Need advice on selecting a grid tie inverter...

Hi All!

This is my first post here, but I've been reading as a guest for quite a while. There are a lot of very knowledgeable people here, so I thought I'd see what the right answer might be.

I recently bought 14 of the Sharp ND-205U1 solar panels. The specs are:

Cell Poly-crystalline silicon
No. of Cells and Connections 60 in series
Open Circuit Voltage (Voc)*................36.3V
Maximum Power Voltage (Vpm)*.........28.9V
Short Circuit Current (Isc) .................7.91A
Maximum Power Current (Ipm)........... 7.1A
Maximum Power (Pmax)*..................205W (+10% / -5%)
Module Efficiency Maximum Power (ηm) 12.6%
Maximum System Voltage 600VDC
Series Fuse Rating 15A
Type of Output Terminal Cable with MC Connector

I bought them used for a good price ($150 ea) and they are about 3 yrs old, but look like new.

I actually bought 16, and mounted 2 of them on my 25' toyhauler along with that cheap Chinese 20 amp MPPT solar controller. At peak times, the max. current output is 18.5-18.6 amps at 15.5v, which is pretty respectable for only 2 panels and a $100 solar controller. LOL

I live in San Diego, and the front of my house faces due south. I plan to install them in 2 rows of 7 ea, with the entire array connected in one string. We get a lot of sunlight, and the temps. are very moderate, so I'm guessing I'm going to get very good output from these panels.

My questions are:

What brand and size of grid tie inverter should I buy? Based on reliability and peak efficiency? Something in the 3KW range? 3.5KW? Should I select it based on rated panel output, or MPPT output?

Would it be more efficient to connect it in one string putting out about 400v, or 2 strings putting out about 200v? Is there a sweet spot, for example where 240 DC is easier to convert to 240v AC?

Our power is pretty reliable, we only lose power maybe once every 2 yrs, for a day, maybe 2, so I'm not too concerned about designing a hybrid system or a battery powered system. I'm mainly interested in reducing my current power bill from $160-$200/month down to zero, or as close as possible. I did hear that SDG&E recently added a $30/mo. charge for solar panel users to help them recoup some of their losses from lost revenue, but perhaps if they didn't charge the highest prices per kw in the nation, there wouldn't be so many people going solar.

If possible, I'd love to find a gently used inverter if anyone has a lead to where I might find one. If not, I'll buy new, but I'd like to make sure I'm not buying a known piece of junk, or one that is below average in reliability or performance.

Thanks in advance!


  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Need advice on selecting a grid tie inverter...

    Welcome to the forum.

    You've got fourteen 205 Watts panels which total 2870 Watts. That's the most they'll produce so it doesn't make sense getting a larger inverter because it will never be able to produce more than the panels can. As such, they will actually average somewhat less output than their rating - around 2300 Watts.

    As far as Voltage is concerned, most central inverters need over 300 Volts to operate properly (they usually have a turn-on point around 280 or so). By the same token they usually have an upper limit of 600 Volts which determines the maximum Voc of the array. With those panels 14 * 28.9 Vmp = 404 for the operating Voltage and 14 * 36.3 Voc = 508 for the maximum. Unless you get cold weather it would work configured that way.

    Another thing you might consider is using micro-inverter like the Enphase. That would be one (199 Watt unit) per panel. This would be especially advantageous if there are potential shading issues. This would be more money than, say, a new Fronius 3000

    Something to think about anyway.
  • SkiDoo55
    SkiDoo55 Solar Expert Posts: 414 ✭✭✭
    Re: Need advice on selecting a grid tie inverter...

    Ran on the Schneider string sizer an a single string of 14 with cool morning temps (25 deg)will push the normal MPPT tracking range fo 195 to 550 VDC for their GT inverters. With you normal average low of @45 deg would be in the green. 2 String of 7 ea won't work it as voltage would be regulated to try and maintain the 195 VDC level. I have had good luck with my Zantrex GT 3.8 240NA inverter. Going to get a second one ordered this Monday for earth day from this site sponsor with the 10% discount.
    I would recommend for your 14 panels a Conext TX 2.8 Inverter.
    Your record low appears to be 25deg in 1913.

    You can go to the Wind Sun site and use the string sizer, putting in you average high and record low temps and see how it comes out.
    Most all of the main manufactures have online string sizing tools.

    Good Luck and enjoy. PG&E so far hasn't hit us with any additional service charges. My monthly bill is @$4.50 with the true-up at the end of the year. Next system to be added soon will zero owing them anything at the end of the year.
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
  • Sulfur
    Sulfur Solar Expert Posts: 62 ✭✭
    Re: Need advice on selecting a grid tie inverter...

    I have had good luck so far with SMA Sunnyboy grid tie inverters, the SMA website has software that will help you size the inverter you would need, but it is probably a SB2500 or SB3000 if you use SMA

    Ebay frequently has used and discounted Sunnyboy inverters
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Re: Need advice on selecting a grid tie inverter...

    I like my Fronius plus inverters. 14 of the Sharp nd-205u1 panels are the optimum amount to use to -4 f on a Fronius 3.0 1 plus inverter that has the 600 vdc limit. I have 2 of the plus inverters and like them over the previous ig models that have the 500 vdc limit. solarvic
  • RandySolar
    RandySolar Registered Users Posts: 2
    Re: Need advice on selecting a grid tie inverter...

    I like my SMA Sunnyboy inverters(5000-us and 6000-us). Been working for years without any problems. For SMA inverters, they recommend panel voltage of 310VDC for conversion to 240AC. Not sure why they don't want 240VDC from the panels for 240AC output, but the SMA minimum voltage to turn the inverter on is 250VDC. SMA has a great track record and are well respected. I would size the inverter so its running at about 80% capacity, just to me on the safe side...but thats just me, I don't like to run things too close to capacity.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Need advice on selecting a grid tie inverter...
    RandySolar wrote: »
    Not sure why they don't want 240VDC from the panels for 240AC output

    The 240 volts AC is a root-mean-squared (RMS) 240 volts. The peak to peak voltage of 240 RMS is about 330 volts. Some inverters require the full peak-to-peak voltage (as a DC voltage) in order to make AC. If the SMA inverter can make 240 volts AC from only 250 DC volts DC, it must have some way of stepping up voltage.

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i