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Can solar plus powerwall run the home during extended power outage

Discussion in 'Tesla Energy' started by BrettS, Apr 17, 2019.

  1. BrettS

    BrettS Member

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    I just got a quote from Tesla for a solar and powerwall system. They quoted me a 14.19kW solar install with two powerwalls and said that it would cover about 55% of my power demand. They can’t go higher because there’s no room on my roof for any more panels. Obviously I use a lot of power, but a significant portion of it goes to my Model S. I often drive as much as 4000 miles a month and nearly all of my charging is done at home.

    I like the option of going solar and the net cost of solar with no powerwalls is about $50/month less than what I’m paying for power now, so that seems like a no brainer. However, adding the powerwalls would increase the net cost to about $50/month more than I’m paying for power now, so I’m trying to figure out if powerwalls make sense for me.

    There is no time of use power rate available here, so the only thing the powerwalls would be useful for is backup in the event of a power failure. The power doesn’t fail here too frequently, but I am in a hurricane zone and when hurricane Matthew came through about three years ago I lost power for three days, so really what I need is protection from extended power failures like that and I’m just not sure that the solar system and power walls will be able to provide enough power 24/7 during an extended outage.

    Obviously I realize that if the power fails then I will need to cut way back on power usage (not charging the car at home would be a significant part of that), but do you think that this system would be enough to keep my AC and fridge running 24/7 as well as some lights and TV’s and other creature comforts?

    Right now I have a portable generator that can be used in an extended power outage, but that requires manual setup when the power fails and it also requires me to store gas for it. The ability for the powerwall to take over instantly would be a huge improvement... especially if I’m not home when the power fails.

    I’m also considering a standby generator, but that’s a big cost as well and I don’t have any gas service at this house, so I would need a large propane tank for that, which would just increase the cost and complexity in making sure the propane tank stayed full.

    I think the powerwall is a much better option, but I’m just not sure that I’ll get enough output from the solar system to power what I need to keep running in an extended outage.
     
  2. JasJ

    JasJ Member

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    Possible, but more likely refrigerator and limited a/c until the clouds clear out, then in full sun after the storm passes you could run A/C most of the time. You are probably not driving as much during the storm, so less charging needs, but you will NOT have enough to really charge the car much at all.
     
  3. Vines

    Vines Member

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    Depends on the amp draw of the appliances. Impossible to say without more data. Not sure how much AC you would use in a storm but then again I have never been a Florida Man.
     
  4. Helmuth

    Helmuth Member

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    I would agree, fridge/freezer, microwave, lights and TV shouldn’t be an issue, limited A/C until it clears up and then with sunshine most likely everything but car charging.
     
  5. BrettS

    BrettS Member

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    Yeah, I can see how more information could be helpful here, so I just went out and noted how many kWh my electric meter shows. I’ll check it again tomorrow at this time and see how many kWh I’ve used in a 24 hour period. I’m not going to try to be crazy conservative with power, but I will avoid charging the car, doing laundry, and using the electric stove. I have an on demand tankless water heater, so that won’t use power unless I actively use hot water. I’ll probably try not to use too much hot water, although I still need to decide if I want to shower during the test period tomorrow to include that in the day’s usage or not. Maybe I’ll shower as soon as the 24 hour period ends and take note of the meter reading before and after the shower so I can see how much power that shower used.

    I feel like that should at least give me some ballpark numbers to work with and might help to better answer this question. Depending on what I come up with I might consider trying to do another day with more effort to conserve power just to see what my bare minimum is.
     
  6. JasJ

    JasJ Member

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    Here is my 10.5kw solar system over the last 4 days... 3 sunny days, one very overcast. No car charging, one 4 ton A/C unit lightly used. If I had batteries, I would have little problem running off grid (this week).

    My MS draws 11kw (40amp/240v) when charging. The 4 ton A/C unit surges at 5kw and settles at 3.5kw. Large Instant Hot Water can have 4x 7Kw heating elements so peak demand at 28kw will require 5-6 powerwalls to run as each powerwall can provide 5kw continuous.

    Peak load is important as well a total storage capacity. I could get by with two powerwalls and make sure I charge the car at 32amp or less, but not while the A/C was running. My Hot water is natural gas.

    upload_2019-4-17_17-33-31.png
     
    • Informative x 1
  7. BrettS

    BrettS Member

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    Thanks for the reply. That is encouraging that your system is providing enough power to live off grid. You also make a good point about continuous load. My large tankless water does indeed have four 7kW elements. However it fires them sequentially as demand increases, so if demand is low it will draw 7kW or less.

