Energy Products

Powerwall plus backup generator?

In the event of a power outage, can you use a generator to top up the powerwall? In the event that either your solar isn't keeping up, or you don't have solar?


  • edited September 2017
    In theory yes but if the battery is fully charged and there is surplus energy from any solar panels, the excess will be shipped to the genset - probably not a good idea. I'd also like to hear an answer from Tesla for this question because I have this exact setup. I have micro-inverter based solar panels from SolarBridge Technologies (bought out by SunPower in Nov-2014).
  • edited September 2017
    So I have a solar panel array (17kW) and a gas-powered back-up generator (22kW) and I want to add two Powerwalls but I want the back-up generator to start when the battery gets low in case of power outage. I am told by my Tesla representative that it is impossible to build an automatic system doing this. In Utah, we sometimes have heavy snow days which block my panels and are prone of producing power outages. Today, my generator kicks in as long as needed. If I put Powerwalls, they will at some point be depleted, and I want the back-up generator to kick in. I cannot believe in this day and age we cannot build such a system. Any comments?
  • edited September 2017
    Such a system could be built (safety is the prime consideration). At the moment, the software/firmware with Powerwall seems a little less capable than it could be but for the average "user" it is pretty good. Alas, there is very little technical documentation describing the current functions but here are a few comments/questions and an overview of PW-2 behaviour I have gathered since my PW-2 was installed about 3 weeks ago.

    1. What triggers the start of your genset currently?
    2. With 2 PW-2's you could draw a max of 10kW (your solar or genset can deliver more than this so is this enough?
    3. I do not know if there is any hardware inside the PW-2 that can provide any genset start/stop signals but would not be surprised if the facility exists (but is not yet enabled)

    4. Providing appropriate isolation is the key to the safety aspect and with the current configuration, the Tesla PW-2 does this by isolating the grid during a grid failure, when this occurs, the PW-2 will combine the available power from your array and the batteries to provide wou with an islanded mini-grid in your home. WHen grid power returns, the PW-2 will see this and return to "normal" behaviour.

    Normal behaviour is:

    1. Provide power from solar if there is enough for your needs
    2. Any excess of solar is first used to charge the batteries
    3. When the batteries are full, any excess is exported to the grid (This is the first issue)
    3b. Some write up's I've seen suggest that if the grid has failed in this situation, the solar panes are disconnected but I do not know for sure.

    4. If there is insufficient solar power to run your home available, any additional power is first taken from batteries and/or from the grid if it exceeds the PW-2 capacity (5 kW per unit). Currently, Tesla don't support more than one PW-2 on a single phase but I'm told this is just a firmware limit at this time (otherwise I'd have bought two of them :-))

    5. You (can) set a "reserve percentage" of power for the PW-2 - this limits the depth of discharge so that battery power stays above this threshold; this reserve value is used as an Uninterruptible Power Supply (UPS) in the event of a grid failure and is otherwise not used to provide power (if you have the backup option)

    6. If you draw power from battery below the reserve percentage (ie from the UPS function) or you raise this setting to a value higher that it was then: Solar and/or grid power will be used to get the battery to this reserve percentage, then solar will be used to try to get back to 100% if available but if over the percentage, the grid is not used.

    So the issues are item 3 above and the method used to start/stop the genset.

    If you want to run a genset, the PW-2 would see this as the "grid" and this may not be good for your genset when it tries to export excess solar. You could isolate the solar array such that there is no power available to send as excess if the batteries are full. In this case, the PW-2 _should_ provide battery power and anything over 5 kW _should_ come from the genset; but it's the co-ordination of this and the genset start/stop that needs to be addressed.

    I have asked if there is a RESTful interface into the Energy Gateway. If there is, then it is programatically possible to monitor and react to the various data and use that to control devices from say an external computer. I am working on something like this for myself in conjunction with my Micro-Inverter based array; but need access to this data from the PW-2 to enable be to integrate these together.

    Anyone else more knowledgeable than me - please feel free to speak up - because I have the same issue but my Electrician has currently advised me to not run my genset with the Solar Array connected [I manually isolate them when I start the genset] (this is why I'm working on my idea). As per item 6 above, I will isolate the Solar array and start the genset before I hit the reserve value, this would then provide power from battery and then from genset and/or battery when the genset is running.

    Note: only having this install for 3 weeks, I have not yet had to start the genset because I have not had to so far but when I did in the past (before the PW-2 install), I first isolate the solar array before starting the genset...
  • edited September 2017
    Surely the powerwall can be configured not to export to the grid? Not all utility companies allow that. The Xantrex XW system from Schneider electric can auto start and stop a generator depending on the battery level, and disconnect the solar when the batteries are full. Hopefully the powerwall will be able to as well.
  • edited September 2017
    My PW-2 is configured to not export _battery_ power to the grid; as indicated above, the item listed as 3. When the Solar array is generating power, and the battery is fully charged the excess Solar power is switched to the grid; if a generator was in use as a substitute (of the grid) at that time then the Solar would be sending power to the generator. This may be a bad idea (depends on the generator).

    If the solar panels were not generating power then this _may_ not be an issue however if a configuration was largely autonomous (as the OP implied) you would not know when this condition might arise. If one was to manually switch components then it could work but of course it's the manual intervention.


    Harry E.
  • edited September 2017
    In some places, you can not export power to the grid by law. Certainly the PW-2 supports being installed in such places by being able to just cut off the feed from the solar?
  • edited July 2018
    Just ordered a large system 45x 350 panels and 3x PW2s. All the other products I looked at could trigger a generator to top off the backup batteries. Went with the Tesla as felt more comfortable with the company and the HOPE they address this.

    Why it makes sense. I have a house in PR and was there for Hurricane Maria. Was lucky enough not to suffer any major damage but neighbors on all sides lost roofs and windows. Have a 20KW generator and 200 gallons of propane. The generator at idle burns ~2gallons an hour and at near full load ~2.5gallons and hour. We had to be Very careful about how we used generator as we had no city water and needed to power pump from our cisterns. Had we had a battery backup that could be charged by this large generator we could have had AC and run a fridge by running the generator for some limited hours a day. We will have many panels now but it still makes complete sense for Tesla to support this. The power from the generator I can guarantee is cleaner than the city as the power fails multiple times a week and brown outs are a daily occurrence. Cant tell you how many times my Led lights are working but everything else is out. When we are gone we unplug everything.

    We ran the generator about an hour a day to shower and cook. Ended up getting out 10x days later and invited our neighbor to move in which they did. Was 4x Months and 1 day before power came back but still goes out daily.
  • edited November -1
    @cjgdiver, looks like you will be getting a sizeable solar array (15,750 kWh) and should do nicely coupled with 3-PW2's. I live in Edmonds, WA and have 13.2 kWh solar system with 2-PW2's and have been operating our home in self-powered mode since turning on our PW2's 33 days ago, and have been completely independent of grid, operating our all-electric home, charging our two Tesla's, charging the PW2's, and are actually exporting 65 percent of our excess solar electricity back to grid. I would imagine that you will also be capable of generating all the electricity you need with your solar/PW2 installation. Sorry to hear about what you have had to put up with over the past 9+ months. Good luck with your solar/PW2 installation. Let us know how things turn out after you become operational.
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