Hi BFL13,
Those are your calculations.
I get 16.9 amps from 256 watts of solar in a fixed flat installation. That works out to 230 watts, so my total losses are lower than 11%.
Now, what does the PWM do when the battery bank is at 14 volts? Oh...it Pulses faster and faster (way faster than any meter can measure), and the width of the pulse is shorter and shorter. So what REALLY is going into the battery bank?
Of course, this is meaningless--because once you are out of bulk charging mode, MPPT controllers change over to PWM to prevent battery over charging. So most of those losses you talk about--don't exist, or are identical between the two types of controllers.
That said, there have to be good reasons to switch to MPPT from a PWM controller, especially with the Grape 40 amp having all the features a person could dream of.
I'm just happy that after 3 grey days that my battery bank was in float by high noon. So nice to see only 3 amps going into 556 amp-hours of bank (and the parasitic load of 1.4 amps taken care of)
BFL13 wrote:
Here is how you calculate your "expected amps" to the battery. Use your own set-ups to get your own numbers but it all works the same way. (Panels aimed at the high sun to get full amps in each case)
PWM---you get the total Isc. (Imp has no meaning with PWM)
So my three 100s, each with 6.2Isc, get me 18.6 amps to the battery. Regardless of panel temperature etc etc.
MPPT- Panel temperature say 50C vs ambient 25C means 10% power drop.
So my 300w worth of panels becomes a 270W right off the top.
Wiring loss array to controller (based on Imp for the wire gauge) say it is 2% (which is pretty good) So now your power is 270 - 5.4 = 264.6w
Now the controller itself has an efficiency between input and output- less efficient when input is higher v than output v--eg 24-12 worse than 12-12. Typical 24-12 is say 95%. so now output watts is 264.6 x 95 = 251.4w (which is why I am ok with my 260w Tracer and my "300w" array.
Now you divide the output watts by battery voltage to get your amps to the battery. You get more amps with a lower voltage battery of course. (So the MPPT salesman always picks an example where the batts are low)
251.4/ 13v = 19.34 amps
251.4/ 13.5v = 18.6 amps (same as with the PWM)
251.4/ 14v = 17.96 amps
So there you see that you don't gain much in actual battery charging by spending big bucks on an MPPT controller. Lots of hype vs actual measurements tells the tale.\
Sometimes you do get a touch more with the higher voltage MPPT in low light but it is sort of trivial in the big picture. IE, in low light you don't get much anyway. A little more than not much is still not much!
I do have MPPT and PWM so I don't care. I just don't get any more amps with the MPPT, but I also have a 24v panel I use sometimes and you can't run that with a PWM, so I have the MPPT that can run it OR the 12s either way.
