Inverter efficiencies and loads

stephendv
stephendv Solar Expert Posts: 1,571 ✭✭
in Off Grid Solar & Battery Systems #1
Are there any published efficiency curves showing how low efficiency can drop with very small loads?
Not sure whether to buy an oversized inverter to provide some future-proofing, or whether I should get one that exactly meets my current needs. Would help to know how much power is being wasted through poor efficiency of small loads.
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  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    #2
    Re: Inverter efficiencies and loads

    I am too lazy to look, but I can say from my own experience, having 40 - 60% eff. at low longterm loads made me replace 2000W inverter with 600W. Now I average 85% eff.

    Do best of both worlds. Power your small longterm loads with small inverter, and large shorterm loads with big inverter set to sleep mode when no loads present.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #3
    Re: Inverter efficiencies and loads

    It a matter of point of view, but I prefer to know what the no load current drain is on the inverter.

    Depending on size and type of inverter it can be anything from 5 watts to 50 watts.
    True sinewave inverters have more idle drain then modified sinewave because they are chopping their output MOSFET much faster and losses due to output filter.

    You can calculate low end efficiency pretty well by knowing % of no load to actual load power. At below 10-15% rated loading there is not much loss due to IR heating of MOSFET's.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: Inverter efficiencies and loads

    Morning Star SureSine 300 watt TSW has a nice little power efficiency chart:

    http://www.morningstarcorp.com/en/support/library/SureSineENG_R2_1_08.pdf

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #5
    Re: Inverter efficiencies and loads

    Thanks guys. Unfortunately running 2 inverters isn't an option because of the long cable runs I'll have from inverter to house, and I wouldn't like to have to switch between 2 different inverters.

    If I had to guess at the peak load distribution I'd say 80% of the time I'll need less than 1kW, 15% of the time I'll need about 2kW and 5% more than 3kW. In a few years time I might have some agricultural machinery that might require about 8kW, but will only be run very infrequency, i.e. a few hours/year so am happy to run this purely off the generator.

    If the suresine's chart is representative of other inverters, then it looks like they hit peak efficiency when running at a 1/3 of their rated output. I was always under the impression that peak efficiency was at a higher load, closer to the rated output(?)

    If this is the case then the best choice seems to be a 3-4kW rated inverter?
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #6
    Re: Inverter efficiencies and loads

    Here a good list

    http://www.gosolarcalifornia.org/equipment/inverter_tests/summaries/
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #7
    Re: Inverter efficiencies and loads

    Great link! Most are GTI's but the XW's and outbacks are there. The XW seems to correlate with the suresine curves, max efficiency at 30% load. The worst it will ever get is 88%, so not too bad really.
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  • jcgee88
    jcgee88 Solar Expert Posts: 154 ✭✭
    #8
    Re: Inverter efficiencies and loads
    Solar Guppy wrote: »
    Here a good list

    http://www.gosolarcalifornia.org/equipment/inverter_tests/summaries/

    Thank you for the list...very interesting!

    I have Enphase M190's, which I see are about 95%
    across the input spectrum, even at low input (that's
    good!). I would expect this because a micro-inverter
    only has to deal with low loads, sub-200w, anyways.

    I did a spot check on central/string inverters, too.
    My observations were that they were also about 95%
    in their prime operating ranges, that they dipped to
    as low as 85% in the low input regimes, and that they
    fully recovered to 95% efficiency when input reached
    30% of the inverters' rated power. On the latter,
    there was about a 3% median efficiency loss in the
    median low input power range.

    I only have about a month of "prime summer month"
    usage on my PV array, so the following is a barebones
    data point. In my case, my array sits below 30% for
    four hours per day (early morning/early evening). To
    this, I must add any time arrays are in shadow and
    when it gets cloudy. Perhaps a good raw estimate is
    that this is all equivalent to 1-1.5 solar hours. Using
    these figures, the micro-inverter architecture yields
    additional harvest of 3% times 1-1.5 solar hours times
    my total PV capacity. While not a huge amount, this
    does add to the other yield advantages that I had
    known micro-inverter architecture offers. As they
    say, every little bit helps!
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  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    #9
    Re: Inverter efficiencies and loads
    Solar Guppy wrote: »
    Here a good list
    http://www.gosolarcalifornia.org/equipment/inverter_tests/summaries/

    Here is a great list! Amazing that some industrial grid-tie inverters are getting 98.3% peak efficiency. I wonder where is the limit due to physics? Has it been reached yet?
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #10
    Re: Inverter efficiencies and loads

    These guys got 99.03% http://www.pv-tech.org/news/_a/pv_inverter_efficiency_record_set_by_fraunhofer_ise/

    Doesn't seem to be a commercial product yet.
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  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    #11
    Re: Inverter efficiencies and loads

    Ofcourse, Silicon-Carbide JFETs. Anyone knows when inverters using these become available?
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  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    #12
    Re: Inverter efficiencies and loads
    AntronX wrote: »
    I wonder where is the limit due to physics? Has it been reached yet?
    The limit is somewhere under 100% due to physics.

    Practically, anything in the 98%+ range is probably going to be as good as you can get. If you can get 95%+ from 10%-100% rated output, you're doing pretty well.
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  • jcgee88
    jcgee88 Solar Expert Posts: 154 ✭✭
    #13
    Re: Inverter efficiencies and loads
    drees wrote: »
    Practically, anything in the 98%+ range is probably going to be as good as you can get.

    Not being an electrical engineer, it's hard for me to
    say what the practical limit is.

    However, as a paying customer, I would have to say
    that the current industry standard of 95% is indeed
    3% lower than I think it should be.

    98% sounds good to me. C'mon inverter guys, get
    cracking!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #14
    Re: Inverter efficiencies and loads

    Believe it or not, many engineers agree that increasing efficiency is a good thing:
    • 5% loss / 2% loss = 2.5x the thermal losses
    That can mean no fans, smaller heat sinks, smaller/sealed units (less dust+bug+humidity problems), lower operating temperatures for longer component life (for every 10C/18F reduction in temperature, there is ~2x increase in lifetime--20C drop 2x2=4x longer live, etc.).

    But, takes more engineering and more expensive components... Always trade-offs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jcgee88
    jcgee88 Solar Expert Posts: 154 ✭✭
    #15
    Re: Inverter efficiencies and loads
    BB. wrote: »
    Believe it or not, many engineers agree that increasing efficiency is a good thing:
    • 5% loss / 2% loss = 2.5x the thermal losses
    That can mean no fans, smaller heat sinks, smaller/sealed units (less dust+bug+humidity problems), lower operating temperatures for longer component life (for every 10C/18F reduction in temperature, there is ~2x increase in lifetime--20C drop 2x2=4x longer live, etc.).

    But, takes more engineering and more expensive components... Always trade-offs.

    -Bill

    Money is an issue. I think "inertia" is the more pressing
    issue here. Most everyone seems satisfied with 95%.
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