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With 2 Powerwalls and plenty of solar, why am I using slight amt of grid power?

Discussion in 'Tesla Energy' started by tij664, May 28, 2018.

  1. tij664

    tij664 Member

    Joined:
    May 8, 2017
    Messages:
    18
    Location:
    California
    My setup is 2 Powerwall 2’s paired with 20 solar Enphase inverters putting out continuous 4.3 kW of AC. I had them installed in April and they’ve been working great. I’m with a local municipal utility in CA and I’m still grid connected. After installation was complete and also days after I tested the battery backup mode and solar charging capability, the batteries would take over flawlessly when I disconnected from the grid through the main breaker.

    Side story here. My utility is, to put it mildly, anti-solar. They’ve changed the rates fairly recently to their version of NEM 2.0. They’re also pretty adamant the rates can change anytime in the future to make your solar investment even less worthy in the future. This basically punishes those who go solar since they added a demand charge and also any excess is credited at paltry wholesale rates which is anywhere from under 1 cent a kWh to at most 4 cents per kWh (the highest credited rate I saw last year). This is on top of the fact that any excess I consume is billed at retail rate: 10 cents off peak to 15 cents on peak. This is the main reason I went with Powerwalls: to not get hosed on demand charges and to avoid giving away excess power generated for peanuts.

    The issue I’m having is that with plenty of solar generation I’m getting this small power drain from the grid. For this month of May I’ve used 6.4 kWh in total from the grid according to the Tesla app. I also check my instantaneous demand on the app and I’ve seen spikes where it goes to 0.1 or 0.2 kW of demand for an instant on a rare occasion. My utility provided meter confirms this too since for the month of May I’ve registered a maximum demand of 0.23 kW. I realize I’m bickering over pennies but shouldn’t the Powerwall register 0 kW of grid power if I’ve never discharged the batteries completely? Same with energy use: shouldn’t I see 0 kWh of use since I’ve never had the batteries anywhere near being discharged? The lowest I’ve gone down to is like 45% since I installed them back in April. I’m just trying to pinpoint where this vampire drain is coming from since I know the batteries coupled with solar can run without grid power. I also generate more than enough to charge batteries and for the home daily consumption.

    Shouldn’t the gateway be disconnected from the grid at all times when there is either enough battery power or solar or a combination of both of them? Are the batteries pinging the grid periodically to reference frequency? Not familiar with the specific switching or inversion scheme on the gateway or batteries but is there any small consumption of grid power going on here for gate currents to drive switching? I’m just trying to figure out how this small power drain from the grid is happening.
     
  2. cwied

    cwied Member

    Joined:
    Jan 13, 2015
    Messages:
    361
    Location:
    San Mateo, CA
    The Powerwalls are always connected to the grid unless there is a power outage. I'm assuming that the powerwalls err on the side of exporting too much rather than importing too much to avoid disqualifying the installation for the ITC. To me it looks like they can't exactly match the house load, so there often is a small current flow to or from the grid when they are in self-consumption mode. Percentage-wise, it doesn't seem significant, though.
     
  3. JHellow

    JHellow Member

    Joined:
    Dec 28, 2015
    Messages:
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    Location:
    Palos Verdes Estates
    In a solar plus battery installation the batteries do not output to the grid, and the grid does not charge the batteries.
     
  4. coolgj

    coolgj Member

    Joined:
    Mar 20, 2014
    Messages:
    18
    Location:
    Atlanta

    Missing data points to answer your main question.

    1. How do you have the Powerwall configure? Backup or consumption?

    2. What is your peak demand kW?
     
  5. GenSao

    GenSao Member

    Joined:
    Aug 3, 2017
    Messages:
    168
    Location:
    Pleasant Hill, CA
    I would not sweat the small power variations between the grid and your home. The powewalls will work to "effectively" have your home off the grid. The spikes in power demand in your home is where the small use of grid power will come from. The batteries may have a very slight lag in filling in the spike demand of power. The grid will fill in this slight gap faster.

    You should leave your home connected to the grid. The Powerwalls like to match the frequency of the grid. IMHO (a hypothesis), the batteries may work a bit harder when they are forced to emulate the grid in a backup situation (higher pings). The extra effort will likely use extra energy. In the unlikely event of battery failure, the grid would power your home.

    Depending on your annual power consumption vs. annual production, most California utilities with Net Energy Metering (NEM) will let you offset the cost of most of your home power use. When you produce more than you consume, the excess production is only credited the wholesale rate of $0.01/kWh to $0.04/kWh. This is fair for the utility since they usually buy power at that rate. Because of this, my system was designed for 90% self consumption. Do you produce more than you consume?

