help selecting inverters - one or two?

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electrofelon

electrofelon

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Location
Cherry Valley NY, Seattle, WA
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Electrician
This is for my system I will be starting soon. I think I am going with 63 235 watt panels, that is 14,805 watts. Kinda an awkward number to string, maybe I can find a place for one more. I am definitely going with fronius inverters since I get a good price on them from my supplier and they do 1kv strings. I dont have software to calculate dc/ac ratio so I am just winging it (anyone want to crunch it for me? :angel:) I am thinking a bit on the lower side since the array will be due south, tilt we be ideal (34 degrees), and I am in the northeast where it is cool/cold. They make a 12.5 which puts me at about 1.2. Or I could go with 2 inverters and there are lots of permutations in the ballpark: 6+7.6, 5+7.6, 5+8.2.... Two inverters is about a $300-$350 increase but gets me 4 MPPT's. Think its worth the added cost?
 
electrofelon said:
Two inverters is about a $300-$350 increase but gets me 4 MPPT's. Think its worth the added cost?
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What are you getting paid per kWh? 12cents? If so my SWAG is that you'll about break even over the warranty period. The MPPTs, and not losing the entire system if one inverter fails, might be worth more than the ratio. Looked at PVwatts for you but their algorithm seems screwed up. :(

Those inverters are not very efficient at 95 percent. Not much point in going below 1.05 on the DC to AC ratio. Might as well get close to that if you're going to bother, ergo the 6+7.6.
 
jaggedben said:
What are you getting paid per kWh? 12cents? If so my SWAG is that you'll about break even over the warranty period. The MPPTs, and not losing the entire system if one inverter fails, might be worth more than the ratio. Looked at PVwatts for you but their algorithm seems screwed up. :(

Those inverters are not very efficient at 95 percent. Not much point in going below 1.05 on the DC to AC ratio. Might as well get close to that if you're going to bother, ergo the 6+7.6.
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I think its about 19,000 KWH/yr. There is a nice rule for my area, take DC system size in watts and mulitply by 1.25 gives you KWH/yr (assuming ideal tilt, no shading, etc).

Just a WAG but guessing I will pretty much never clip at 1.1-1.15.
 
Anyone familiar with PVsyst want to take a stab at explaining to me how you put in an inverter with two mppt inputs? I can't for the life of me figure it out. There is even an example in the help file but they lose me.
 
electrofelon said:
Anyone familiar with PVsyst want to take a stab at explaining to me how you put in an inverter with two mppt inputs? I can't for the life of me figure it out. There is even an example in the help file but they lose me.
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It's been a long time since I've dealt with PVSyst, but aren't there inverter specific parameter files one loads into PVSyst? It seems to me that if you were to load the parameter file for an inverter with multiple MPPT inputs you'd have separate ports for them.
 
ggunn said:
It's been a long time since I've dealt with PVSyst, but aren't there inverter specific parameter files one loads into PVSyst? It seems to me that if you were to load the parameter file for an inverter with multiple MPPT inputs you'd have separate ports for them.
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It loads the inverter, and seems to know it has 2 unbalanced mppt's........however its setup such that there isn't just 2 fields that you enter your string data for each....seems you have to make a sub array for each mppt and......I just can't get it to work :(
 
electrofelon said:
It loads the inverter, and seems to know it has 2 unbalanced mppt's........however its setup such that there isn't just 2 fields that you enter your string data for each....seems you have to make a sub array for each mppt and......I just can't get it to work :(
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Sorry; that's about all I remember about PVSyst. I won't ask, but if you're using a hacked version of this pretty expensive software it might not be fully functional. If you're using it legitimately, why not ask their tech support for help? Back when I was using it I found them to be pretty responsive to questions.
 
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ggunn said:
Sorry; that's about all I remember about PVSyst. I won't ask, but if you're using a hacked version of this pretty expensive software it might not be fully functional. If you're using it legitimately, why not ask their tech support for help? Back when I was using it I found them to be pretty responsive to questions.
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Good idea. Fyi they offer a free demo version for 30 days. I think even after that you can use it free with reduced capabilities.
 
So Ive been playing around with it a bit. Basically the one 12.5KW inverter is the way to go. Thats a 1.2 DC/AC ratio. I am looking at a min $450 premium to go past the 12.5K unit (12.5 K is $2650, 15K is $3107, a 7.6 and 6 would be $3160). Depending a little on what I put in for albedo, I am looking at about 1% loss from clipping with the 12.5. The system will make 21 MWH/yr (everything was with 64 panels) so I will lose 210 KWH/yr. So 15 year payback. Oh, none of those figures include shading, havent gotten to that yet. There will be a little in the morning and evening, but nothing major. So thus I havent been able to assess the gain from more mppt's. Still, I doubt it will make a significant difference. Plus, I have 68 panels already, so they are kinda "free" which makes it tough to justify spending real dollars to trim some clipping losses when I could just add a few more panels.
 
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