Inverter size relative efficiency.

spacebass
spacebass Registered Users Posts: 87 ✭✭
in Solar Beginners Corner #1
Thank You Bill for last wisdom.

If a brand range of inverters have the same efficiency (92%) per model how can using a bigger inverter in the range cause more conversion loss than a smaller one ?
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
    92% would be maximum efficiency.  Operating efficiency could be much lower.

    For example, you could have an inverter using 40w just being on, running a 5w load.  40w x 24hrs is ~1000wh vs a 5w load (5x24 =120wh), so 12% efficiency.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #3
    The efficiency will differ depending on load, at low or high loads the efficiency won't be at maximum, usually it's somewhere in between. Often in the specifications it will be stated where in the performance curve this peak occurs, so using an oversized inverter to power small loads may likely cause it to run inefficiently, whereas a smaller unit could be in its sweet spot with the same load, idle consumption aside.

    Obviously every inverter has its own specific characteristics and not all will provide detailed information but simply the peak efficiency. To test this use a DC clamp on ammeter to measure the battery current preferably with resistive loads on the inverter, then measure AC output current output, with a simple conversation ratio 10:1 for 12VDC  to 120VAC  for example, you will be able to determine the losses. Try various loads to understand where this sweet spot is, probably a waste of time for the most part but interesting none the less.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #4
    On page two of the specifications for the MorningStar 300 Watt TSW 12 volt inverter, they have a nice curve of efficiency vs wattage (this is an "official" MorningStar link):

    https://2n1s7w3qw84d2ysnx3ia2bct-wpengine.netdna-ssl.com/wp-content/uploads/2014/02/SureSineENG_R2_1_08.pdf

    https://www.morningstarcorp.com/products/suresine/

    For larger AC inverters, many will have parallel switching units that add up to the high wattage output... I think I read one place that at least one of the Schneider/Xantrex XW units would turn off some of the parallel switching sections to reduce power use and increase efficiency at lower AC output loading.

    Anyway, here is a ~6 kWatt Context XW+ manual. You can go to page A-9 or ~132 for an efficiency graph of the AC inverter:

    https://www.solar-electric.com/lib/wind-sun/Conext-XW%2B-NA-Owners-Guide.pdf

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