slightly different solar qx
Am looking at solar on a small class A. I currently have 3 group 27 batteries (rv/marine) for the house. (prior owners had just installed 2 new, and I had one from a trolling motor setup) they will...
Forum Discussion
wanderingaimlessly wrote:
I'm not apparently making my question clear.
To take a different tack,
To charge a 300 Amp hr, 12 volt battery bank
Would 3, 100 watt ,solar panels such as
renogy 100 wired in parellel,
provide a substantial upgrade, or downgrade as compared to a single REC 325
The gain or loss due to voltage difference is my question.
Or is this a case of 300 watts is 300 watts,,voltage be damned?,,
To answer the question I think you keep asking.... The higher voltage (in and of itself) of the 325w panel doesn't magically get you something extra (i.e 2x).
I think what you are missing is this simple equation...
Panel Voltage x Panel current(amps) = Watts
So on the 100w panel, it is ~17.9v x ~5.72amps = ~100w
And on the 325w panel, it is ~34v x ~9.56amps - ~325w
It is the combination of voltage and amps that gives you watts.
The 325w higher voltage panel with the MPPT solar charge controller will be inherently slightly more efficient because it will utilize ALL of the available voltage (via converting from higher input voltage to the exact voltage, and higher amps, needed by the battery at whatever charging stage it is at). The PWM solar charge controller (on the Renogy 12v panels for example) will not (it will simply pass the non-scaled current through to the batteries).
Under an ideal scenario (which never really happens as outlined in some of the other posts talking about inefficiencies, etc), you could see...
On the 325w MPPT system: 34v / 13.6v = 2.5 x 9.56amps = 24.125 amps.
Whereas on the 100w PWM system: 17.9v simply gets chopped to 13.6v and the charge controller delivers 5.72 amps. By comparison, if you could somehow magically put together 3.25 of the 100w panels it would deliver 3.25 x 5.72amps = 18.59 amps (compared to the 24.125amps with MPPT).
Again...this is just for illustrative purposes to make a point (MPPT is more efficient).
My strong suggestion would be for you to string 2 of the 325w panels together (...and even 1 more if you have the space). A 325w panel will likely only be able to add ~80-90AH per day back into the batteries (on a sunny day). Adding just one more 325w panel gives you 160-180AH generated per sunny day (much better). If you want to save some $$, here is a link discussing used panels:
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30175150.cfm
Most panels should last 20-30 years which is a pretty long time, so buying used panels as old as 5-10 years old is not an issue.
Good luck!
Chris
