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Array Oversizing

Discussion in 'Energy, Environment, and Policy' started by nwdiver, Feb 27, 2015.

?

What's your solar DC/AC ratio?

  1. <= 1

    7 vote(s)
    36.8%
  2. ~1.2

    10 vote(s)
    52.6%
  3. ~1.4

    2 vote(s)
    10.5%
  4. >= 1.6

    0 vote(s)
    0.0%
  1. nwdiver

    nwdiver Well-Known Member

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    I'm helping a friend with what's probably going to be a 15.6kW array tied to a 11kW inverter. That's an oversize ratio of ~1.4. That seems to be the sweet spot with module prices <$1/w. I'm curious as to how common this practice has become... Array Oversizing. UL1741 inverters are tested to a ratio of 2.
     
  2. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    #2 FlasherZ, Feb 27, 2015
    Last edited: Feb 27, 2015
    I think most of it is anticipating how long you'll be operating at peak.

    One of my arrays is 30x300 (9 kW, pointed S @ 175 deg) fed into 8 kW inverter. The second array is 16x315 (5.04 kW, pointed SE @ 140 deg) fed into a 4 kW inverter. The third array is 8x315 (2.52 kW, pointed E @ 85 deg) fed into a 2 kW inverter. The fourth array is 7x250 (1.75 kW, pointed SW @ 215 deg) fed into a 2 kW inverter. I've tried to keep them at 1.25 or below.

    There are times during the summer day when the first array will reach 8400 W generation for about an hour and you can hear the fans blazing on the SMA 8kW inverter. I can't imagine running a higher oversubscription for several hours without shortening the life of the inverter.
     
  3. ChadS

    ChadS Last tank of gas: March 2009

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    My second system (installed in December) was 4.86kW into a 4kW inverter, so just over 1.2. I didn't pick the numbers (not enough knowledge), the solar company did.
     
  4. nwdiver

    nwdiver Well-Known Member

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    The inverter doesn't dissipate the extra power... it increases the input voltage lowering the current, this simply lowers the power being pulled from the array. I believe the UL1741 limit of 2 is based on a fault condition. If there are no faults you should be able to tie an 8MW array to an 8kW inverter... as long as the input voltage is <600v. You would just be at 8kW from sunrise to sunset :wink:
     
  5. RichardC

    RichardC Cdn Sig & Solar Supporter

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    Unless you have diverse panel orientation, or a feed-in tariff limit or other regulatory constraints, I don't understand why you would go above 1.2 ratio. As reflected in the following (from the panels that first cleared themselves of snow), even a 1.2 ratio is leaving a significant number of electrons on the roof on sunny days.

    AC Production.JPG
     
  6. nwdiver

    nwdiver Well-Known Member

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    There are multiple reasons to oversize your array... utility restrictions are probably the most common.

    I was surprised how little is lost on an annual basis. For example... 14kW array tied to a 10kW inverter will produce ~23367kWh/yr. Upgrading to an 11kW array will yield ~250kWh more per year. It's ~$300 more for that extra kW.... sure it's only $300... but you're only making an extra ~$25/yr... that's a 12 year payback.

    Why supersize?
     
  7. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    Sure - sorry, didn't mean to imply that it sank the power; instead, meant to imply that running at 100% load for many hours per day was likely to have an impact on the lifespan. The fans running are a side-effect of running the hardware at 100%, not to cool sinks. To Richard's point, flat-spotting the curve will lose some production, but I'm looking for more longevity from the equipment.
     
  8. nwdiver

    nwdiver Well-Known Member

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    It's really heat not power that degrades the electronics inside an inverter... I realize that more power = more heat but that pales in comparison to environmental factors. An inverter operating with an ambient temp of 20C & 70kWh/day will probably live longer than one operating with an ambient temp of 23C & 50kWh/day. Most inverters are 96 - 98% efficient. @ 10kW that's only 200 - 400w of heat. The actual 'guts' of my inverter are probably <5% of it's mass... discounting the transformer >50% of the weight is thermal mass. Dissipating heat appears to have been the top design priority.

    It would be interesting to see a geographic representation of MTBF for inverters... I would expect there to be a strong correlation between temperature and failure.
     
