (they started out at $1000.00 per watt) it made a lot of sense to use a more sophisticated controller called MPPT because it was economically sensible. Today, with panel prices as low as $0.35 cents per watt, it may make more sense to use a good PWM type controller.

 

It makes sense to shop around for the best price on panels. Generally speaking, either they work, or don’t work. The only requirement is that they must have bypass diodes. Some panels even have diodes between each and every cell.

 

My own system is small and is from 2005. It continues to provide me with power, though since I now full time, I don’t have enough wattage to provide for all of my needs.

 

The output from panels varies inversely with temperature. I.E. when the temperature goes up the output goes down. The cooler the panel is the greater the output.

 

There are semi-flexible panels available. So far, they exhibit lower output per square foot than more traditional panels. In some situations folks have found they may “cup” in the heat. They tend to be more expensive per watt than traditional panels.

 

To extract the greatest harvest, panels need to follow the sun. Unfortunately tracking systems far exceed the cost of the panels. My general feeling is that I would not wish to climb up on my roof to lay tilted panels down in even a light rain, let alone a wind storm, so I prefer a fixed flat installation. If you want more harvest, add an additional panel.

 

The only time I’d recommend tilting is if all the available roof real estate has been used up and more harvest is needed. This is also where an MPPT charge controller may shine.

 

The sad fact is that high voltage panels may be considerably cheaper than nominally 12 volt ones. If the system is going to be larger than 800 watts it may be worthwhile going to MPPT.

 
 

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