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How long does a Powerwall support a home during a power outage?

jboy210

Supporting Member
Supporting Member
Dec 2, 2016
5,509
3,497
Northern California
Hi,

All of us in Northern California are aware that power is no longer something we can take for granted. If a home has a Powerwall how long can it provide power for basic support like minimal lighting, fridge, fans for furnace?

Thanks.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
10,854
12,581
Riverside Co. CA
What's the size of house if you don't mind me asking and do you charge you're car also?

You wont be charging your car (which has a 60-75 or even 100 kW battery) with your powerwalls which have 13.5kW each, especially in an outage situation. As @Vines said in another thread, its like "using a AAA battery to charge a D battery" (I believe thats what he said).

The size of the house doesnt really matter, its the size of the electrical loads. Normally a large house = more load but small houses can have big loads too, depending on what they are running.

Check your electric bill for last month, and see what your average daily usage is. then figure you might cut some of that out in a power outage situation (or maybe you dont want to) and size accordingly.
 

patrick40363

Active Member
Mar 25, 2014
1,178
617
Cali
Hi,

All of us in Northern California are aware that power is no longer something we can take for granted. If a home has a Powerwall how long can it provide power for basic support like minimal lighting, fridge, fans for furnace?

Thanks.
We were out for 96 hours and we have 2 powerwalls. We have solar so we never lost power even though all my neighbors were out. I suggest you add solar if you are considering powerwalls.
 

jkoya

NA2 NSX
Nov 21, 2018
3,626
1,569
Northern CA
We were out for 96 hours and we have 2 powerwalls. We have solar so we never lost power even though all my neighbors were out. I suggest you add solar if you are considering powerwalls.

Hi - What size is your solar system and how long does it take to recharge the two Powerwalls ?
 

woferry

Member
Mar 4, 2019
406
478
San Jose, CA
Each Powerwall 2 will charge at a maximum rate of 5kW. So it's really a matter of how much your solar produces, subtracting how much the house is consuming. The rest will go into the PW to charge it up, up to the maximum charge rate. As the PW nears 100% its draw starts to ramp-down (a requirement in Lithium battery charging), this is why the PWs start to throttle/disable solar when the grid is down and the battery is almost full, because they can no longer draw the full 5kW/PW and there might end up being too much current sourced by the solar inverters and nowhere for it to go.

So if your solar is producing 1kW but the house is drawing 1kW, you won't be charging the PWs at all. If the solar is producing 10kW and the house is drawing 1kW, then there's 9kW available that 2 or more PW's will easily soak-up until they get close to fully-charged (at which point the PWs will shut down the solar and the the house will draw from the PWs until they're ready to start drawing full-power from the solar again). Each PW stores around 13kWh[*], so if the full 5kW is available it should recharge in around 3 hours (and if the PW wasn't close to empty it will charge in less time), but in the morning while the sun is still rising you probably won't have the full power available, so it will charge more slowly. And if your solar doesn't produce enough over what the house draws then it may not reach full-charge over the course of a day.

[*] Because of efficiency loss in charging/discharging, it probably takes more like 15kW/PW to go from totally-empty to fully-charged.
 

bob_p

Active Member
Apr 5, 2012
3,728
2,922
We just went through planning for our pending solar panel/PowerWall configuration.

The monthly electricity bills make it easy to estimate the average daily usage (KWh/day) - but those are only averages, with electric usage likely fluctuating from day-to-day.

We also have access to smart meter data - so I pulled the usage over the past12 months - in 15 minute increments. Using that data, it was easy to look at the periods of highest electric usage. For those periods, I then identified what was running at that time. In our case, during those periods we had 2 EVs charging, pool pumps, ovens, full-size refrigerators, air conditioning - as the major loads. By removing the EV charging and pool pumps, the remaining load should be within the power that could be supplied by the PowerWalls we're planning to install.

Based on actual energy usage, including an estimate of the largest electricity loads (for us - charging our S & X, and 3 pool pumps), we're confident we'll be able to run off grid for days (after a hurricane), with the possibility we might have to manually adjust usage for a few of our remaining devices (adjusting HVAC systems, not running ovens, …).
 

MorrisonHiker

Well-Known Member
Mar 8, 2015
10,251
9,991
Colorado
We just finished an extended grid outage test with our solar and Powerwalls earlier this week. Even with 8.5" of snow one day and charging multiple Teslas throughout the week, we were able to go over 8 days off-grid without any issues. This was with 4 Powerwalls, 16.5 kW of solar and the usual sunny Colorado days.

After passing 200 hours off-grid, we finally turned grid power back on during an unusually cold, three day October snowstorm. We received three Storm Watch notifications while we were off the grid. According to our neighbors, power actually went out at least twice during the week that we were off the grid.
 
Last edited:

patrick40363

Active Member
Mar 25, 2014
1,178
617
Cali
Hi - What size is your solar system and how long does it take to recharge the two Powerwalls ?
5KW system with 2 sets of panels. One is on the south side of the house the other west. Took all day to get back up to 85% after starting at approx 32% in the morning. It depends on the sun but even with the size of my system it was no problem. When power came back up the batteries were topped off from the grid. Very happy I have the system.
 

jboy210

Supporting Member
Supporting Member
Dec 2, 2016
5,509
3,497
Northern California
We just finished an extended grid outage test with our solar and Powerwalls earlier this week. Even with 8.5" of snow one day and charging multiple Teslas throughout the week, we were able to go over 8 days off-grid without any issues. This was with 4 Powerwalls, 16.5 kW of solar and the usual sunny Colorado days.

