Energy Products

3.8kWh Solar setup in LA - Do I really need a powerwall?

edited November -1 in Energy Products
Just wondering your thoughts on if it's worth it to get a powerwall along with a small solar setup.
I'm questioning if the powerwall would even be able to charge daily for use during peak hours.
Would I be better off just sending the excess back t the grid?
Anyone have some thoughts on this?


  • edited November -1
    If you are expecting service disruptions, then yes. Get the PW. Here in the Northern California fire zones (PG&E), I expect outages to increase dramatically. Also, my PW's included with my solar installation get the Federal tax incentive.
  • edited December 2019
    It's rarely "worth it" to get a Powerwall (PW). Solar...yes., unless you value a PW as "insurance" against outages, which is especially important if you have medical devices in the household, like a C-PAP. (Fortunately we're not there, yet.) It's expensive "insurance", but it's why I got two PWs here in "Earthquake Country", Southern California. Even without earthquakes, my PWs have protected my refrigerator and freezer food, as well as my wine collection, during a planned outage of 10 hours, and several shorter, unplanned outages in the 10 months since installation. I've watched the e-grid become less and less reliable during the 50+ years I've lived here. You simply cannot count on it to be here any more.

    In the States, it's almost always against utility rules to routinely charge a PW from the grid. You'd want to do this in order to "time-shift", filling the PW with energy from low rate times to export it during high rate times in order reap cost differential between the two, but it's not enabled in the PW firmware here. It *is* enabled in other locations, like Australia.
  • edited November -1
    Thank you both for the valid answers. Just what I needed.
    Gregbrew, since you're also in SoCal, do you think a small system like this would really be able to re-charge a single Powerwall daily? (solar only of course - not from the grid). I don't get many power outages here in Burbank so the PW would only be to provide energy during Peak high cost hours and maybe into the night.
  • edited December 2019
    I agree with @gregbrew. Hard to justify it financially...
    I have one PW and a small 5kW solar. I save about $400-$500 per year with load shifting in San Diego... I have spreadsheets to track my power consumption and I calculate the savings based on the PW data...
  • edited December 2019
    I have 5kW of PV and two PWs. It took two days to charge the PWs after installation in late January/ Early February this year, which is during my low production months. My PV array is situated almost ideally, on a South-facing pitched roof. Having a large South-facing roof was one of the top criteria for picking this house.

    One PW will hold 13.5kWh, which a 3.8kW array can probably fill in Burbank during most of the year. Depends on the array orientation, though. In the dead of solar Winter (pretty close to right now), my 5kW will still produce about 20kWh if it's clear. It can be close to zero in the rain. During Summer, it routinely produces 30kW. You can extrapolate from that for 3.8kW, but understand that I'm subject to a pretty mean marine layer most mornings until 10AM or so. You probably don't get much of that in Burbank, which should help.

    You really need to look at your e-bills to get a feel for your routine daily usage (by the hour), if you intend to use a PW to time-shift grid usage to the high power rate evening hours (TOU). Also understand that due to Physics (2nd Law of Thermodynamics) the energy transfer into and out of a PW is not free. You're converting electrical energy into a chemical form (and vice-versa) in the batteries. There's about a 10% energy loss in this in-out energy conversion process. Take that into consideration in your calculations, if it's close.
  • edited December 2019
    I wish this forum had an editing feature: The "30kW" in the second paragraph should read "30kWh". The batteries hold *energy* (kWh), not power (kW).
  • edited December 2019
    @bcmusic, you should be able to see your solar productions during sunlight times how much is being produced and how much returned to the grid. If your powerwall is down, that will be used to recharge. How quickly depends on this and who you set up the PW, backup only or to supplement the usage as needed.

    No, there is no cost benefit to this other than a piece of mind when grid is down. An auto backup system may be cheaper but it too depends on fuel of some kind.
  • edited December 2019
    You're replies are much appreciated you guys. Thanks for taking the time.
    I feel like I can easily get by without it. I think there's just a part of me that wants one (like when I got my model 3).
    I'm leaning towards canceling the powerwall part of my purchase (assuming I still can). Thanks again for the info/advise.
  • edited December 2019
    I just went through this exercise of justifying the powerwall purchase after I placed the order today. Looked at discussions on reddit and various forums. I just realized that with net metering 2.0, getting a PW isn't worth it. It's because the excess kwh generated by the solar panels can be exported and "stored" in the grid (instead of storing in the PW which costs $11,000 less fed credit and waitlisted SGIP) and used in the form of generated bill credits. Sort of like storing your rollover minutes (excess kwh generated) in the cloud (the grid). I cancelled the PW order and will just get the solar panels.
  • edited November -1
    The Powerwall is bada$$ cool.

    Forget the economics of it. It is just freakin' awesome, pure and simple as that.

    I get an incredible amount of satisfaction just turning off my connection to the grid and going pure off grid mode. As soon as my system was installed, but the net meter was not yet installed, I was off the grid for a few weeks. I didn't want my system just sitting around being unused.

