NOCT wattage ratings??

ywhicywhic Posts: 612Solar Expert ✭✭
I see this NOCT wattage ratings on some panels spec sheets...

For some of the 180w 24v panels I am looking at, it lists NOCT as 133w 32.9vmp, 4.05 imp (40.9 VOC and 4.3amp ISC)

This is a big difference from the rated 180w (36 vmp * 5 imp [44.8 VOC and 5.29 ISC]).. its looks like 72% effiecency rating..

I know the NOCT is a test bed for certain panels to assist (I guess) how good a panel is under real standards here in the USA..

Are the numbers real.. should I be super concerned with a 180w panel that only puts out (by the test) 133w??

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: NOCT wattage ratings??

    Don't worry about it.

    In one respect the NOCT (Normal Operating Cell Temperature) rating is just as arbitrary as the STC (Standard Test Condition) rating in that it relies on controlled circumstances to produce the output. Controlled circumstances don't exist in the real world.

    Consider that with a PWM controller the maximum output is going to be limited by the battery Voltage (only the MPPT can make use of available "extra" Voltage to increase current) and that you will not see or indeed need maximum panel power most of the time.

    You may have seen the repeated use of "typical panel derating of 77%" around here. That's usually what the panel + charge controller can put out on average over the "hours of equivalent good sun". Therein lies another clue: normal cell operating temperature is not the temp the cell will be operating at throughout the 4-5 hours of charging time. The climate you are in will have an effect on how well the panels can radiate the heat they produce, but even within that framework the cell temperature is not constant throughout the day. It can in fact go down at the "hottest hour" because usually by then the demand for charging has lessened and the panel doesn't produce as much current (which makes heat).

    All the numbers used in electronics are arbitrary standards. Any one on its own is meaningless; it has to be taken in context with the others. So Wattage ratings for panels are pretty much only a guideline that are useful only when applied to the rest of the system to determine the end result. That end is having enough power to recharge the batteries, and the main concern there is being able to meet the minimum requirements. If you shoot for more than those minimum requirements you'll come up with a functioning system. If you try to marginalize everything you will not.

    A real world example: my panels at 3200 feet of elevation run from 80 to 84 percent of their nameplate (STC) rating.

    None of he numbers are absolute. All value have range and tolerance. The trick is knowing which way to round the numbers so you don't come up short of what you need.
  • Eric LEric L Posts: 262Solar Expert ✭✭✭✭✭✭
    Re: NOCT wattage ratings??
    It can in fact go down at the "hottest hour" because usually by then the demand for charging has lessened and the panel doesn't produce as much current (which makes heat).

    That's interesting Cariboocoot. I had thought (based on a vague grasp of the physics involved) that the opposite might happen; as the batteries reach full charge and the panels produce less, the panels would grow slightly hotter because that 14% or so of solar radiation that they had been converting to electricity would now be absorbed/re-radiated as heat. Although current makes heat, most of it is conducted away from the panels during full charge, I'd think. So now I'm wondering what I'm missing....
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: NOCT wattage ratings??
    Eric L wrote: »
    That's interesting Cariboocoot. I had thought (based on a vague grasp of the physics involved) that the opposite might happen; as the batteries reach full charge and the panels produce less, the panels would grow slightly hotter because that 14% or so of solar radiation that they had been converting to electricity would now be absorbed/re-radiated as heat. Although current makes heat, most of it is conducted away from the panels during full charge, I'd think. So now I'm wondering what I'm missing....

    Eric, As I understand the physics of photovoltaic panels, a panel with a load will run cooler than a panel without a load. The effect is very small and not of much practical significance. Basically, the radiation that hits an unloaded panel is converted to heat by the panel. If the panel has a load, some of that radiation energy is exported (to the load) and the panel will run cooler. A small proportion of the 'exported' energy from the loaded panel will heat up the conductors within the panel.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • stephendvstephendv Posts: 1,571Solar Expert ✭✭
    Re: NOCT wattage ratings??
    ywhic wrote: »
    I know the NOCT is a test bed for certain panels to assist (I guess) how good a panel is under real standards here in the USA..

    The ratio of STC to NOCT will be pretty much the same for the same type of panel. So using NOCT to compare the same type of panel from different manufacturers is just as useful as using STC. Where you can really see a different with NOCT values compared to STC is between different panel types, e.g. CdTe thin film vs. Poli.
  • VicVic Posts: 2,920Solar Expert ✭✭✭✭
    Re: NOCT wattage ratings??

    vtM,

    Some NOCT data which you may know or have seen:
    http://pvcdrom.pveducation.org/MODULE/NOCT.htm

    And as you know CA, and perhaps NREL publishes data for currently available PV Modules. CA uses PTC, which is similar to NOCT data, but PTC is at 1 Kw/SQ MTr, FWIW, but may help you narrow your module search:
    http://www.gosolarcalifornia.org/equipment/pv_modules.php

    Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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