    In fact, it has four circuit breakers and it will function with only one of them on, so that’s what I do when I’m running on my small generator. One 7kW heating element is enough to provide hot water to one shower. And by turning off the other three circuit breakers I can ensure that it won’t draw more than the generator (Or powerwall) can handle if someone does accidentally call for more hot water while someone is showering.
     
  8. Maximapolak

    Maximapolak Member

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    I went with the powerwall because, if the power does go out, I still have water, electricity, etc. It's like having a generator, and when the sun has set, I can still benefit from the solar.

    I say go for it for the $50 more per month.
     
  9. Dan123

    Dan123 Member

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    You will get enough output from the solar to run essential things. You can always monitor it and cut down if necessary. Refrigerators and lights consume very little power, and the main power hog is AC.

    The Mayor of South Miami went off grid for a week with, I believe, two powerwalls

    In a trial run for hurricane season, South Miami’s solar-powered mayor went off the grid for seven days
     
  10. BrettS

    BrettS Member

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    So I finished my test and in the past 24 hours I used 43kWh of power. Again, this was with no car charging, no hot water, and no major appliance use, but I left the AC running as normal and didn’t really attempt to conserve power elsewhere other than what I would normally do (turning off lights as I left the room and such).

    So looking at my solar quote from Tesla it shows that October through February the system will only be generating 500kWh to 1000kWh per month. That works out to an average of 17kWh to 33kWh per day, so I wouldn’t have been able to run my house for the past 24 hours on that output. However, I also wouldn’t have been running the AC nearly as much in the winter either, so I might still be OK. The rest of the year ranges from 1500kWh to a bit over 2000kWh per month, or an average 50kWh to 67kWh per day. During that time I would have had no problem running for the past 24 hours and I would have even had a bit of power to spare.

    The other thing that I realized as I’ve been thinking about this is that sizing a solar system is a little different than sizing a generator. With a generator the big constraint is the amount of continuous power it can supply. For example with a 20kWh generator I could run my oven and take a shower at the same time. With a 10kWh generator I could still run the oven and then take a shower, but I couldn’t do them both at once. With solar, though, only a finite amount of power is produced each day, so I might not be able to run the oven and take a shower in the same day if my power demand for everything else has been high.

    So all that said, I’m feeling reasonably comfortable that I would be able to run off the grid indefinitely with the solar system and power walls as long as I can avoid charging the car and stay somewhat conservative with the large appliances. There might be some times in winter where it could get a little iffy, but that’s not during hurricane season, so I’m less likely to be without power for days at a time during the winter as well.
     
  11. Dan123

    Dan123 Member

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    Brett, you might also look into trying to improve the energy efficiency of your home. Are there any air leaks? Perhaps additional insulation? How old at the AC compressor and the water heater? Perhaps they are old enough where you can justify upgrading with a new energy efficient model. Are all of the lights LED? Impact windows reduce energy loss. As well as painting your roof with an energy reflecting coating.
     
  12. jsimon7777

    jsimon7777 Member

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    With 10kW of solar and 4 powerwalls, we can definitely live off grid in the summer. Not so in the winter. I should test out living off grid, but then I'll lose all that power I could feed into the grid.
     
    • Like x 1
  13. hodad66

    hodad66 Supporting Member

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    Interesting, I am new to the forum & have an appointment for a Tesla Survey on Tuesday.
    I live in central Florida, in a small house & my main reason for the solar / powerwall install
    is post hurricane comfort.
     
    • Like x 1
  14. xhaltsalute

    xhaltsalute New Member

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    Location:
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    We have a 13.95 kWp PV system and 2 Powerwalls in central NC. We also use more electricity than average as we have a hot tub, freezer in the basement, and a large wine refrigerator. I think you will be hard pressed to be able to use AC during a hurricane if you lose grid power because you would deplete the Powerwalls very quickly and the PV system will not be generating all that much electricity because it will be cloudy - it's amazing how much less energy is produced on a cloudy day vs. a clear day... Also, if you are not home to throw the breakers to high-consumption but non-essential circuits (for us, the hot tub), you might be screwed because they'll draw down the stored power in the Powerwalls very quickly.