    The day the county inspector approved my system, I went off grid. For me, I love the feeling of not needing Duke Energy, which is truly an evil company and their customer service department deserves every bad thing that happens to their senior VPs.
  • edited December 2019
    Considering 17.6 KW Solar system with 2 PW2. Just here to hopefully learn more about benefits of PW2 before contract obligation.
  • edited December 2019
    Did the installer approve your project? 17.5 kW is too much for only two Powerwalls. Technically, the output of your solar needs to be less than the total capacity of the batteries. Each battery can sink 5kW, so your system would require at least four PowerWalls...
  • edited December 2019
    I think Tesla requires one PowerWall for each 7.5kW of solar for this very reason... They do allow some overhead due to the fact that full output is never really achieved and your home will be sinking some power too... So, your system would be ok with probably three Powerwalls minimum...
  • edited December 2019
    I'm having trouble finding any documentation from Tesla stating that there's a limit on how much PV can accompany Powerwalls. I believe it boils down to how much backup capacity you want, and how long you want to wait to refill your batteries, up to the power I/O limit of each PW, which is 7.2kW peak, 5kW continuous. If the PV output is higher than the PWs can take, the TEG should be able to deal with it, either by shuttling the excess to the grid, or cycling the PV as in an outage with full batteries and sunlight.
  • edited December 2019
    @Passion2Fly: "I think Tesla requires one PowerWall for each 7.5kW of solar for this very reason."

    Where did you see this? I'd never known that and knowing what I currently know sounds arbitrary or purely regulatory. What about PW installations with no solar?
  • edited December 2019
    thegateway can only control the solar inverter ON/OFF. At least for the older ones like mine...
    So, if the solar output is higher than what the PW can sink (which varies with the battery's internal temperature), the gateway will just shut the solar off... probably, towards the end of the day when the production drops, the gateway will turn your solar back on... unfortunately, you missed a good amount of solar energy production...
  • edited December 2019
    That link looks like an installation performance recommendation rather than an actual requirement or hardware limitation. I've seen nothing that indicates that the TEG can only manage up to 7.5kW of PV per Powerwall.
  • edited December 2019
    Interesting discussion. My understanding from a schematic viewpoint: House = load... batteries are wired as source for this load, and the batteries are connected to the inverter and the grid to 'balance' the load through the TEG. The controller in the TEG will 'sense' all these variables and determine best practice for the current (no pun intended) configuration. When the batteries are full, the power still 'floats' through the PWs to maintain the house load. Which is why you can see the app show a draw from the array and the batteries during the sundown transition. Cool Beans, eh?
    Which is why they (TESLA) originally spent quite a lot of engineering resources to determine the 'best' setup for each unique user if installing a paired system (contract signed in October 2017 and my PTO was July 2018 - whew).
    They're shifting away from that design concept with a menu of standard sized systems now, but that's another story.
  • edited December 2019
    bcmusik: Part of the answer to your initial question is another question: What can you afford?

    If you will have to finance the Powerwall, and you don't really need it, then don't buy it. If you are financially stable, and can afford to spend some $$ on a backup system, it may be worth it.
  • edited December 2019
    Don’t have a schematic handy but perhaps this photo of our TEG wiring will help explain all the connections.

    The “microgrid” panel (installed left of TEG - not shown) combines the Powerwalls and inverter with separate breakers for each. The microgrid panel connects to the TEG on the large bottom lugs, which also connect to the main breaker panel of the home. The utility grid connects to the upper lugs on the TEG.

    So everything (grid, PWs, inverters, home) is physically connected together when the grid is up, with the TEG controller monitoring everything via CTs and the Neurio box and orchestrating the energy flow as required.

    When the grid fails, the TEG just disconnects the grid and everything else just keeps on humming. So simple, so elegant.

    Haven’t done much research on restrictions for matching the various capacity of the PV, inverter and PWs but it stands to reason that one should install a PV generator with somewhat higher power output than the max expected consumption load of the home during the day (so the PWs can also charge) but not much higher. Anything more is wasted unless one has net-metering and the local utility pays good rates to buy the excess generation.

    Tesla or a good solar dealer/integrator will usually guide the customer to make the right selections in this regard.
  • edited December 2019

    As much as I like the Tesla PowerWall, I wouldn't qualify their TEG as the "invention of the century"... It actually acts as an ATS (Automatic Transfer Switch) which has a been around for a while with backup generators... The battery management software (inside the battery) is really the brains of the operation... The TEG is an ATS + communication box via the Neurio (between the battery and the outside world)... The Neurio also senses the currents via the CTs...

    Something else you need to understand is that the TEG cannot control the direction of every single electron. So, the PW is not ONLY "charging" from the solar. It is charging from Grid + Solar but the CTs make sure that the charge going into the PW is exactly equal to the output of your solar system, so they can "claim" that the PW is charging from "solar"...

    The principle is simple: Kirchhoff’s Current Law
    “The algebraic sum of all currents entering and exiting a node must equal zero”
  • edited December 2019
    Sounds like we’re splitting hairs here and we both understand how the system works. Enjoy your microgrid!
  • edited December 2019
    I was wondering how much do you actually generate annually with small and medium system in southern California. My average daily usage bases on last 12 months is 30 kwh. Powerwall is looking bleak on ROO give how expensive installation costs are. I was thinking to subscribe small for $65/month and wondering how much my bill be offset? annually I can only get 6000kwh, cost per kwh comes to $0.13/kwh plus .for solar subscription. On TOU Prime, except 4-9 pm rate from SCE is 0.13/kwh any way.

    What has been experience of others who have usage about 30kwh/day and using small solar system? Are you seeing any savings?
  • edited December 2019
    In your case, leasing is too expensive. The same system (4 kW) installed with tax incentives will produce ~$0.07/kWh over 20 years... but you need to pay upfront...
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