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Does solar output add to the load capacity of a powerwall?

Discussion in 'Tesla Energy' started by Flyguy, Sep 26, 2019.

  1. Flyguy

    Flyguy Member

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    I have been thinking about adding two powerwalls to a 6.9 kW solar system. During peak summer solar hits about 6kw max. If two powerwalls are not quite enough to run our AC on their own, does the output from solar add to the load capacity? Or does the LRA requirement need to be met by the powerwalls independent of solar output to work?
     
  2. Pale_Rider

    Pale_Rider Member

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    The way I understand it, and I could be mistaken as I have only had my PW since July, when the grid is down, everything runs through the power walls. So the PW, shuts off the solar until they drop to a certain capacity and then turn it back on to top themselves off. You won’t get more out of the system than what the PWs can provide directly, so in your case, 60amps and about 10kw continuously.
     
  3. ewoodrick

    ewoodrick Well-Known Member

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    No.

    What if you lose power at night?
     
  4. wwhitney

    wwhitney Member

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    Instantaneously, yes. If the grid is out, the solar system is producing 6 kW, and each Powerwall can produce 7 kW surge power, the microgrid should be able to handle a short-term 20 kW load.

    But the Powerwalls control the solar production (via frequency shifting) based on their charge state, without any eye to maximizing peak load power capacity. So if your Powerwalls happened to be almost full, the solar would be off, and your peak load power capacity would be only 14 kW.

    Cheers, Wayne
     
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  5. Ampster

    Ampster Active Member

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    #5 Ampster, Sep 27, 2019
    Last edited: Sep 27, 2019
    I was under the impression that the Powerwalls are able to modulate the solar so that the solar can run your loads during the day after the Powerwall is charged. Also both can be done simultaneously depending on loads and solar production. I dont have a Powerwall but that is how my hybrid inverter works. In my case that is how it works whether the gid is down or not.

    I do understand the limitations of AC coupling and perhaps when the batteries are full the Powerwall may limit the solar production.
     
  6. nwdiver

    nwdiver Well-Known Member

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    .... then obviously your solar would not help... but if there is sun then solar will assist and you can maintain a higher load than without it. The same effect occurs on the grid. Solar and wind resources effect 'available capacity'. Your home is a mini-grid. If you have 5kW available from solar and 5kW available from PWs then you have 10kW of power available to run loads. If the sun dips behind a cloud for more than a few seconds your 'grid' will 'drop-out' just as if you tried to overload the PWs.

    Yes. That would be the tricky part. And grid-tie inverters have a 5 minute counter before they try to carry a load again. So if you try to pull more from the PW than they can carry the grid-tie won't kick on in response.
     
  7. Dan123

    Dan123 Member

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    Yes, solar output adds to the max powerwall output. With 2 Powerwalls + Solar, I was able to use 16-18KW for several minutes.

    The problem is that solar is unreliable, and you will not be able to reliably turn on the AC. You will have to manually monitor when the solar is producing enough, and them flip the AC breaker on.

    The other problem is that a 140 LRA would exceed even the 2PW plus solar. You will still need to get your LRA down to about 80.
     
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  8. Dan123

    Dan123 Member

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    This depends on the inverter, and whether it supports it. But this ramp-up and down capability, even if it works, it will not be fast enough. If there is, for example, a surge in demand, Powerwall has to supply the demand instantaneously. Then it starts to reduce the frequency in steps (usually takes 10s of seconds). Then the inverter will start to ramp up solar production. This whole process takes several minutes - way to slow to respond to a surge from an AC motor.

    So in practice, what's available for starting an AC motor: 14K (2pws) + solar - other home loads
     
  9. Ampster

    Ampster Active Member

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    I was not referring to surge loads from motor start ups. I was thinking more of the question of whether the Powerwall would modulate the solar to full capacity of the solar inverter. As Dan123 mentions the ramp down speed is the issue. The risk in that situation with AC coupling is that if the load drops precipitously and the Powerwall is fully charged then the output of the solar has no where to go and the Powerwall cannot ramp down the grid tie inverter fast enough. This may be way over explaining the technical issues but might explain why the Powerwall might limit solar production when fully charged and the grid is down. Because hybrid inverters are DC coupled they can ramp down solar to match loads. I have heard from off grinders that the Powerwall is not suited for off grid.
     
  10. Dan123

    Dan123 Member

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    In the situation when the PW is fully charged, the solar is producing something, and the home load drops suddenly, the PW should instantly increase the frequency to 62.5, which should cause the solar to shut down in less than a second.

    In reality, in off-grid mode, the Powerwall will not go above 95%.
     
  11. Flyguy

    Flyguy Member

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    Thanks for replies everyone. It sounds like technically, the answer is yes, if the LRA start-up needs of an A.C. exceeded the powerwalls, but did not exceed the powerwalls + Solar (at that moment) then it should work. The caveats are: 1) if powerwalls are fully charged and have shut down solar production then the solar production would not be available. 2) Solar must be supplying an appropriate amount of power at that moment to the currently charging powerwalls. 3) Other household needs are not using most of that solar power being produced at that moment.

    I ask the question because if its really hot out, your solar is producing all day long, to the point your powerwalls are full, but you couldn't run A.C., seems like a waste. Since long outages are fairly rare in my hood (unless PG&E starts hosing us with "safety" outages) then I have no problem with the need to actively manage/monitor the system for a window of opportunity to turn on the A.C. system.
     
  12. ewoodrick

    ewoodrick Well-Known Member

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    Yep, A/C on, cloud comes over, capacity goes over, entire system shuts down.

    Great Solution!!!
     
  13. Flyguy

    Flyguy Member

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    Agreed, that is not a good solution. Unless the powerwalls can handle the full load of the AC while running and solar output was only needed to meet the LRA requirement to start.
     
  14. ewoodrick

    ewoodrick Well-Known Member

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    And since it is common for compressors to cycle on and off in 15 minutes.....
     
  15. Dan123

    Dan123 Member

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    The problem with most central AC units is that they cycle on/off 2/3 times per hour. So even if you start them, you will need need to find a way to stop it from cycling on/off. Perhaps there are thermostat settings that would prevent cycling.
     

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