    One disappointment we have with the Powerwalls is that we thought the "storm watch" feature would charge them from the grid when a warning was issued for our area, so they'd be 100% available if we lost power. That has not happened for us - they charge from the grid only for very specific warnings (like high winds, which we *never* have warnings for). We are most likely to lose power due to thunderstorms or winter storms (trees falling on electric lines). So, that feature seems fairly useless for us and we need to manually adjust the stored power if storms are forecast (so, not what we thought we were signing up for).

    Lastly, if I wanted to charge my EV during an outage (have a Level 2 EVSE at home) and the sun wasn't fully shining, I'd have to very carefully time it so that the draw load wouldn't exceed the max output limits of the Powerwalls. It would have to be almost the only thing drawing power during the time it's charging. Maybe if we had more than 2 Powerwalls, we'd have to think about all of this less, but that's an awful lot of money to spend...
     
    • Helpful x 1
  15. Dan123

    Dan123 Member

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    Can't you control the power used for charging? Tesla has the option to lower the maximum power, other EVs probably too. Or you can charge with 120v outlet, slower charging, but still charging and won't overload the powerwall.
     
    • Informative x 2
    • Like x 1
  16. bob_p

    bob_p Active Member

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    We're looking at the possibility of getting enough panels/PowerWalls to provide around 50% of our average daily power.

    Should we be faced with an extended power outage, we should be able to reduce consumption enough to survive on the panels and PowerWalls for an extended period by eliminating all but essential items (no EV charging, turn off pool pumps, …).
     
  17. gpez

    gpez Member

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    Had this exact conversation as we were putting in our system!

    We put a new NEMA 15-50 plug in the garage as part of our Powerwall + PV install for the likely potential of a future EV. Since we were getting only 1 Powerwall the 15-50 would not be allowed backed up loads panel so we thought that during an emergency where the grid wasn't available 120v charging would be sufficient to get by.
     
  18. Joelgjr

    Joelgjr Member

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    Or you can do what I’m doing and add a ground mount array after your roof is full. We have 6kw on the roof that was preinstalled when we bought the house and am putting an additional 21 kw down the hill. I was surprised to learn that ground mount cost less than roof mount for labor costs
     
    • Like x 1
  19. winfield100

    winfield100 Active Member

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    @BrettS
    im in Cape Coral Sanibel area
    i have 11,655 system, de-rated to 9,906 (0.85) so just under 10kW so i avoid a boat load of extra permits, 4-5x insurance, PITA's n such
    I have about 95% LED lighting, on demand tankless heater.
    I probably make 150 - 175% of my needs (also 19,000 gallon pool so those expenses)
    installed 12/10/2018
    Dec 747 kWh
    January 631 kWh (legal 1/12/2019 so actually more around 1,000)
    Feb 1,300
    Mar 1,600
    April 1,800

    PV Watts v5 says you should generate 21,600kWh/year, >1,400 in december
    PVWatts Calculator

    i have 37 Q.Peak 315w panels, 2 arrays, 1 faces east, 1 faces south, virtually no shading, all have microinverters, 1 microinverter was replaced after 2 weeks,
    im looking at 16,000 - 17,000 or so/year
    I really want the next version that will __island__ w/o batteries and getting battery
    Hurricane Irma took out elec for over 70 hours (security system went down for that long, yay,) Eyewall was about 20 miles east of us

    What does your electric bill say you use? go online, get 1 years data, get consumption data. you are paying for it
     
  20. CWNE88

    CWNE88 Member

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    Location:
    Australia
    You might have a rare but possible problem with 14.19kW of solar, and 2 Powerwalls. The Powerwall has a 5kw power rating (in/out). If you're producing 14.19kW from solar, and you're grid connected, then that excess 4.19kW above the combined Powerwalls rating of 10kw will go out onto the grid. No problem. But, if you're off-grid, and it's a bright sunny day, the Powerwalls will not be able to handle that power and I believe they will shut down in that situation.
    They will not adjust frequency to shut down solar production like they do in high state of charge situation. At least that's what I've been told, and kind of makes sense.
    Ideally you'd match the solar capacity to the Powerwall capacity.
    Also, I did a week-long test of off grid and made a YouTube video on it. (CWNE88 is the channel name)
     

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