Watts needed

cliffy49,

Never apologize for asking questions!

Here are some reasons to go with a larger capacity controller.

It is quite likely you may wish to expand the system in the future by adding panels. If the controller is maxed out--then there is a extra (large) expense for doing so.


In addition, electrical equipment that is "pushed" to the edge of its capacity may be stressed a LOT more and fail prematurely. There is a cloud lensing effect that may cause the panels to output 25% more than usual. That may damage a controller that is teetering on the edge of being too small.

In at least one type of controller there are three modules that work together. First one cuts in, then as solar production increases another cuts it, and so on.

I've already given my reasons to wire in parallel.

I hope you will be happy with the system! It is the best money I spent on my RV.

StirCrazy wrote:
I would run them in series on a MPPT controler if you can. the beausty of the MPPT is the conversion to a lower "charging voltage and that nothing is lost except for the efficency. also running in series means higher voltage lower amprage which lets you run a smaller wire with less line loss than you would get with a lower voltage, but higher amprage, on the same wire size. of course like PT and others have mentioned you have to be aware of the effects of shading so you have to place your panels a little smarter. another side benifit of running in series is the morning and evening charging will start sooner and finish laiter and you will get a higher amprage during that time than you would in parralel as you have a higher voltage hiting the controler.

the panels you referenced you could easy run two in series on that, but if it was for myself I would go with at least a 40 amp MPPT
controler.

Steve

You are at least the second person to say that I should use a 40 MPPT controller. To be honest, I can not understand why a 40 and not a 20. At the bery best, this panel is only going to be putting out a little over 8 amps if wired in series and 17 if wired in parallel.

Can you please explaiin to me what I am missing in all of this. I apologize for the questions but my solar knowledge is zero and I am trying to learn.

I would run them in series on a MPPT controler if you can. the beausty of the MPPT is the conversion to a lower "charging voltage and that nothing is lost except for the efficency. also running in series means higher voltage lower amprage which lets you run a smaller wire with less line loss than you would get with a lower voltage, but higher amprage, on the same wire size. of course like PT and others have mentioned you have to be aware of the effects of shading so you have to place your panels a little smarter. another side benifit of running in series is the morning and evening charging will start sooner and finish laiter and you will get a higher amprage during that time than you would in parralel as you have a higher voltage hiting the controler.

the panels you referenced you could easy run two in series on that, but if it was for myself I would go with at least a 40 amp MPPT controler.

Steve

Okay another question for the experts. Since most of the controllers I have looked at will not accept any wire larger than 8 gauge, would it be okay to use on of these adapters?






This would be for running wire from the controller to the battery.

Shading is a killer for most panels when wired in series. I observed a significant drop with 2*245W when one of them was only partially shaded. Don't remember now, but the percentage of drop was much higher than percentage of shaded area.

Wiring 2*285 in parallel would require #8 cable if distance is 25ft or longer. With cable #10 at 25ft he'll have to put up with voltage drop 3.1% - a little higher than the recommended 3%.

cliffy49 wrote:

BFL13 wrote:
PT, why not have two 285s in series?

I have the samew question.

Because of shading. Because the voltage on the panels is already high enough that it won't lead to wire looses. Because the MPPT may function with better efficiency at a lower voltage. Because there will be lower heat loss within the controller.

That makes it a "walk" in baseball.

It will be hard to find 45A MPPT that doesn't accept 100V, but you should always check, of course.

Efficiency will decrease (by 5-10% maybe?) when the voltage increases to 80-90V, but you get the convenience of pulling a thin wire and the ease of wiring in series, just plug one pigtail into another. Running 2*285W instead of 2*220W will dwarf these losses.

BFL13 wrote:
PT, why not have two 285s in series?

I have the samew question.

PT, why not have two 285s in series?

cliffy49 wrote:


Am I correct in assuming that a mppt charge controller that will accepts 100 volts and 20 amps would work with these panels.

I am currently trying to get a better understanding of charge controllers so your help is greatly appreciated.

It depends on the controller. Mine, when purchased in 2005 was top of the line. It only accepts 50 volts input.

There are *always* trade offs. The higher the input voltage the thinner the wire can be--BUT the MPPT efficiency may be lower.

One famous make says it will accept 100 amps. And so it will--but only if the battery bank is 48 volts.

In my opinion I would NOT do two 285 watt panels in series.