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Compass Marine How To | all galleries >> Welcome To >> Installing A Small Marine Solar System > Understanding the Specifications & PWM vs. MPPT
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Understanding the Specifications & PWM vs. MPPT

Solar panels are most often rated in three general tests, STC which means "Standard Test Conditions" and NOCT which means "Normal Operating Cell Temperature". These two tests define its general operation parameters. There is also a test called PTC, which includes a cooling wind speed, and 113F temps, but it is rare to see a panel rated at PTC.

A solar panel will also undergo an Isc or "short-circuit current" test at 1000 watts per meterČ of irradiance. The Isc test is just as it is described, the panel is physically short circuited under ideal testing conditions.

Seeing as solar cells are current-limited the short circuit test defines the maximum current it can deliver under ideal conditions of 1000 watts per meterČ. While it's theoretically possible for the panel to exceed the Isc, this would be extremely rare and you would need a very cool day and better than 1000 watts per meterČ irradiance levels to do this. This occurrence would be so rare that the NEC and NFPA use a panels the Isc rating for sizing over-current protection.

The solar panel will also have a specification for VOC or "Voltage - Open Circuit". This just defines the maximum voltage the panel will develop with zero load and 1000 watts per meterČ of irradiance.

STC - Standard Test Conditions:

The STC test is conducted at 25°C or 77°F and the panel is given an irradiance of 1000 watts per square meter. These are conditions you will rarely see in the real world yet these are the ratings they sell the panel with. In other words a 140W panel at 77F with 1000 watter per square meter of irradiance can deliver 7.91A oc current. Will you regularly see this, heck no. Can you see this certainly.

NOCT - Normal Operating Cell Temperature Test:

In the NOCT testing, or as I tend to call it realistic testing, the panel is tested at a more reasonable temp of 45°C of 113°F. In this testing scenario the irradiance is also reduced to 800 watts per meterČ of irradiance.

What the chart shows is two ways to look at the specifications. One with a PWM controller and a 50% discharged battery and one with an MPPT controller and a 50% discharged battery. Both STC and NOCT data have been used for the calculations. Because nominal 12V panels are usually 17V to 18V rated, and marine batteries charge at 12.0V to 14.8V you will not see the rated "wattage" from the panel unless you use an MPPT controller.

"But RC I once saw my panels exceed the STC current rating and I only have a PWM controller?

While not totally impossible it would be a very rare occurrence without an MPPT controller. If you combine a very cool day with better than STC/perfect irradiance & a very deeply discharged battery you may be able to approach or even very slightly exceed the Isc rating. As panels get cooler, and we have perfect irradiance, we can see a panel slightly exceed its STC rating even with a PWM controller. Exceeding Isc/short circuit well that is much tougher..

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