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Tesla HPWC - impact to standby generator sizing?

Discussion in 'Charging Standards and Infrastructure' started by Magellan55, Dec 6, 2018.

  1. Magellan55

    Magellan55 Member

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    not sure if this is the right forum, but I have a Tesla Wall Connector and I'm looking to get a standby generator for when we have power outages. Electrician says we need to include the 90A circuit in his load calc, which puts me into a very expensive water cooled generator. anyone have a similar situation? Anything I should know about the Tesla Wall Connector that would let it be controlled by the transfer switch? I know these HPWC's can talk to each other and load balance, so I was hoping maybe they can be controlled by other devices to ensure they don't overload a gen. not much on the Tesla website.
     
  2. miimura

    miimura Active Member

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    Don't back up the Wall Connector that has a large breaker. Install a smaller 240V circuit (NEMA 6-20 or 14-30) and use the Mobile Connector when you absolutely have to charge on the generator. Also keep in mind that the car is not guaranteed to charge successfully on a generator. In the past, Tesla cars have been relatively picky about the frequency and voltage stability and many traditional generators can't hold that steady under varying loads.
     
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  3. Magellan55

    Magellan55 Member

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    It's not about charging from the generator - I have ICE cars so that would be silly to use up propane that way. This is just to satisfy some kind of code or generator manufacturer requirement. They say since it's hard wired, they have to include it in the load calcs, even if I'll never plug the car in during an outage.
     
  4. mongo

    mongo Well-Known Member

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    Assuming you are talking a whole house generator setup, there may be a status line from the automatic transfer switch that the electrician can use to run a 100A contactor for the HPWC which would only enable it when on grid power (at the expense of the coil power usage).
     
  5. ewoodrick

    ewoodrick Active Member

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    Honestly, you will probably be happier with the larger generator. When you start moving away from lawn mower engines, they become a lot more reliable. Dramatically oversizing a generator is not a bad thing.
     
  6. miimura

    miimura Active Member

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    If he's running on propane and not natural gas, then the efficiency becomes an issue. A large generator at relatively low loads will be spending most of its fuel overcoming friction instead of generating electricity.

    If you don't want to charge on the generator, the best way is to somehow get it excluded from being powered by the generator. If you're wiring the transfer switch to back up the whole house, that may not be so easy. Most of the time, people move loads out to a critical loads panel. In your case, you might move loads out of the main panel that are NOT backed up.
     
  7. mongo

    mongo Well-Known Member

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    May be able to run a separate feeder or tap from the meter to a 100A service rated panel/ disconenct then to the EVSE, thus bypassing the transfer switch.
     
  8. Magellan55

    Magellan55 Member

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    Thanks for all the ideas. Miimura's idea might be what we end up doing. I already have a subpanel with critical circuits, that's manually switched and run off a portable gasoline generator during an outage. But there's only about 50A worth of circuits on it. So we'd have to move most of the others over to it, as my wife wants the AC and washer/dryer to work during an outage. We were out over 3 days with hurricane Michael this fall, and no being able to do a wash put her over the edge. Since everyone was out, the closest laundromat that had power had lines around the block. Well that, and having to go get more gasoline and refilling the tank in the middle of the night, etc.

    I was just looking for an easy solution, and whole-house seemed the way to go until we hit this snag with the HPWC. Part of me is wondering if I'm getting a runaround, so I was hoping to find another Tesla owner that installed a whole-house generator with a HPWC to see if they had the same issue. I also sent a question in to Tesla.

    As for generator, I was looking at 20kw, which is a 1L 2 cylinder engine - not quite a lawn mower engine. Heck the 16HP engine on my 7.5kw portable gen is way better built than a lawnmower's. And miimura is correct propane usage is a big factor (besides the extra $5000 the bigger gen would cost). I only have a 500 gal buried tank, which would last about 5 days at half load with the 20kw model, not including any I use for hot water or the furnace. The liquid cooled 25/30kw ones would drop that to 3 days. I was even considering dropping to a 16kw model just to squeeze out more time on the tank.
     

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