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First actual loss of grid - success for Powerwalls!

Discussion in 'Tesla Energy' started by Bigtanuki, Sep 23, 2017.

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  1. Bigtanuki

    Bigtanuki Member

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    We lost power this morning. This was the first real test of the Powerwalls. We only lost power for 5 minutes, but hey, no computer crashes, no UPS alarms. My neighbor a quarter mile up the road, called moments after the lights flickered. He had 3 Powerwalls installed about a week after mine. You would have thought we were a couple of kids. Very cool.
     
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  2. nwdiver

    nwdiver Active Member

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    Glad I'm not the only one that gets excited now when the grid fails :cool:
     
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  3. Krugerrand

    Krugerrand Well-Known Member

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    That's so awesome. Love the enthusiasm and glad to hear the PWs worked.
     
  4. bonnie

    bonnie Oil is for sissies.

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    @Bigtanuki -- great news! Can't wait til mine is installed.
     
  5. Discoducky

    Discoducky Active Member

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    How long do you think you could go without power? I would think at least a few days with only essential items running.
     
  6. idoco

    idoco Member

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    This is from a post I wrote in one of the Irma threads that may help.
    --------------
    We don't have a Powerwall but do have ~40kWh of lead acid batteries as backup (about 6 Powerwall equivalents or half the size of the avg Tesla battery pack) with 10kW of solar panels on the roof. This provides electrical backup for our house when the power is down.

    Based on my experience, including a week when we were out of power from hurricane Sandy, here are my conclusions. With just powering critical circuits (fridge, well, septic, TV/internet, air handler, motors on propane furnace, and a few small appliances) this setup can go indefinitely with normal sunlight. Without sunlight (snow cover, cloudy days) then two days before batteries discharge.

    Notice there is no AC or electric heat on the critical circuit list. They would eat up the battery pack and solar production in no time. Same thing with charging a car. Anything more than a trickle charge would be a huge drain on the system.


    Post Irma and My Roadster
     
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  7. nwdiver

    nwdiver Active Member

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    I've got ~20kWh of Golf Cart Batteries for backup. I can keep the AC going but I've got a small house and a very efficient ductless system. Heat is a bit more of a challenge especially if it's below freezing since the heat pump REALLY struggles.

    I wish my refrigerator had a VFD. I don't notice my AC cycling buy my lights dim every time the compressor in the fridge kicks on when I'm operating off-grid.

    The only thing that would limit my ability to run off-grid is the cycle life of my GC2s :)
     
  8. Jeffgtx

    Jeffgtx Member

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    Did you get a notification from the app that the grid went down?
     
  9. Bigtanuki

    Bigtanuki Member

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    No notification of loss of grid. That's on my wish list for features.
    We have spent a lot of effort on energy efficiency over the last 3 years and the climate is very temperate here on the central coast of California. Our 2700 sq/ft home uses about 12 - 15 kWh/day. With our solar we should be able to run indefinitely. We use wood heat in the winter rather than the furnace most of the time. Last winter we averaged almost 11 kwh/day during December from our 4.1 kW solar system.
     
  10. arnis

    arnis Member

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    If we exclude heating/cooling devices and electric vehicles, in a modern homes there is almost significant load.
    Most devices consume up to 100W - computers, LCD-screens, fridges/freezers, fans, lights, speakers.
    With 11kWh storage, that's 100 hours of 100W load (some losses).
    Anything that makes heat (stove, AC, radiator, microwave oven, kettle, laundry/dish washing machines,
    dryers) consumes around at least 10x more. And older bigger vacuum cleaners.

    So in case of unknown period of grid failure, it is reasonable to stop ALL heating appliances.
    Microwave oven is the only reasonable appliance to cook small portions of food.
    Stand-alone kettle is fine for making water for coffee/tea. Make only as much as you drink immediately!
    There is no excuse to use AC. And heating with resistive elements is totally useless, it will drain whole
    Powerwall within few hours. Though not heating blankets.


    PS: incandescent bulbs are thermal appliances as they are better at making heat than light:)
     
  11. Parzival

    Parzival Member

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    I sized my system to be able to run A/C. It gets hot enough here in the summer that a few people literally die each year because they don't have A/C. If you don't pay your electric bill, they won't turn off your electricity in the summer.

    I will certainly cut back usage during an extended outage, turn off the water heater, minimize cooling, etc., but I still need to be able to charge the EV and cook meals. I had to deal with a 15-day grid loss after a hurricane a few years ago. As long as a hurricane doesn't rip all the PV strings off my roof, I won't have to go through that again.

