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In Bay Area, CA with PG&E NEM 2.0, I'm confused on what size system to install

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  • #16
    PVwatts doesn't account for daylight savings time, so an adjustment to the hourly output table is necessary to accurately estimate TOU revenue.
    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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    • #17
      Originally posted by maximus96 View Post

      This particular installer was one of many companies that submitted a quote, but not the one I chose for the install.

      The panels are southeast facing and all the wattage numbers pretty much drop off after 5pm for most months out of the year.

      Yes, using 2-9pm for peak time period, so I only get about 3 out of 11 or 12 hours of production in peak.

      Yes, the table shows average daily generation based on the annual estimated generation from PVWatts. The daily kwh usage is the average daily use in July per my latest August PG&E bill. The bill shows about 18% usage in peak and 17% in partial peak, rest in off-peak.
      I am also in the Bay Area and have EV-A rate plan with NEM in case you are still wondering. Based on the info you provided, you may only need a solar system that covers 70% of your consumption since off-peak EV charging makes up pretty high portion of your consumption You should target your gross NEM charges at true-up equal to the minimum charge (~$120).

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      • #18
        Originally posted by solardreamer View Post

        I am also in the Bay Area and have EV-A rate plan with NEM in case you are still wondering. Based on the info you provided, you may only need a solar system that covers 70% of your consumption since off-peak EV charging makes up pretty high portion of your consumption You should target your gross NEM charges at true-up equal to the minimum charge (~$120).
        Based on the 1st year production guarantee, I'm at 76.4% of my past 12 months of usage. How did you arrive at the 70%? Thanks

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        • #19
          Originally posted by maximus96 View Post

          Based on the 1st year production guarantee, I'm at 76.4% of my past 12 months of usage. How did you arrive at the 70%? Thanks
          If my understanding is correct, your last estimate shows $7.86 daily generation credit vs $7.46 daily consumption The daily generation should be $7.13 to get to ~$0.33 daily NEM charge for ~$120 annual minimum charge. So, your generation target should change from 11,080 Kwh to ~9,900 (11,080*7.31/7.86) which is ~70% of your consumption (~13,900 Kwh). My own experience with a system that covered ~75% of consumption is that I ended up with several hundred $'s of NEM credit at true-up that was wasted. Also, my off-peak usage is smaller portion of overall consumption than yours. You also mentioned your panels may generate 10% above estimate. So, it seems you may only need ~70% coverage. Keep in mind the actual result for any year will be different due to weather which can have up to 20% impact. Good luck with your system.

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          • #20
            Originally posted by maximus96 View Post

            Based on the 1st year production guarantee, I'm at 76.4% of my past 12 months of usage. How did you arrive at the 70%? Thanks
            Question not directed at me, but FWIW:

            If you are on T.O.U., depending on when (the time of day) your usage occurs, and when your system generates power, you may be able to offset, say 80 % of your bill by offsetting, say 60% of your total usage.

            That can happen, for example, if you have low use during peak pricing periods and those peak pricing periods are when your system is sending high(er) value electricity back to the grid. Simple and incomplete example: On one day, you generate 10 kWh during peak pricing, and use 10 kWh at super off peak. That's it. Now, do that for 30 straight days of a billing period.
            Also say, for simplicity, that POCO rates are $0.40/kWh for the peak rate and $0.25/kWh for the super off peak rate.

            At the end of 30 days, you will have generated 300 kWh and used 300 kWh. On the old tiered rates, that would be a wash.


            Under T.O.U., again, very simple example and not to be used for planning, the 300 kWh you use would generate a bill of : 300 kWh * $0.25/kWh = $75.00, while, because entirely generated at peak pricing time, your system will be generating revenue to offset use of : 300 kWh * $0.40/ kWh = $120.00.

            Knowing this during design phase, that is before purchase, a savvy potential PV user might say " Ok, hat's great. So, how big(small) can I make my system and still get awy with a zero bill under T.O.U. tariffs ?"

            Well, the simple answer for this very simple example : You want to offset a $75.00 bill. Looks like, for this very simple example, you can do that by a system that will only need to generate :

            $75.00/$0.40/kWh = 188 kWh.

            So, while under the tiered rate tariff you needed a system that generated 300 kWh for the period, under T.O.U. tariff you'll need a system that only needs to generate 188/300 = ~ 63 % as much electricity, or be 63 % as large.

            Now, be careful - VERY careful ! This simple example is only meant to show the reasoning behind possible smaller system sizes that are perhaps possibly lower in a caricature form by example to highlight how a T.O.U. tariff might allow system size reductions. It's more complicated than that in reality. For one thing, actual savings can turn negative and result in larger bills depending on use patterns and how the POCO peak times coincide (or don't coincide) with when a system will be generating power. For another, rates and particularly for this discussion, times of peak/off peak rates will be changing in the future in unknown ways. Best to keep flexibility in mind.

            One way to get a SWAG, if you don't yet have PV, is to download prior use from the POCO (the green button data), plug in POCO rates for all 8,760 hrs. in a year on a spreadsheet (takes a bit of work, but not terrible as it's a lot of repetitive copy/pasting), multiply the two for an estimate of annual bill, and then do the same for how much power a 1 kW system will generate for you using PVWatts hourly output using the same hourly rates. If you plan on a straight T.O.U. tariff without tiers laid over it, the system size estimate in STC kW is simply the annual bill divided by how much revenue a 1 kW system will generate, that result multiplied by how much of your bill you want to offset. Again, overly simple as it leaves out min. annual bill, NBC charges and probably some other stuff, and it won't work for most existing users because for them, the green button data has use and generation mixed together, and, again, the method won't be as easy to use for mixed tiered/T.O.U. tariffs (but it can be done - I'm still working on that one, but maybe it's time for a pit drip), but it does have its uses.

            On the 8,760 hr/row spreadsheet (or 8,760 X 4 if you use the 15 minute data): It sounds like a PITA and in some ways it is, but it really isn't that bad, and I've found it a very good tool, not only to make sense of the mess, but also as a base to start organizing the POCO lack of clarity on how their billing systems, times, rates and tariff policies function. Being helpful and forthcoming in those things does not seem to be a POCO attribute.

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            • #21
              Thank you SolarDreamer and J.P.M. Looks like I have some more work to do.

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              • #22
                Originally posted by maximus96 View Post
                Thank you SolarDreamer and J.P.M. Looks like I have some more work to do.
                For anything I may have put out, you're most welcome. No pain, no gain. FWIW, my experience in such matters is the time and effort vs. savings ratio can be quite high from doing the details. You will get your money's worth with some efforts at self education.

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