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Best time to charge: SDGE w/ solar

Discussion in 'California' started by Ohmie, Oct 11, 2019.

  1. Ohmie

    Ohmie Member

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    San Diego, CA
    I recently installed solar (two months ago) and bought a Model S. After reviewing my bill from SDGE I'm trying to figure out what the best time to charge is (most cost effective).

    My basic assumption is that I should charge the car when I'm generating excess Kwh, which is during the day from sunrise to just before the peak power period starts.

    It looks like I'm charged $0.55/kwh for net energy used during on peak hours used. I think it's safe to say I should avoid charging in the peak window (4-9 pm).

    I'm charged $0.29 for super off peak (midnight to 6 am I believe). Because we use some net energy during super off peak periods (usually AC, after 9 pm), I assume that means I should avoid charging during super off peak.

    In theory, I would also be paying $0.29 for off peak energy usage. But I generate excess power (about 400/kwh per month) during off peak. So right now I am not charged anything for that period. Instead I receive a credit for power generated during this period.

    With regard to excess energy generated (I generated 350 - 400 kWh extra per month, before the Model S), SDGE always pays a much lower rate than they charge the customer. It was $0.23/kwh for the Aug 9 - Sep 9 period, and $0.09/kwh for the Sept 10 - Oct 10 period.

    By process of elimination, I believe it's best to charge during the daylight ours of the off peak period because (1) I generate significantly more energy than I use during this period and (2) SDGE undervalues any excess power generated during this period. If I understand the bills correctly, it will reduce my off peak power generation to approximately zero each month. But since energy produced during that period is always worth less to me (SDGE pays me less than it charges per kWh), I should use up the excess energy produced during this period.

    Is my thinking correct on this?
     
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  2. Randy Spencer

    Randy Spencer Active Member

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    If you have an energy meter (I use the Sense Energy Monitor) that shows you when you are generating excess power that your house is not using, THAT is the best time to charge. I plug my car into a 120v outlet that has a remote switch on it. This allows me to power that outlet ON when the house is generating too much power so I can put some of that power in the car. You can also do that with the Tesla App. But charging at 240v draws a lot more than my solar generates, though you can set how much you draw on the dash of the car, sadly not in the app.

    I actually have another electric car and I can charge them BOTH at Peak Solar, so from noon to 2 pm they are both on. I hope to automate this in the future. There is software (New Wall Connector load sharing protocol) that you can use to control the output of the Tesla Wall Charger based on the amount of solar production, but you have to program it to work with your solar setup. Also, I charge with a 40 amp original UMC. If I could get the software to talk to my Sense and control the charging based on unused power that would otherwise go to the grid, it might be worth buying the wall charger, but I have a long way to go to figure that out.
     
  3. Ohmie

    Ohmie Member

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    Hmmm. Sounds like this is more complicated than I thought. If I understand what you are saying, even during peak solar production SDGE may charge me for power, if the power used by my charger exceeds the power being produced by my solar system. Another option would be the TOU plan that charges only $0.09/kwh for super off peak. But it comes with a fixed monthly charge of somewhere just under $20 (I forget how much exactly). I don't think I would break even on the fixed charge at the amount of power I currently consume in the S.

    If I'm understanding how this works, lets say I can generate 4/kwh per hour from my solar system and my 240v charger charges at 8/kwh per hour, then I would be charged for about half of the energy consumed during the charging period. That's still better than charging at any other time of day.

    Further, if I slow the charging rate to 4/kwh per hour, I may be able to charge solely by solar and be charged nothing (or close to it).

    Does that sound correct?
     
  4. Troyin909

    Troyin909 Member

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    Hmm we have SCE and I’m not sure if they run the program the same way as SDGE but, I’d think it would be similar. What we do is charge at night after 10 just to keep it simple, cause for one that is super off peak and we’re on a TOUA plan, but if we need to add a little more range we don’t even sweat it at home, we just plug it in. We generated an extra 4,000 kw last year so I’m still playing around with how we use our dual house Ac, pool pumps, and charge 2 MS. The way it seems to work is like a bank we use what we use and at the end of the 12 month cycle we either owe extra or don’t. Now they still charge us if we generate power too cause that’s what they do, but at the end of the day we use a lot of energy and my bill has not gone over $38 in the past 12 months and we live in Corona ca.
     

