Panel Wattage Rating Question (Midnite Mystery?)

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

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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

Mexicowanderer,

Regina, Sk, Canada

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

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

Home?

Gatorville?

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).

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.

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.

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

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