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frankvw avatar image
frankvw asked

MPPT calculator: STC or NOCT rated power?

I've been running several configurations of PV panels through the MPPT calculator, with encouraging results. However, going through the PV panel specs I note there is a dramatic difference between the maximum rated power at Standard Test Conditions (STC) and Normal Operating Cell Temperature (NOCT). The former will never be achieved in the real world (1kW/m2 solar energy; cell temperature 25 deg. C.; Air Mass 1.5). The NOCT temperature of the (JA Solar) panels I'm looking at is 45 +/-2 degrees C which seems far more likely when the sun is up.

I'm at the South African south coast where the temperature never drops to freezing point; about 5 deg. C is the coldest it's ever gotten. When the sun comes up every surface exposed to it quickly heats up to at least 45 deg. C.

Which leads me to wonder: if I plug the Pmax @ STC into the calculator it prescribes an MPPT that is about twice the cost of what I need for the Pmax @ NOCT, even given that I'm using the specified voltage and current temperature coefficients.

To illustrate: the specs for the panel I'm looking at give a Pmax @ STD of 500W but at NOCT it's only 378 (which seems much more realistic).

Seeing as STC is a theoretical lab situation never achievable in practice, shouldn't I work on the basis of Pmax @ NOCT rather than Pmax @ STC? I mean, why worry about whether or not the MPPT can handle a power rating you'll never achieve anyway?

(Needless to say I am still working on the basis of Voc and Isc @ STC since these are the highest values and the only way to ensure the voltage and current limits of the MPPT will never be exceeded.)

Your thoughts?

// F

mppt calculator
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nickdb avatar image
nickdb answered ·

Power isn't the sizing criteria, that is Voc and Isc. The calculator allows "over-panelling" anyway to account for the difference based on standard condition, which is what it is expecting (Wp - the maximum electrical capacity that a solar cell can yield under ideal circumstances).

You can also adjust the temp ranges.

In SA, we do get cold days, with wind chill sometimes even negative, though this generally doesn't correspond at times of major PV production.

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frankvw avatar image frankvw commented ·
That was my guess as well: Voc and Isc are the "hard" limits, while Pmax is more of a sensibility thing taking real world conditions into account. I'm expecting things far closer to Pmax @ NOCT than Pmax @ STC. I mean, how are you going to have any power coming out of your PV panel without the sun immediately heating the cells up to above 25 deg. C?


(Incidentally, I'm a bit appalled at the industry's habit of advertising PV panels at a power level that is typically a full 33% higher than what you will actually get... But that's just me.)


I'm at the KZN south coast (Munster) and I can see the ocean from here. Even taking wind chill into account, the chances that things will drop to below 5 deg. C are a big fat zero. Even so, in my chosen configuration I've got to drop the temperature to 10 below zero in order to reach the MPPT's limits. (Ain't gonna happen!) So I'm safe in that respect.

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kevgermany avatar image kevgermany ♦♦ frankvw commented ·
Be aware that you don't need huge amounts of sun to get the voltage. But as the MPPT loads the panels, in poor light current drops off very quickly. The MPPT does a lot of work to extract max power from the panels, and will see panel max voltage while this goes on.

5C is well worth checking in the calculator, might surprise you.

Power ratings are useful for sizing the system and the good thing about STC values is that it's common standard to use for comparisons.


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nickdb avatar image nickdb ♦♦ frankvw commented ·
The family used to have a holiday cottage in Munster, happy days.
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