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  5. Ohmie

    Ohmie Member

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    Is your super off peak rate lower than your off peak rate? Mine isn’t. SDGE charges the same for off peak and super off peak. The only way to get a lower super off peak rate is to pay $16/mo. Back of the envelope, I think I’d need two BEVs to make the $16 fixed cost pay off.
     
  6. MountainRatMat

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    Location:
    Orange County, CA
    How can they do this? Charge you for making power? Are they the mafia?
     
  7. ucmndd

    ucmndd Active Member

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    Your explanation of charges and your reasoning doesn’t line up with any understanding of how net metering works that I’ve come across.

    Full disclosure, I’m in PG&E territory.

    For me, PG&E credits us for any over production at the current retail rate I’d be paying at that time. So, excess production gets credited to me during peak periods at ~$0.55/kWh. Then I charge my car at night during super off peak periods, and buy that electricity back for $0.11/kWh. So that 1kwh I sold during the day puts 4kwh in my car overnight.

    The only time PGE “buys” excess power from me is once a year - if I’ve produced more power than I’ve consumed over the *entire year*, then they true up with me and buy that excess generation at some piddly wholesale rate of like $0.03/kWh.

    So with that arrangement, it’s obviously beneficial to let the excess production during the day go to the grid and buy it back when charging at night (“sell high buy low”).
     
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  8. Troyin909

    Troyin909 Member

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    This is what TOUA plan looks like for us with SCE.

     

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  9. Ohmie

    Ohmie Member

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    Yea SDGE doesn’t pay you the retail rate for power generated. It pays a wholesale rate. So far I’ve been paid between $0.09 and $0.23 per kwh, which is well below the rates they charge me.

    PS Sorry for the confusion over the term “excess.” I was referring to the unused power that I sell back to the grid. (I.e., that which they pay me the wholesale rate)

    As to the excess power they pay for at year end, SDGE pays $0.04/kWh.
     
  10. David L

    David L Member

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    Location:
    San Diego, CA
    Since you just installed solar, I'm going to assume that you're on plan DR-SES.
    • Summertime super off-peak rates are about 5.5 cents less than off-peak rates. For that reason alone, it's best to charge during super-off-peak times.
    • As @ucmndd explained, net metering bill credits are given at the full retail rate for overproduction. You're mistaken that SDG&E pays a lower rate for bill credits. Note that I say bill credits, which aren't refundable as cash at true-up time and are different than the wholesale rate that SDG&E pays for excess generation at true-up time.
    • Wintertime rates only span a 2 cent difference, so it doesn't matter much when you charge. If you really want to optimize, it's actually a little tricky to determine when best to charge. Before reading further, let me first say that the optimization would save you a negligible amount of money, so it's really not worth worrying about.
      • Since you just got solar, you're under NEM 2.0, under which you're charged non-bypassable charges (NBC) for energy consumed from the grid. Solar bill credits cannot cancel out NBCs, which run about 2-3 cents per kWh. Therefore, if you charge when you're not generating, you may end up paying 2-3 cents per kWh, even if you have NEM credits to cover all of your usage. The good thing is that NBCs count toward the monthly min usage charge of about $10, so monthly NBCs less than the monthly min charge doesn't affect your bill.
     
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  11. Ohmie

    Ohmie Member

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    Long story short, you guys are right. After reading what you said, I called SDG&E thinking my bill was in error. Turns out they only apply a portion of your credit each month, just enough to zero it out. The remainder is on page 5 saved for future use.

    Thanks all for your help and patience!
     
  12. Ohmie

    Ohmie Member

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    Also, in case anyone else runs across this thread in the future, the answer is:

    Charge during super off peak to get the lowest rate possible.
     
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  13. mnml

    mnml Member

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    Location:
    Ladera Ranch, CA
    I'm on SDG&E and have solar as well. Which plan did you end up with? I'm currently on DR-SES-G. I rarely charge at home since my work offer free charging.
     
  14. Ohmie

    Ohmie Member

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    I haven’t quite decided yet. I need to run the system - with the car charging as well - to see if the TOU-5 plan (I think that’s the one with the fixed charge of $16+/- each month) make sense. Even then it will only make sense if I can use the compensation for power generated to offset the fixed fee.
     
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