Panel ratings

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Cariboocoot
Cariboocoot Banned Posts: 17,615 ✭✭✭
in Solar Beginners Corner #1
I am constantly amazed at the number of 'newbies' who get suckered into buy one 130 watt panel by some salesman and expect this to do the trick. They seem to come under the impression that a panel puts out full wattage dawn to dusk (sometimes they think they work at night too).

Right now I'm sizing up a secondary system for my proposed 'shop' at the cabin, and am counting on the real world performance numbers of my existing system as a basis. Generally speaking, peak output in full sun is 75% of the panel rating. The over-all performance is more like 60% over daylight hours.

Is this typical? Or have I got something horribly wrong in my orientation? Only twice have I seen output approaching 100%, and once it went over.

On a brighter note, I've just re-charged and equalized two old Trojan batteries from a previous system and they came up nicely! SG reading is 1.240 & voltage is at 6.4. Very happy with Trojans ... not so happy with the new Interstates which seem to be capable of only 50% of their rating.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Panel ratings

    There are lots of variations in systems and operating points...

    PWM controllers need panel Vmp matched very closely to battery voltage + controller drop to get near maximum watt rating.

    And, for all types of DC charge controllers, the battery needs to be discharged enough to accept full charging current and not cut back to absorb or float state in the solar charge controller (by the way, this is proper and normal operation).

    To see true "full" PV panel output throughout the day--you really need to watch a Grid Tied system with MPPT type inverter (as far as I know, all GT Inverters are MPPT--maximum power point tracking--type). A GT inverter is always trying to output 100% of available panel energy to the utility grid (essentially, an "AC Battery" of unlimited capacity)

    On my 3.5 kW peak rated (panel STC rating with Xantrex GT 3.0 inverter), 3kW PTC rated (by California)... I rarely get around 3,000 watts peak (few times per year), and most of the time (when randomly walking by my inverter) get around 2,500-2,700 peak (this is the measured output wattage--so includes panel power - inverter losses--also, must be a very clear, cool, and typically windy day for maximum output--sometimes bright clouds, but but exposed sun can give me high readings too):

    3,000/3,500 = 87.5% of panel STC rating to GT 240 VAC Output
    2,700/3,500 = 77% (somewhat often)
    2,500/3,500 = 71.4% (pretty much peaks all of the time, minimum)

    And, before everyone writes and says their XYZ system does much better than mine--Solar Guppy (a solar design engineer that posts on this forum, and has his own forum at www.Solar-Guppy.com) has measured most of the currently shipping GT inverters (and off-grid charge controllers) available in the US...

    He has found that Xantrex tends to be a rare mfg. where their readings agree within 1-2% of laboratory type/calibrated equipment. Many other brands are off by upwards of 5-10%---and guess what, the error is almost always in the +5-10% side of the range (showing more power than actually is being produced).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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