Enphase VS string inverters

heynow999heynow999 Solar Expert Posts: 80 ✭✭✭✭
I know this has been talked about many times on this forum. My opinion is that they both have there place. If you have some shading issuses you could probably benefit from Enphase, no shade go with a string inverter.

Here is the problem. I am an installer in Ontario, Canada. We often get customers with not ideal roofs for solar with shading issuses so we reccommend Enphase. I can see online from some past installations we did that the Enphase systems seem to work well in some not ideal roofs.

Enphase are currently in short supply so we basically can't get them at a reasonable price and we expect that to get worse. What I am looking for in some criteria to use where we should use Enphase, so that we can look very carefully whether we can use a string inverter instead. Perhaps we could use some shading percentage to deciede like if there is more than %15 shade we need Enphase (just making up that number)

I guess the real question is how bad does a site have to be before you would consider it not a good site for an array using a strinng inverter?

Thanks

Peter

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Enphase VS string inverters

    My two cents...
    1. Panels should be on the same plane and roughly same mounting (i.e., mixing flush against roof deck and top of pole--different temperatures may cause issues if 2 or more parallel strings due to Vmp-temperature mismatches).
    2. Look at the number of panels shaded (typically morning and/or afternoon). And look at the minimum/maximum inverter input voltage range... Will the array start and inverter if 1/x panels shaded, and will the inverter handle the Voc-cold with all panels in full sun).
    Looking at many inverters, the "wide range" GT inverter seems to be ~200 to 600 VDC and the narrower range seem to be ~250-450 VDC...

    Both sound like a pretty wide range of input voltages, but when you look at the typical extremes of Vmp-hot and Voc-cold--they can be almost 1:2 range on an unshaded string (i.e., Vmp-very-hot = 220 volts and Voc-freezing=440 volts; while Vmp-stc-nominal may be ~293 volts).

    If you take a string of 10 panels with Vmp=34.5 volts and assume 2 of them are shaded morning/evening--Rough numbers:
    • 10x 34.5v = 345 Vmp-nominal 10 of 10 panels
    • 10x34.5v * 3/2 = 517 Volts Voc-freezing 10 of 10 panels
    • 517v * 1/2 = 258 volts Vmp-very-hot 10 of 10 panels
    • 258 volts * 8/10 shaded panels = 206 Vmp-hot 8 out of 10 panels
    So I would be looking for a GT inverter that had an operating range of 206 Vmp-hot 8/10 panels to 517 Voc-cold with 10/10 panels. That is a wider operational range than some central GT inverters have.

    Obviously, you can play with 7/11 panels or 9/10 and see what would work for a specific installation.

    The above numbers are very rough estimates to give you an idea of how I would approach the problem. You would have to use a real solar panel string calculator for your inverters to get exact numbers (and probably expecting 1-2 volts string drop on each series shaded panel).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Enphase VS string inverters

    Check out solaredge.com. With this next-generation inverter, you get the best of both strategies. Individual module optimization handles shading and gives best efficiency, central inverter gives string sizes of 8 to 25.
  • FatawanFatawan Solar Expert Posts: 71 ✭✭✭
    Re: Enphase VS string inverters
    heynow999 wrote: »

    Enphase are currently in short supply so we basically can't get them at a reasonable price and we expect that to get worse. Thanks

    Peter

    I wonder if they are stopping production of current model to gear up for the fourth generation model. Makes sense---if Gen IV is better(and eliminates the electrolytic caps?), then they won't sell many Gen III without a big discount. When I called a while ago, they said Gen IV comes out "end of Q2".
  • xiphiasxiphias Solar Expert Posts: 52 ✭✭✭✭
    Re: Enphase VS string inverters

    Might also check out the DC optimization approach by Tigo Energy.
  • dreesdrees Solar Expert Posts: 481 ✭✭✭
    Re: Enphase VS string inverters
    xiphias wrote: »
    Might also check out the DC optimization approach by Tigo Energy.
    Anecdotal evidence, I know - but I've heard of an install which had 2 of their optimization units fail within 4 months...
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