Basic solar PV starter system options

I'm new to this forum and seeking advise on adding a PV system to my house. Actually, I have too many questions but the main question I have is if I should proceed with this project since it does cost a good amount to get started. The basics first: I have a 13 yo home that is well built - 2,800 sq ft living with about 3,800 total area. My bills run between $250 to $450 per month. Cost of power is currently 16.1 cents per kwh. In July i used 2,900 kwh. Live in south Texas (lots of sun).
Can I get a decent invertor like the Xantrex 3.8 GT and connect only about 4 panels of 120 to 200 watts each to get started? Later, as budget permits I would like to add more panels - like 4 at a time.
Thanks in advance for any advise. I do appreciate it.


  • BB.BB. Super Moderators, Administrators Posts: 33,080 admin
    Re: Basic solar PV starter system options

    The answer is maybe...

    First, the Inverter has conversion losses, at full load my Xantrex GT 3.0 kW is about 60 watts at the lower loads (actually it is pretty constant across the range from 1kW to 3kW). So, if you put on 480 watts STC rated of panels, you will only generate about 350 watts peak.

    The other issue is that Xantrex GT family need about 195 VDC to "turn on"... By the time you add the operating at 95 or 104 F (warm summer days), you are looking at a minimum of eight 36 VDC panels in series to give you about 200 VDC minimum.

    If you have lower voltage panels (like 24 vdc or 12 vdc rated panels), the minimum number of series connected panels will needed will go up too...

    For example, I picked Shell SP 130-PC panels and see that 1 string of 8-12 panels will work (install eight, then add up to 4 more for 16 total). Then two stings rewire to two strings of 8 (16 minimum, add 2, 4, etc. up to 24 total), etc...

    Play with the Xantrex GT calculator here to look at how this all works:

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Basic solar PV starter system options

    Thanks for the advise here and on my other posting about the PV heat issues.  I can see two things so far but I still need to learn so much more.
    1 - I need to learn more or hire someone to advise me.  Closest contractor/vendor that I have found on the Internet is about 300 miles away!
    2 - I need to shoot for a bigger system than what I had in mind.  I wanted 4 panels to start with but I will get 8 or 10 or 12 panels to start.

    I take it that a "string" is a line of panels connected in series (more e - same i).  Then at a later time I "string" another string - in series - and connect this string to the first string in parallel (pump up the amps this time).

    Any recommendation for panels?  I found some BP 120s for $540.  Also found Sanyo at good prices.

    Thanks again,
  • BB.BB. Super Moderators, Administrators Posts: 33,080 admin
    Re: Basic solar PV starter system options


    Yes, a string is a series connected set of panels (say 8, 9, 10, 11, or 12 in the example I gave). String #2 will be connected in parallel to string #1. And both strings need to have the same number of panels in series (output voltages have to be the same to parallel two or more strings). In theory, you can reconfigure the panels into a new series/parallel configuration... But you will have to play with rewiring existing panels and integrating the new (and modifying your DC to inverter feed connections). Certainly possible--but it is extra work (especially if mounted on a second story roof).

    Personally, I would get the largest "high voltage" panel, within price reason, I can afford. Fewer wiring connections required.

    It is not that hard to do--and a local licensed electrician can do the wiring to make sure it is safe and to code (and probably required by the utility and/or local gov. agency if you are doing Grid Tie). But it would not be much fun as a do-it-yourself project if it is your first time. Mounting on a roof keeps it out of the way and safe from "others". Ground mounted is much easier to work on.

    Mine came with 2 x 10 panel strings of BP 4175 (175 WATT) ~30-32 vdc (actual, not rated voltage).

    Solar Guppy (Solar Grid Tie Inverter design engineer):

    Suggests staying away from Sanyo using Si/Asi (and possibly UniSolar Asi too) as this technology had lots of long term life problems (even BP gave up).

    BP has the longest (positive) track record... The rest are probably OK as all are using same Si type technology. Mono-Crystaline is about 1-2% more efficient--but probably not worth much of a premium unless you are very space constrained.

    Basically, buy your panels per watt. And, if you plan on adding to your string, make sure that you can get additional panels of the same (or very close) ratings. Otherwise, you will have difficulties getting your strings and panels matched for optimal output (parallel output string voltages must match and in-string current of the panels should match).

    Take a look at his website--you may need to register to view/post at his site (I never had any spam from him--so his site seems friendly). He is very quick at answering questions too.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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