    I'll have to replace my water heater within a few years, and when I do, I'll get a heat pump model. I accidentally turned my water heater's circuit breaker off yesterday afternoon, and when I turned it on this morning, it used 5kWh. I guess that means I use 6-7kWh a day just for hot water. Crazy!

    My primary concern now is that I currently have 300-600 watts of always-on phantom loads. I can only account for about 100 watts (home network gear and refrigerator). I already identified one culprit, an old UPS with a no-load consumption of about 70 watts, but it's time-consuming to hunt them all down.
     
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  12. mongo

    mongo Member

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    #12 mongo, Oct 2, 2017
    Last edited: Oct 2, 2017
    Regarding water heater usage: One kwh is 3412 BTUh, neglecting standby losses, if your heater is set to 120 degrees F and your cold water temp is 50 degrees, 6 kWh would be 6 * 3412 / 8 (lbs per gallon) / (120-50) = 36.6 gallons.
    Besides efficiency, another benefit of a heat pump water heater is that it will act as a small air conditioner in the summer. A solar water heater panel could also cut down the energy needs.

    Worst case, if you still have city water during a power outage, you can run it through a radiator with a box fan attached to cool things down. Even running the shower on cold will help.

    Edit: That's 138.5461 Liters for persons outside the States.
     
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  13. rypalmer

    rypalmer Member

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    #imperialsystemgore
     
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  14. idoco

    idoco Member

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    Could you be more specific on sizing?Specifically # of BTU's of A/C, kW of panels, and kWh of batteries?

    A 2000 sq ft home requires an approximately 3 ton unit. This is roughly 36,000 BTU's requiring ~10kW per hour of power. So even running only 10 hours each day you would need 10kWh production. Back of the envelope panel requirements...20kW or about 80 panels.

    That's a big consumer installation. Probably bigger than the roof on a 2000 sq ft house. Now if you're only thinking of using a small room A/C unit, say 8000BTU, then I think it's doable with a normal consumer installation.
     
  15. mongo

    mongo Member

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    You're missing the coefficient of performance / SEER from your numbers. 10kWh of resistance heating is 34k BTU. A 3 ton 16 SEER 34,800 BTU ac unit pulls 15.2 A (fan and compressor) * 240 V = 3.7 kW (actual operating is less, this was the nameplate amps).
    Example datasheet: https://www.alpinehomeair.com/related/SS-GSX16R25.pdf
     
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  16. zanary

    zanary Member

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    I've lost over 248 hours of grid power in the past two weeks Only because I'm forceable taking myself off the grid by flipping the main breaker...

    I'm waiting for Time of Use Load Shifting to arrive by December 31st in a PowerwallOS update, so until it arrives, I do it manually each day (I don't have solar, only two Powerwall that run the entire house's load [excluding the 60amp Tesla Wall Connector for my Model S/Model X]).

    I can tell you going on and off the grid is seamless, without issue. No impact to anything running in my house at all.

    Going back on grid does take almost 5 minutes from the time when grid power comes back online. This is similar to the Tesla Wall Connector as not to overload the grid with HUGE spikes after a power failure
     

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  17. idoco

    idoco Member

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    Thanks for the additional info.

    But that leads to a somewhat off topic question. Why does an RV A/C unit of 1/3 the BTU's require a generator 3.5kW (roughly the same power requirements for the 35000 BTU unit you listed)? Which would be more in line with the numbers I originally used.

    Going Off Grid: What Size Generator Do You Need? | Airstream
     
  18. mongo

    mongo Member

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    Starting draw of the compressor is way higher than running for a non-inverter controlled AC induction motor. For the unit I was referencing, the locked rotor (non-spinning/ and proportional to startup inrush) current draw is 77 amps and running is 14.1 amp. Circuit requirement is a time delay fuse or HACR breaker to not trip during startup.
     
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  19. Parzival

    Parzival Member

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    My house is 2300 square feet, and I have a 4-ton 16-SEER 2-stage A/C, consuming about 2kW/4kW (measured from Tesla app) depending on stage. Runtime consumes about 30kWh on really hot summer days. I should be producing 60-70kWh on those days. I'm getting about 60kWh/day on sunny days right now from my ~ 12kW DC system. I keep the house at about 80 degrees as an away temp, and usually 76 at home, maybe lower if it's too humid. Three Powerwalls give 40.5kWh usable after roundtrip losses.

    Overall, this seems to work well, but I've only had the system for a few days. In that time, I've not used any energy from the grid. Obviously weather could necessitate sacrifices when running off the grid.
     
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  20. idoco

    idoco Member

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    I might have to re-think about not having my A/C compressor to my critical load panel.
     

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