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Arbitrage

Discussion in 'Tesla Energy' started by montreid, Apr 10, 2016.

  1. montreid

    montreid Member

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    Location:
    san diego,ca
    Question: Can the powerwall discharge to the grid? Arbitrage is the key question.

    Owner who 'right sized', installed PV systems that optimized TOU+Net metering to be near zero at tru-ups. For me, we've since added 2 EVs and a pool and now drawing about 15kWH in the evenings (though still 500kwh positive during the solar year for net metering).

    Scenario 1:
    Super Off Pk hours -- Overnight/Early morning: Recharge EV cars and Powerwall to 100%
    Pk hours SUN shine - PV system runs net positive and sells to grid at Pk rates
    Pk hours negative - PV system insufficient -- Powerwall runs household needs
    Off Pk hours - Powerwall continue to run household until super off pk hours

    Scenario 2:
    Super Off Pk hours -- Overnight/Early morning: Recharge EV cars and Powerwall to 100%
    Pk hours SUN shine - PV system runs net positive and sells to grid at Pk rates
    Pk hours negative - PV system insufficient -- Powerwall runs household needs
    Software(or manually) toward end of Pk rate -- Powerwall DISCHARGES to 0%
    Off Pk hours - Grid draw at lower rates.

    --IIRC, when we started PV grid-tied systems in the past, we were explicitly NOT allowed to have batteries to avoid Scenario 2 altogether.

    Thoughts?
     
  2. Ampster

    Ampster Member

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    #2 Ampster, Apr 14, 2016
    Last edited: Apr 15, 2016
    You would need to set up the inverter to discharge to the grid and the inverters are not quite ready yet. I believe you would need permission from the utility to do that. There are some large megaWatt systems in Irvine that do that in conjuntion with the utility and get paid for adding load when there is overgeneration, and shedding load when the grid is short on resources.
    Fo a small system behind the meter you may be able to take advantage of arbitrage without discharging to the grid. In that sceneries you would run your house loads off the batteries, which would allow all or most of your solar production to be credited at peak TOU rates. You could then charge your batteries during the super off peak rates. I think that is what you implied in sceneries 1 an 2. I am not sure I understood the different between 1 an 2.
     
  3. Aussie

    Aussie Member

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    Was Norcal / Now Australia
    I don't know your legal situation in terms of feeding and discharging from the grid. However, I think a single Powerwall might not be the right product for either scenario.
    Firstly, it only has 3.3KW in power so charging takes at least 2 hours if you have the perfect sunshine. If you are drawing more than 3.3KW power, you will still have to use electricity from either solar or in the evening or on cloudy days from the grid. Secondly, the Powerwall really has a limited cycle life, in your heavy use scenario there might be other product more suited for your need. I don't know how the warranty policy is in the US, but in Australia Tesla warrants that after 1500 cycles (so in your scenario easily after 3 year) the product only has 4.6KWh in capacity left. You will potentially reach end of life of those cells before 10 years are up.
    Regardless of your scenario, you might need more than one Powerwall for your needs or a product which has significantly better warranty conditions.
     
  4. montreid

    montreid Member

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    Thanks for the responses. Interesting that the warranty wears that low. The stated purpose of the Powerwall is daily cycling.
    they show complete power cycle drainage to 0%.

    Sorry Scenario 1 should have been like showed in youtube. The battery is Charged by the solar array -- not overnight. This is the distinct problem on how it's being sold/marketed as shifting the production evenly (hence not any advantage for netmetered folk in CA and other netmetered states).

    But if can charge the battery overnight at super low rates, then is feasible option.

    Scenario 3 would be to hook up the EVs upon returning home for the evening and discharge them during the shoulder period to support home use (why not? same tesla batteries-- right?); then recharge at the super low rates between 12midnight-5am.
     
  5. Ampster

    Ampster Member

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    #5 Ampster, Apr 23, 2016
    Last edited: Apr 23, 2016
    Only problem is Tesla controls the relay that let's your connector access DC. They probably won't do that because they don't want people filling up at a SC and discharging at home.
    To your other point about the Powerwall and inverter only charging from solar PV, that is so you can get the same tax credit as solar. If the inverter can be programed like hybrid inverters, then you can play the arbitrage game. Your tax situation and inverter capabilities may determine your optimum approach.
    Also, about the video showing batteries going to 0%. They are referring to the Powerwall "available" energy window, not the actual capacity of the batteries. It is the same concept as in our EVs, where the "available" energy is probably only 80% of the actual battery pack capacity. That way, as hard as we may try, we probably can't overstressed the batteries.
     

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