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Need Help with Inverter
#1
Tesla aka Solar City just finished up installing my 9.1 kW array of 28 (325 watt panels). To go with this they installed a Delta Solivia 7.6 TL Grid Tie inverter. Since the max AC out is 7600w I am wondering how this is going to work. I called Delta and they said this is called over sizing and is done to optimize the panels, meaning that the panels will be more efficient in low light levels. I am not sure I am buying into this idea, so at full sun I will only be generating 7600 watts and the rest lost to clipping. Shouldn't I be owed a explanation of the system, I am purchasing and not doing a lease. Here is why I worry:

Initial Design was:
23 (325 watt Panels) for 7.48 DC
2 - Delta Solivia 3.8 TL inverters (Max AC Out 7600W)

I asked for larger system that would cover 110% of my current need:
28 (325 watt Panels) for 9.1 DC
1 - Delta Solivia 7.6 TL inverter (Max AC Out 7600W)

I am worried that I added 5 panels for no gain. At agreement time I gave them a bill that showed I use 10,325 kWH yearly, they are only going to guarantee 9,474 kWH. The PVWatts calculator is telling me 11,896 kWh/Year based on my zip code. I want to get this resolved before the utility installs the net meter and gives me the blessing to turn it on and I find the system generating at 7.6 kWH. My panels are facing 150 degree south with no obstructions. Should I be asking for an inverter that more closely matches my DC input????

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#2
You're probably never going to see 100% output anyway. I think the amount you're going to gain by having the extra panels on cloudy/average days is going to outweigh the amount being lost during the hour of the day when the sun is at the perfect angle to even get close to 100% output.
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#3
I agree. You will never get the advertised output and it will only slip over the years. The rated output is for cold days. Hot days will lower your yield. Add a little garbage in the air and you get even less. If you could afford it, the extra is nice padding.
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#4
The problem I see is the first design they matched the DC to AC, the second design the is 120% DC to the same AC level. Should I be talking to the system designer or a gopher?
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#5
Some inverters can be oversubscribed like this with up to to 10-15% too large a solar array, in fact I'm pretty sure my SolarEdge inverter states that 15% is normal procedure and accepted by the manufacturer of the inverter.

20% oversubscription may be just outside what's recommended by the manufacturer though I think.

Have you looked up the design guides from the manufacturer of the solar inverter, it should say so in the design guide how much oversubscription they would recommend.

Also check the max string voltage as I don't think you can safely oversubscribe that one without the inverter letting out it's magic blue smoke!
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#6
(09-07-2017, 04:46 AM)Robert Baumer Wrote: Tesla aka Solar City just finished up installing my 9.1 kW array of 28 (325 watt panels). To go with this they installed a Delta Solivia 7.6 TL Grid Tie inverter. Since the max AC out is 7600w I am wondering how this is going to work. I called Delta and they said this is called over sizing and is done to optimize the panels, meaning that the panels will be more efficient in low light levels. I am not sure I am buying into this idea, so at full sun I will only be generating 7600 watts and the rest lost to clipping. Shouldn't I be owed a explanation of the system, I am purchasing and not doing a lease. Here is why I worry:

Initial Design was:
23 (325 watt Panels) for 7.48 DC
2 - Delta Solivia 3.8 TL inverters (Max AC Out 7600W)

I asked for larger system that would cover 110% of my current need:
28 (325 watt Panels) for 9.1 DC
1 - Delta Solivia 7.6 TL inverter (Max AC Out 7600W)

I am worried that I added 5 panels for no gain. At agreement time I gave them a bill that showed I use 10,325 kWH yearly, they are only going to guarantee 9,474 kWH. The PVWatts calculator is telling me 11,896 kWh/Year based on my zip code. I want to get this resolved before the utility installs the net meter and gives me the blessing to turn it on and I find the system generating at 7.6 kWH. My panels are facing 150 degree south with no obstructions. Should I be asking for an inverter that more closely matches my DC input????



I don't know much about residential solar, but for utility-scale solar overbuilding is common.  I've seen sites that have been overbuilt by as much as 80% (most have been in the 20-40% range).  That particular plant had 10 MW of inverters and 18 MW of panels.  At first glance this seems wasteful, and an 80% overbuild is extreme, but there are genuine benefits.

1.  The cost of panels (at least for utility-scale) is relatively small compared to the cost of the entire project.  In other words, adding extra panels is relatively cheap.
2.  As the sun rises and sets, you produce more energy and for a longer period of time.
3.  You produce more energy in the winter and when the weather is cloudy.
4.  You can more easily reach your peak power output as the panels degrade over time.

You might be wondering why you don't just size up your inverter, too.  Inverters are expensive and sizing up an inverter is probably not worth the cost.  It would likely be more expensive than the value of the energy you lose due to clipping.

P.S. The reason the site that had an 80% overbuild didn't add more inverters is because they were limited by their PPA.  They were contractually limited to producing 10 MW.  They overbuilt so they could produce more in the winter and on cloudy days.
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#7
Also more panels mean your inverter will turn on earlier in the morning and shut off later at night because it reaches the minimum voltage to turn on sooner and most inverters run more efficient at their full capacity. With oversizing you reach max capacity sooner and more often.
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#8
Its normal to do that. Its not when the sun is brightest you generally need or want it. You want it when there is low light Smile

In Sweden most of the systems are oversized by atleast 20% compare to what the inverter takes.

its also worth mentioned that it is very very very very rare that you ever reach 100% of what the panels can do. I would say 70% of what they do is more common Smile
Follow me! >> YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcomming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#9
(10-03-2017, 07:12 PM)daromer Wrote: Its normal to do that. Its not when the sun is brightest you generally need or want it. You want it when there is low light Smile

In Sweden most of the systems are oversized by atleast 20% compare to what the inverter takes.

its also worth mentioned that it is very very very very rare that you ever reach 100% of what the panels can do. I would say 70% of what they do is more common Smile

I suppose this probably true for residential because most homes were not constructed with the optimal orientation for solar energy collection.  Utility-scale sites I have encountered routinely reach 100% on a cloudless day, but they also spent millions to make sure every panel is at the optimal orientation.
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#10
For followers yes you can reach 100% with cooling. But with static mounting you will not reach it many seconds per day. Its like early morning when its cold and such Smile
Note that the 70% number is an avg over several hours and not just 20 minutes middle of the day.
This also depends on where you live and all of course. In Sweden we dont have many hours per year where we reach 100% on static mounted panels.

And this thread is about home-mounted panels and also static.
Follow me! >> YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcomming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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