BFL13 wrote:
With PWM you get Isc value for amps to the battery. Flat on roof will not get as many as aimed.
With MPPT it is all about watts. Let's say you have 300w aimed (series or parallel makes no diff) You lose 10% of your watts due to panel heating. So now you are at your 270 watts. Lose say 2% wiring loss from array to controller (which is pretty good) Now at 264.6 watts. Controller efficiency say 97% (it could be more like 95%) so now at 256.7 watts output.
Amps to battery is output watts/ battery voltage (assuming no wiring loss on that leg)
258/13 = 19.8
258/13.5 =19.1
258/14 = 18.4
With panels flat on roof you start with less than 300 then lose the 10% for heat and go from there.
The PWM does not lose amps from heat, in fact amps go up slightly, so you don't lose. Still get same as whatever Isc is at the time.
Only good reason to get MPPT instead of PWM for that sort of work is if you get one 300w panel instead of three 100s. Now you must have MPPT to get the buck converter in it so you can go from 24v panel to 12v battery.
And, in the above calculation where controller efficiency is mentioned, don't forget the efficiency is lower when dropping voltage, so 24-12 loses more than 12-12. That might be a good reason to stay in parallel with the three 100s and MPPT.
However, it can be wiring gauge that makes a practical difference to your installation plan. Just don't expect a bunch of amps that won't ever happen in real life.
Well we will see. The higher voltage will have less resistance. I payed 89 bucks for the mppt and I can go from series to parallel if I want to get to the nitty gritty. But knowing that I was seeing 10 amps before if I see 18 to 20 now I will call it a win! :) It is not always extremly hot when I am camping. Many times it is nice and cool which would negate the panel heating loss. All in all in will be a interesting experiment. Solar is fun that I know for sure.