  9. neroden

    neroden Model S Owner and Frustrated Tesla Fan

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    Well, I guess THAT makes sense. But barring "trouble from the utility company", it seems odd to me. My inclination would be to get all the power I can and sell the excess (if the utility wants it) or store it in batteries (if the utility doesn't). But I suppose you could put the battery storage on the DC side of the inverter. Actually you probably should, thinking about it....
     
  10. nwdiver

    nwdiver Well-Known Member

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    If you're triggered by oversized arrays... look away. :)

    It’s time to start wasting solar energy

    'Last year, the 49 MW Tobacco Valley Solar in Connecticut began operation with a 1.84 inverter ratio (meaning it produces 84% more electricity than its connection to the grid can output during peak production).'
     
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  11. iPlug

    iPlug Member

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    Economics and grid needs are making this a better argument every year. Less peak electricity when it is not needed and more on the early morning and later afternoon shoulders when it is needed makes sense.
     
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  12. SageBrush

    SageBrush REJECT Fascism

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    #12 SageBrush, Dec 31, 2020
    Last edited: Dec 31, 2020
    Funny headline ;)

    I've mostly come around to your view when the marginal inverter upsizing is expensive and the panels are cheap. Do you think that situation will last ?
    Utility scale is a different animal because they often deal with a grid connection bottleneck. In that case the profit optimization is to keep the pipe full.

    I would not be quick to say that what is good for utility scale PV is good for residential.
     
    • Like x 1
  13. nwdiver

    nwdiver Well-Known Member

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    I expect it to only accelerate. I can see a time when it will cost you money to export for a few hours mid-day. The wholesale price of electricity already goes negative
    occasionally near noon in places with high levels of solar penetration. This will probably get worse. Oversizing arrays will probably get a lot more popular if clipping noon output could actually save money or at the very least that lost production is next to worthless anyway.
     
  14. mspohr

    mspohr Well-Known Member

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    My older arrays are undersized. Newer installations are oversized (~20%) since panels are so cheap now.
     
  15. SageBrush

    SageBrush REJECT Fascism

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    #15 SageBrush, Dec 31, 2020
    Last edited: Dec 31, 2020
    From our friends at Renvu

    Screen Shot 2020-12-31 at 11.07.08 AM.png

    If I was putting up 6 or more kW, I think the largest of these inverters would be an easy choice.
    Going by your earlier post that undersizing 1 kW loses 250 kWh a year, that is 3,750 kWh over a 15 year inverter lifetime. If the marginal inverter cost is $100 to collect that energy you end up paying 2.66 cents a kWh
     
  16. iPlug

    iPlug Member

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    As an example, if it costs roughly the same to get 25% more solar panels installed as increasing inverter power capacity 25%, the former option is much better. More kWh net generated and more importantly, even more generated at times when it is more valuable to the grid. Properly priced TOU plans would encourage that at the residential level.

    Understand this example is a huge exaggeration and inverter costs still net and incremental increases are substantially below that at this time.
     
  17. mspohr

    mspohr Well-Known Member

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    That's a choice I made on one of my arrays. Since the extra cost for the larger SMA inverter was only $100, I went with it even though I didn't need the capacity at the time. Gives me the option to add another string in the future for just the cost of the panels.
     
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  18. SageBrush

    SageBrush REJECT Fascism

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    By the way, PVWatts lets you set the DC:AC ratio in the advanced settings. Default is 1.2:1
     
  19. nwdiver

    nwdiver Well-Known Member

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    There's some 'step changes' you start running into. The first one is rather minor. Upgrading from ~3.8 to 6kW inverters is cheap. To get to 7.6kW requires a larger disconnect but that also doesn't cost too much. Over 7.6kW you need a 'line-side tap' which is why 7.6kW inverters are so common. The next cap is 48A or 11.4kW. More than that and you need a disconnect >60A which is uncommon for some reason.

    The project we're working now is ~35kWdc and ~31kWac. We're using (4) 7.6kW inverters because as you point out the cost difference between a 6,7 and 7.6 is minor so might as well go with the 7.6. The interconnect we're using can support up to 160A so might as well use it. Check out the size of the AC disconnect you need to accommodate 128A on the AC side...

    Screen Shot 2020-12-31 at 11.35.46 AM.png

    So basically my design philosophy is sorta based around these step changes of 16, 32, 48, 80 and 160A.
     
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  20. SageBrush

    SageBrush REJECT Fascism

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    LOL !
    Looks to be about two Molly's

    (apologies if I mangled her name.)
     
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