Panel Wattage Rating Question (Midnite Mystery?)

[BFL13]

I am confused about how they do the panel wattage at 12v vs 24v for a controller.

EG, the usual blurb seems to be that output wattage rating for an MPPT (EG Morningstar 60 amper is 800/12v and 1600w/24v for your array.

I don't understand why they have that for output when the array is input.

However, I gather ?????? that the controller has the same amps limit (self -limiting) to 60 amps no matter what the battery voltage is, so if your battery system is 12v then you get 12 x 60 = 720w as your limit (800 here, so amps limit must be a touch higher?) and with 24v system, you get to have 24 x 60 = 1440W of panels except they allow 1600w.

Similar with many other controller brands which have twice the panel wattage allowed for 24v system (meaning battery voltage AFAIK) than for 12v systems.

Then we get this Midnite Mystery--
http://www.midnitesolar.com/pdfs/spec_sheet_classic.pdf

Where the output amps is less for higher voltage systems. So what is the rating for panel wattage at different voltage systems? You can't double the watts if the amps rating of the controller does not stay the same going from 12 to 24 eg.

[MEXICOWANDERER]

As an aside...
If a MOSFET has x Vf @ 14 volts and an identical Vf @ 28 volts, same amperage for both, then a device can handle double the wattage. Elementary my dear Watt-Sun

[pianotuna]

.

[BFL13]

CA Traveler wrote:

BFL13 wrote:

If at 14.8v battery and the output watts limit is 800w can you ever see 60 amps? I think it would be limited to 800/14.8 = 54 amps but I don't own one to have noticed that.

The MPPT60 is limited to 60A. There is no stated input/panel watt limit.

It's clearly stated that if the panel watts should result in exceeding 60A output the CC will derate to 60A.

OK I see now what MS does as explained here in Q1--they use 13.33 battv

http://support.morningstarcorp.com/wp-content/uploads/2014/07/MPPT-Technology-Primer.pdf

[pianotuna]

Mexicowanderer,

Regina, Sk, Canada

However, there is some one at my USA address to pick up mail.

[MEXICOWANDERER]

Don. Your location 05/05/2017 (?)

Home?

Gatorville?

[pianotuna]

60 amps is the output. I imagine wire size is the issue, among other things.

Input wattage (and voltage) are the other two factors.

Morningstar claims their controllers can just ignore any extra input (within reason).

I've always based input wattage on output amps X voltage, so the sixty amp can possibly tolerate 800 watts.

My Blue Sky, on the other hand, wants a 25% cushion so that 30 amps becomes only 22.5 amps (X 12.8 = 288 watts). As I have 256 I'm unable to add more panels.

I'm possibly going to throw caution to the winds this next summer and do a rack on the roof with 1600 watts of panels. I'll probably go with the 40 amp Grape controller(s).

[CA_Traveler]

MrWizard wrote:
i'm only taking educated guess's, but that what it seems like

as voltage input/output goes up they have derated the max amps

to compensate for the internal stress produced by the higer voltage

Certainly possible. It's also possible that the MPPT 60 could produce a few more amps than 60 at 12V but they just use a fixed cutoff for all battery voltages.

I'm sure a lot of factors are involved including technical and non technical product considerations.

[BFL13]

Makes sense to me! I can see when PWM specs have a spec panel limit, that is to keep the output amps from going too high. Most PWM brands are not self-limiting for amps. If you go over the amps rating the controller heats up and will eventually cook itself.

I now suspect but don't know, that the other MPPT controllers are more like the Midnite for how they really work except the Midnite specs are more clear about that? Folks who have them and have watched their monitors would know about that I guess. The other brands like MS must simply ballpark the array size as was suggested in replies above, so you can see what sort of array size "goes with" that controller but don't have to be exact.

It is confusing when the numbers don't come out right--makes me think I am missing something that might matter for my own MPPT set-up which has some over-panelling to it.

The critical Voc limit does mean you have to be exact and needs the numbers to come out right for cold temps too. So that is the real limiter on array size/arrangement with MPPT-- not the total wattage in the specs which are just broad brush numbers it seems.

[MrWizard]

Wattage ?

they could be calculating total wattage handling, heat and internal wattage dissipation

out amps a 24v is less than at 12v for the same watts, but internal load on the components involved could be more, with the higher input voltage into those solid state parts

in other words, its NOT linier its not the same amps for each voltage
, like just about every other mfg, not 800, 1600, 3200

i'm only taking educated guess's, but that what it seems like

as voltage input/output goes up they have derated the max amps

to compensate for the internal stress produced by the higer voltage

[BFL13]

I am not dissing their specs, I am trying to understand them.

[CA_Traveler]

BFL13 wrote:
I can see how the wattage doubles if you hold amps the same and double the voltage. But that Midnite has lower output amps ratings for each rise in voltage. I didn't see where they show any panel wattage amounts.

So what? These are very sophisticated electronic designs that are NOT linear. Midnight is likely the top dog of CCs and I wouldn't doubt their specs for a second.

Don't know why you do. 😞

[BFL13]

The MS 60 has a "nominal" panel watts limit of 800/12 and 1600/24, but does allow for "over-panelling" because it has that 60 amp output control. 800w/12v = 66.7 amps. They have a note suggesting over-panelling can be good for you.

I can see how the wattage doubles if you hold amps the same and double the voltage. But that Midnite has lower output amps ratings for each rise in voltage. I didn't see where they show any panel wattage amounts.

if you do have 800w rating of panel as input, then with 25C above ambient for panel temp, like 25c/50C then you have 10% power loss: 800-80=720w and
720/12 = 60 amps but you can have other circumstances.

[CA_Traveler]

BFL13 wrote:

If at 14.8v battery and the output watts limit is 800w can you ever see 60 amps? I think it would be limited to 800/14.8 = 54 amps but I don't own one to have noticed that.

The MPPT60 is limited to 60A. There is no stated input/panel watt limit.

It's clearly stated that if the panel watts should result in exceeding 60A output the CC will derate to 60A.

[MrWizard]

Are we taking about pwm or mppt
For Pwm I chose to go by total amps not panel watts
There is voltage drop, not amperage drop I calculate Panel max amps times 14.xx
And use that for max input watts

But for mppt, the controller does volts to amps conversion, so I would use the max watts rating of the controller listing

I am on my tablet and did not read the minnite solar pdf
Might be nothing more than a typo