Please help me with solar setup
I have been reading a lot about solar on here and finally decided to take the plunge or come to the dark side as some would say. Sunny side makes more sense, but whatever. :) We do not use a lot of...
Forum Discussion
BFL13 wrote:smyke wrote:
I haven't decided yet.
My head needs to stop spinning. ;)
I was going to ask for suggestions on better controllers in the $100 range.
Thinking of also going 24v with MPPT since i found some quality (at least to me) panels on dmsolar.com. but I am not really sure how to read the specs on those panels. Somehow i think that 8amp from 250watt 24v panel must be "better" than from 150w 12v panel.
Am i making sense?
No. :)
With PWM you get the panel rated Isc as your amps to the battery when the panel is aimed at a high sun. This might only be for a minute a day at high noon while the rest of the day you get fewer amps with the sun being lower and off to the side of a tilted up panel facing South.
With MPPT you ignore the rated Isc and the rated Imp and only look at the watts. The amps to the battery are what is left over after you get your "output watts" and divide that by the battery voltage of the moment.
It is very wishy-washy how many amps that turns out to be, but generally it is less than advertised. The MPPT sellers like to brag about how many amps you will get at battery voltage of 12.1v, but don't like to talk about how many you get at 14.1v
The critical factor is that your MPPT 24v input watts is entirely dependent on panel temperature which gets hot in the sun and kills the panel voltage and so its watts and so the "input watts" to the controller.
Subtract whatever the controller loses from its inefficiency bucking that 24v down to 12v, which also depends on temperature (hot is bad) and now you get your output watts to take some amps from.
But you are still not out of the woods, because the higher the battery voltage, the fewer amps you can get from the same watts!
So now you are going crazy, with the sun going up and down messing with your amps, plus now the battery voltage is climbing, messing with your amps, and there goes your MPPT "advantage" over PWM.
The only excuse to have MPPT is if you need to go 24v panels because they are cheaper per watt, or because you have roof real estate issues (although many find it is easier to go several smaller 12v panels in between roof obstructions, than find a spot for a big 24v panel or two.
Not to mention opportunities to aim the panels or not, where it is easier to move around (aim) smaller 12v panels in a portable configuration than big 24v panels to set out and put back (where?) every stop.
You need to get your camping scenario sorted out, and go from there.
If you can't do that then just get a little solar now and learn from that. Then get the "real" solar set, now you know what works for you. That is allowed (recommended , in fact), and it might even turn out your "learner set" can be part of your final set.
/\ This is the path I chose to follow. It's worked out well for me.
Ended up with a great 12V 8.5amp 150W poly solar panel, a good PWM adjustable voltage charge controller good for 20 amps, and, for my purposes, it fits in perfectly with my 150 AH Trojan T-1275 battery. Very good match.
For portable, 1 watt of solar panel per ampere hour of battery is a good working ratio, provided you have an adjustable V PWM charge controller.
