Considering Options to expand NEM 2 system before the change to NEM 3 - SDG&E

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  • so_cal_burbs
    Member
    • Jul 2016
    • 33
    #1

    Considering Options to expand NEM 2 system before the change to NEM 3 - SDG&E

    I have 24 LG 335W panels connected with P400 optimizers to a SolarEdge 7600H inverter (8.04 DC total). I'm on NEM 2 and just reached my fifth year of operation so will be required to move to TOU plan (although SDG&E hasn't made me yet).

    Since spec'ing the system we've added two EV's to the household and the demand for electricity is far higher than I had projected with the original system. We appear to have the opportunity to lock in the NEM 2 rate of return for any system/addition we get filed for approval by Mid-April so i've started shopping.

    I'm in 92131 and the current roof install faces 142 degrees. I'd like to add 3-4kW DC to the roof.

    My panel is 200A and is about maxed out (maybe one more slot) so adding a separate system in parallel has extra costs involved compared to leveraging the existing system. But I know I want another 40-50amp circuit put in for a second EV charger so there might be value in just rolling a main panel replacement/upgrade into the Solar work. But I suspect it will be cheaper to wire in a subpanel for the new charger and stick with what I have.

    I'd like to leverage a smallish (about 121 sq feet after fire code setbacks) roof facing SW at around 233 degrees to add as many panels as will fit then put the remaining on the current 142 degree roof. If I can use 400W panels there I'm hoping 6-7 there and the remaining 1-2 added to the existing set.

    I've tried to access the SolarEdge design tool but can't get to it. My monitoring account errors out and the application online requires a tax EIN to prove you're more than a self installer i guess.

    I know that my inverter can accept 11,800kW-DC total in two strings. I know that I will get clipping if I get close to that. But PVWatts says I will lose about 5%ish of annual output (by changing the advanced setting of DC to AC Size Ratio) which I can live with. I know that Solaredge has a compatibility list of optimizers and new install would need to use compatible models.

    Things I'm not sure about:
    1) Are there limits to adding larger panels (400W) to the existing system at the string/optimizer level? For instance does it matter if one string is loaded to the max while the other is not? Seems like I've read here there is no problem with this practice however local installers seem to think it is best practice-not sure if that is a remanant of older inverters or actual fact.
    2) What are the options for how to tie new panels into existing strings? Can they splice in mid-conduit run? Current setup penetrates roof then runs in conduit through attic then outside, into garage into the inverter. Is there best practice where to tie in other panels? New ones are on different roof (closer to inverter).
    3) Feels like oversizing this system leaves the clearest path to adding batteries in the future but i'm not sure if that is true. Is there a significant advantage to adding a battery to one big system compared to one medium/one small system if the total output is the same? Aside from avoiding clipping and warranty hassles are there advantages of a new/separate system?

    So far the pricing is coming in around $3.70 to $4.00 per DC kW to add into existing system which seems like it should come with an inverter. But maybe things are just high due to the surge to beat 3.0.
    Tags: None
    • CharlieEscCA
      Solar Fanatic
      • Dec 2016
      • 229
      #2
      I'm probably missing something, but it seems to me that this is going to be a long payback - and I would personally not go forward. Chances are very high that it's your two EV's that push you beyond what you cam generate. If that's the case, just using EV-TOU5 will push down the price of those kWh to the point of recovering 3000 * $3.70 on the low end to 4000 * $4.00 on the high end, payback is going to be long and slow. Add to it the uncertainty of what SDGE will do in terms of rates and time periods (in the past five years alone the changes in rates have messed with you, haven't they?), it's going to be a crap shoot investment.

      And I think you will get a lot more clipping of 11 to 12 DC total on that 7600 inverter particularly in than the 5% loss you envision.

      This is all just my opinion, so do as you think best.
      8.6 kWp roof (SE 7600 and 28 panels)

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        • so_cal_burbs
          Member
          • Jul 2016
          • 33
          #3
          Hey Charlie, thanks for sharing your thoughts. I will look at my excel sheet again and look for errors as well as try to model the additional production. I put in my 2022 SDG&E Green button info as well as the EV-TOU5 rates and after accounting for the $16/month fee my 2022 bill looks to be about the same as on my Tiered rate plan was in practice. I just used the published rate sheets and didn't account for the fee per kW delivered so there is some grey area but I did expect it to be lower since I had set the primary EV to start charging at midnight several months ago in anticipation of doing some modeling. I haven't yet tried to model new production against the TOU rates.

          As far as clipping perhaps someone can clarify if I'm way off base. It does feel low to me but at the same time there are numerous clipping threads here that come to the conclusion that lost production from clipping is far lower than the threads creator expected.

          In PVWatts if I put in my proposed new system size of 11.36kW, Premium modules, system losses at 10. tilt 20 and azimuth of 142 and in Advanced put DC to AC Size Ratio to 1 I get a result of 19,455kWh/Year. If I take 11,360/7600 I get a ratio just shy of 1.5. When I put that into PVWatts and recalculate it says I will get 18,565 from the system. So 18,565/19,455 is .954 or 95.4%. I have been assuming that PVWatts is calculating the clipping based on the ratio here. Also by placing the bulk of new panels facing 90 degrees further west my expectation is that the clipping will be slightly less than modeled since they will reach peak production a couple hours apart. If I'm twisting the PVWatts model too far or have made an error please let me know.

          I totally agree that it is a bit of a crap shoot since we can't predict what is next. And yes, today's energy marketplace is not what I expected to see five years ago. It is far more expensive and I fear it will keep going up.

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