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Wall Battery Custom Reserve for Power Outage

I recently had a residential system installed and it is up and running. 5.28 kW solar array with Panasonic panels and a single Tesla Powerwall 2.

My custom power reserve is set at 30%. I am trying to decide what might be the best setting to maximize the economy of the solar array and the useful life of the battery as well. We are 0.5 credit to 1 from the power company here. What are others using as the power reserve % and why?. Have others had good or bad experience with the feature on the Powerwall to increase backup in the event of weather events. Locally, we don't have a lot of power outages absent a weather event.

Thanks for any and all opinions.


  • bpbp
    edited March 2020
    The answer depends upon multiple factors...

    Assuming the PowerWalls are similar to the S/3/X battery packs, as long as you avoid completely discharging the batteries or keeping PowerWall at very low or very high charge for extended periods, the PowerWall batteries should do much better over 10 years than the 70% charge level warranty. The reserve % shouldn't have much impact on battery life.

    If you plan to use the PowerWall to provide off-grid power, then you want to set the reserve % so you have at least a few hours of power during an unplanned outage. Though with a single PowerWall, that may be difficult to achieve.

    If you have a net metering plan that provides a 1-to-1 buyback for solar energy, then the grid is essentially the equivalent of a large battery pack - so the reserve % should be based on how much power you want in reserve for a power outage.

    But if you don't have net metering or it's not 1-to-1 buyback, then you'll want to minimize how much power is sent to the grid and maximize how much power you are getting from the solar panels and PowerWall - and set the reserve % lower.

    We don't have net metering with our 15.4 KW solar/4 PowerWall system, so we want to minimize (if not eliminate) any excess power being sent back to the grid (for which we get no credit). Initially we set the reserve % to 40 - and after several sunny days with the PowerWalls fully charged and power being sent back to the grid, we've lowered the reserve % to 20.

    Best strategy is to observe how your system is working, and adjust the % based on your actual experience - which may take a year to figure out what strategy works best.
  • edited November -1

    So far we’ve set our reserve to 10% in the winter months as our objective is getting as close as possible to 100% self-powered. We need all the PW juice to run the home during multiple consecutive days of cloudy/rainy weather and low solar generation. I’m expecting to increase the reserve at other times of the year as the weather changes to mostly sunny.

    In our case we can’t send energy back to the grid and there are no time-of-use rates. In your case the half-credit net metering arrangement is an incentive to keep/use your own energy generation and send as little energy back to the utility grid as possible.

    If your priority is lowering the electric bill it sounds like your reserve should be set low - maybe 5-10%. A higher setting makes sense if your priority is having as much backup energy as possible during utility grid outages. Maybe experiment to find the right balance for your use case...
  • edited March 2020
    As the above contributors have implied - there is no single answer. I live in Northern California and have been subject to some pretty impressive outages - the major reason for adding two powerwalls to our existing 10 kW solar. In the past year, the utility feed has failed over a dozen times ranging from a few minutes to 31 hours. I have used a seasonal limit - 20% in the summer (when there is abundant sunshine to recharge) and 50% in the short days of winter. I have never had the powerwalls fail to provide full electricity during an outage with those settings. I recently decided to go with a single setting year round - 30% and have been monitoring production and usage to make sure this is sufficient - it feels like it is so far. Oddly enough, the summer problem is excess production - during my extended 31 hour October outage, my solar shut down several times and I changed my vehicle charging to mid-day to soak up the excess.
  • edited March 2020
    You really need to look at your power consumption relative to your storage capacity. We use approx 15kWh a day so we can run the whole night and still have 10-40% in the powerwall the next day. I’ve set our reserve very low (5%), just enough to ensure the powerwall has some level of power to enable restart of the solar system should we lose poeer at the worst time (just before dawn)
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