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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

    3 vote(s)
    23.1%
  2. ~1.2

    9 vote(s)
    69.2%
  3. ~1.4

    1 vote(s)
    7.7%
  4. >= 1.6

    0 vote(s)
    0.0%
  1. nwdiver

    nwdiver Active 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 Petroleum is for sissies

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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 Active 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 Active 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 Active 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 Happy Model S Owner

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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....
     

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