    The batteries can minimize the non-bypass charges that come with NEM 2.0 ($0.02/kWh to $0.03/kWh). But the "in and out" energy loss of 10% (90% efficiency) would diminish the saving. You could actually come out negative. I would calculate the efficiency loss based on the lowest utility rate (usually the NEM overnight rate) you are on. For example:

    In my case, Non-bypass charges (NBC) in PG&E land are:
    $ 0.0141 Public Purpose Program Charge
    $ 0.0002 Nuclear Decommissioning Charge
    $ 0.0013 Competition Transition Charge
    $ 0.0055 Department of Water Resources Bond Charges
    -----------------------------------------------------------------------------
    $ 0.0211 / kWh Total NBC​

    In PG&E land, there are several rate scheduled, using the best and worst:
    EV-A Summer Rate: $0.0211 (NBC) - $0.12330 x 10% = $0.00881 / kWh Savings
    ETOU-B Summer Rate $0.0211 (NBC) - $0.23181 x 10% = ($0.00204) / kWh Cost​

    I have a similar setup. Two Powerwall 2’s and 20 Enphase M250 inverters paired with 20 SolarWorld 285 W (SW 285 Mono) solar panels.
     
  6. Sylvia Else

    Sylvia Else Member

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    May 28, 2018
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    Location:
    Sydney
    When you have solar as well as a powerwall, the powerwall is trying to supply or consume just the difference between the solar output and the load. To do that it has to measure both, and it doesn't react instantaneously. What I've seen is that when there's an appliance that repeatedly and abruptly changes its demand, the powerwall briefly allows grid input when the load increases, and causes grid output when the load drops. I've seen this behavious with an electric oven that's switching itself on and off because of its thermostatic control, and also a washing machine when it stops and starts its motor. Whether such effects could really amount to 6kWh in a month is hard to say.
     
  7. tij664

    tij664 Member

    Joined:
    May 8, 2017
    Messages:
    18
    Location:
    California
    I have it in self-powered mode with 0% reserved for outages. My peak demand so far this month for the home is 11.2 kW. When that happened the solar was putting out about 3.5 kW. At that moment the batteries kicked in 7.5 kW with the grid kicking in 0.2 kW to meet the demand.

    This is probably the most logical explanation. The grid filling for a momentary lag from the batteries could very well explain it. Just curious, does PG&E have demand charges? Most demand charges I've seen are usually applied to larger commercial or industrial customers. In my case, demand charges are $2/kW in the summer, which certainly negates some of the solar savings if you only had solar (no Powerwalls).
     
  8. GenSao

    GenSao Member

    Joined:
    Aug 3, 2017
    Messages:
    168
    Location:
    Pleasant Hill, CA
    PG&E has a SmartRate option for non-TOU residential users. A brochure is available. There are some other options found here.

    Otherwise for TOU users, under Electric Rule 24, third-party Demand Response Providers (DRPs) such as OhmConnect pay you to reduce demand on a requested hour. Many users in this forum use this service. I have been using this service strategically with the PW. The CA PUC has a list of providers as well.

    If you are interested in signing with up with OhmConnect please feel free to use my referral link (not required).
     
  9. bkp_duke

    bkp_duke Active Member

    Joined:
    May 15, 2016
    Messages:
    1,291
    Location:
    San Diego, CA
    What you described in your OP is standard for the PW2s. I have three of them, and usually I can see a 0.02 pull or push to the grid. Most of the time, I trickle out a little to the grid.

    The wholesale rate you get back is also standard per NEM 2.0 - all the utilities in CA give you only the wholesale rate for net annual generation. This is put in place by the CPUC specifically to prevent residents from over-spec'ing solar systems so as to try to make money off it. I over-generated 4800 kWh last year, and I think I got something like $95 for it. Won't be an issue this year, as we will have added more demand from a newly installed pool, but you get the idea.
     
  10. ewoodrick

    ewoodrick Member

    Joined:
    Apr 13, 2018
    Messages:
    894
    Location:
    Buford, GA
    It is very possibly using some grid power as the big appliances kick in, like the air conditioner. I'd also question if some of the inverter may be pulling power from the grid, just to maybe even measure if the grid is there. At 8 watts an hour, and a few cents in cost, it is indeed not much.

    The rate that you receive when generating electricity is indeed the wholesale rate, that's normal. It's like trading in a car when you buy a new one, you don't get the same as what you would pay when buying the car, the seller has to make money. And in this case, the differential goes both the producer of the power and the maintainer of the lines. It ain't cheap to keep power lines on poles.
    At least you have an energy provider that allows sale back, not all utilities do.
     
  11. TomDiego

    TomDiego Member

    Joined:
    Jan 28, 2018
    Messages:
    39
    Location:
    San Diego, CA
    What utility is that? You list your location as "California". Why not just indicate what utility you're talking about and list the city you live in as your location? Makes it so much easier to address questions.
     

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