Forum Discussion
412 Replies
jrnymn7 wrote:
12thgenusa,
"300 watts is more than 245 watts. At 13.5 v output it means 4 amps difference"
This was directed to JiminDenver's comment that he only sees a few more amps out of a pair of 150s compared to a 245.
Four 250w/24v panels = 1000w.
Six 160w/12v panels = 960w
1000w - 960w = 40w.
40w / 13.5v = ~3a
At 13.5v that's only a 3a difference, and yet a 960w/12v/series array produces 12 amps less than the 1000w/24v/parallel array. So where did the extra 9 amps go? That's a 20% loss.
Where is your documentation for the 20% difference?
The comparison being discussed is 12v series (44v) vs. 24v parallel (36v), using mppt. The Rogue comparison is between 12v parallel (17v) vs. 24v parallel (36v). So while the Rogue publication may actually show mppt works a little better with 12v panels, at low amperages, it does not address the issue at hand.
This comment intended to address the notion that higher voltage input to lower voltage output is much more inefficient than low voltage input to low voltage output. It is slightly less efficient, but minimally so. The actual documentation shows that 24-v panels to 24-v system is most efficient, 12-v panels to 12-v system is second best and 24-v panels to 12-v system comes in third. However, there is only slight difference between the last two.
"Panels in series has nothing to do with it"
Real world observations seem to indicate that 12v panels wired in series, coupled with an mppt controller, will produce less output than a single 24v panel of equal wattage.
Again, show me real world documentation of this, not "I know a guy . . ." anecdotal stories.
The controller does not know series or parallel. It only sees voltage and current.jrnymn7 wrote:
"Otherwise it's Imp and Vmp (maximum power) for MPPT for either series or parallel panels."
Thanks, yes, now I recall BFL explaining that.
So, in that case, the comparison being discussed is 12v in series (34v) vs. 24v in parallel (36v), using mppt. But if it's all about wattage, why is there 20% less output when using 12v panels in series? Are some of the watts not usable?
24 V in parallel is 28 to 30V, not 36V.
With a 12V panel, you end up with 14.4 to 14.8V in bulk mode, out of 17.0V for the panel under full load, about 21 to 22 V open circuit.
With 24V panel, you end up 28 to 30 V Open circuit... check the panel specifications for Voc and Imp as well as Vmp... the ratio of the loss is less Voc to Vmp in a 24 volt system.Four 250w/24v panels = 1000w.
Six 160w/12v panels = 960w
1000w - 960w = 40w.
40w / 13.5v = ~3a
At 13.5v that's only a 3a difference, and yet a 960w/12v/series array produces 12 amps less than the 1000w/24v/parallel array. So where did the extra 9 amps go? That's a 20% loss.
that is at the panel, at 100% efficiency
now factor % of loss per panel by 2 more panels 6 vs 4
now wiring losses,
4 panels in parallel, vs 3 series sets in parallel
install both sets, run identical loads on both simultaneously , and measure the output
they might be closer together than that (9a-20%) loss , where did you get that number
my 505w system does not give me 505w
amount wire, how its wired, are panels tilted, panel age (mine were all bought used and are a flat install) etc...
you can't argue theoretical on paper vs real, you can compare on paper, or installed, but not one to the other
unless of course, you want to compare, paper math of the install to the real world results
each system is entirely different- It sounds like you're trying to compare apples and oranges. Panel wattage is determined in lab conditions and have a variance. Real world conditions involve weather, panel pointing, wiring and controller losses, technology, etc.
jrnymn7 wrote:
"Otherwise it's Imp and Vmp (maximum power) for MPPT for either series or parallel panels."
Thanks, yes, now I recall BFL explaining that.
So, in that case, the comparison being discussed is 12v in series (34v) vs. 24v in parallel (36v), using mppt. But if it's all about wattage, why is there 20% less output when using 12v panels in series? Are some of the watts not usable?
Not following closely, am camping with limited data plan.
I suspect there could be something in the IV curves for each panel type. Some 12s, usually monos, have higher Voc and lower Isc than others, mostly poly, which may affect their Vmp as a combo?
I don't know if the 24v panels have this variety. Need how their Vmps could be compared with whatever the 12s have for the same wattage of panel. I don't know, ask Ken if he ever gets back here :)- "Otherwise it's Imp and Vmp (maximum power) for MPPT for either series or parallel panels."
Thanks, yes, now I recall BFL explaining that.
So, in that case, the comparison being discussed is 12v in series (34v) vs. 24v in parallel (36v), using mppt. But if it's all about wattage, why is there 20% less output when using 12v panels in series? Are some of the watts not usable? - 12thgenusa,
"300 watts is more than 245 watts. At 13.5 v output it means 4 amps difference"
Four 250w/24v panels = 1000w.
Six 160w/12v panels = 960w
1000w - 960w = 40w.
40w / 13.5v = ~3a
At 13.5v that's only a 3a difference, and yet a 960w/12v/series array produces 12 amps less than the 1000w/24v/parallel array. So where did the extra 9 amps go? That's a 20% loss.
The comparison being discussed is 12v series (44v) vs. 24v parallel (36v), using mppt. The Rogue comparison is between 12v parallel (17v) vs. 24v parallel (36v). So while the Rogue publication may actually show mppt works a little better with 12v panels, at low amperages, it does not address the issue at hand.
"Panels in series has nothing to do with it"
Real world observations seem to indicate that 12v panels wired in series, coupled with an mppt controller, will produce less output than a single 24v panel of equal wattage. jrnymn7 wrote:
Voc is used to make sure that the controller hardware can tolerate that much voltage should it occur in some low light or other situation. Voc means "open circuit" and that means no amps or power.
CaTraveler,
I thought you use Voc when talking series and mppt?
Otherwise it's Imp and Vmp (maximum power) for MPPT for either series or parallel panels.- Voc voltage open circuit...no load
Vmp voltage making power ...With load
Isc amps thru short circuit across panel
Imp amps into a load aka charge controller our battery or whatever jrnymn7 wrote:
"I'm seeing two 150w panels in series only producing a few amps more than my 245w panel. On a better controller I know of a 960w system using 160w panels in pairs for mid 40's producing 48a while I would expect 60a from four 250w 24v panels."
That sure seems counter-intuative. One would think the 12v's in series, having a higher combined Voc, would result in even more wattage being used by the controller. I can see if the mppt was rated for 50v, and you threw 46 volts at it, but most buck converters boast ~98% efficiency, when operated within their duty cycle.
Does it perhaps have to do with the 12v 160w panels being in series, and thus their output would become 160w / 44v = 3.6a, as opposed to ~9a Isc?
Not counterintuitive at all. You guys need to start thinking in watts instead of amps and Isc. 300 watts is more than 245 watts. At 13.5 v output it means 4 amps difference.
Panels in series has nothing to do with it except it reduces line losses and allows one to make use of the advantages that the MPPT controller offers.
I don't know about other controllers, but Rogue's published efficiency curves show that at a 20 amp output, 17-volt panels to a 12-volt system are about .2% more efficient than 34-volt panels, both right at 96%. At a 10 amp output, 17-volt panel input is about 1.2% more efficient. So at 20 amps 34-v panels "lose" 4 mA more than 12-v panels. At 10 amps the difference is 120 mA. Line losses for bringing 12-v power to the controller are certainly much greater than that.