After passing 200 hours off-grid, we finally turned grid power back on during an unusually cold, three day October snowstorm. We received three Storm Watch notifications while we were off the grid. According to our neighbors, power actually went out at least twice during the week that we were off the grid.

Amazing!! You must be very popular with neighbors when the power goes out.
 

jboy210

Supporting Member
Supporting Member
Dec 2, 2016
5,509
3,497
Northern California
We were out for 96 hours and we have 2 powerwalls. We have solar so we never lost power even though all my neighbors were out. I suggest you add solar if you are considering powerwalls.

Definitely adding solar. This is all part of a roof replacement + solar upgrade. Still trying to decide whether to do the Tesla roof V3, but leaning that way.
 

MorrisonHiker

Well-Known Member
Mar 8, 2015
10,251
9,991
Colorado
Thanks for the replies. Looks like people are getting some great backup support with their powerwalls.

I ran across this backup time estimator, How long will Powerwall last in an outage? | Tesla . Looks cool. But, could anyone comment on how accurate it is?
It seems overly optimistic to me and not complete enough since it doesn't allow you to indicate multiple cars, multiple refrigerators, etc. I think a more useful tool would be for you to just input your hourly or daily kWh usage. Most people could easily determine their daily kWh usage and then use that to figure out how long Powerwalls would last them. Also, they should allow you to indicate the size of your solar system and expected weather. While we might only be able to go two or three days with our Powerwalls, we could go indefinitely if there's enough sunlight.

Amazing!! You must be very popular with neighbors when the power goes out.
One of my neighbors is amazed and lets me know whenever his power goes out. I always have to tell him that we didn't notice any outage. ;) Whenever I brag to my other neighbor about generating our own electricity, she just says that she'll sit by her gas stove. She doesn't understand that there's a difference between gas and electricity because our utility normally provides both. I've explained to her a dozen times that our solar electricity has nothing to do with natural gas but she doesn't get it. o_O

Definitely adding solar. This is all part of a roof replacement + solar upgrade. Still trying to decide whether to do the Tesla roof V3, but leaning that way.
We considered the solar roof last year but at the time, it cost at least twice as much as regular solar panels. Also, there was a long wait. If the cost and wait times have been reduced over the past year, then I'd say go for it!
 

jkoya

NA2 NSX
Nov 21, 2018
3,626
1,569
Northern CA
5KW system with 2 sets of panels. One is on the south side of the house the other west. Took all day to get back up to 85% after starting at approx 32% in the morning. It depends on the sun but even with the size of my system it was no problem. When power came back up the batteries were topped off from the grid. Very happy I have the system.

Thanks for the info - much appreciated !!!
 

Kant.Ing

Member
Apr 21, 2016
700
469
Pacific Coast, US
We've never really noticed any outage. Garage door operates as normal. We also added a couple small UPSs over computer devices and internet infrastructures to remove the last bit of data/comms errors. With solar to replenish the battery on a daily basis, the utility is our backup.
 
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jboy210

Supporting Member
Supporting Member
Dec 2, 2016
5,509
3,497
Northern California
We've never really noticed any outage. Garage door operates as normal. We also added a couple small UPSs over computer devices and internet infrastructures to remove the last bit of data/comms errors. With solar to replenish the battery on a daily basis, the utility is our backup.

Just curious why you need the USPs for the computer and internet. What sort of data/comm errors did you see?

We work from home and do software development and training and one reason for buying the PowerWall would be to ensure we did not have power issues impacting our work.
 

gpez

Member
Apr 25, 2019
735
602
USA
Just curious why you need the USPs for the computer and internet. What sort of data/comm errors did you see?

We work from home and do software development and training and one reason for buying the PowerWall would be to ensure we did not have power issues impacting our work.

@jboy210 the Powerwall is fast to switch over, much faster than a generator that needs spin up time, but it is not the single-digit millisecond response times that even your cheap your under-the-desk UPS provides. In ideal conditions the Powerwall can take at least 100ms-200ms to switch over, I've experienced single digit seconds delay during high PV production. Even a 100ms blip can cause sensitive electronics, like your PC, to have issues. I have all of my home electronics (PC, TVs, servers, Xbox, etc) behind a UPS for this reason. Probably overkill but I'd rather have happy electronics ;)
 
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jboy210

Supporting Member
Supporting Member
Dec 2, 2016
5,509
3,497
Northern California
@jboy210 the Powerwall is fast to switch over, much faster than a generator that needs spin up time, but it is not the single-digit millisecond response times that even your cheap your under-the-desk UPS provides. In ideal conditions the Powerwall can take at least 100ms-200ms to switch over, I've experienced single digit seconds delay during high PV production. Even a 100ms blip can cause sensitive electronics, like your PC, to have issues. I have all of my home electronics (PC, TVs, servers, Xbox, etc) behind a UPS for this reason. Probably overkill but I'd rather have happy electronics ;)

Thanks. Makes sense. We have UPSes for most of our gear already. And have not suffered system failures when the PG&E provided power went out. I just wanted to be sure there was not something special about the switchover to power provided from the PowerWall.

However, good reminder to get a UPS for the TV, Xbox and other "critical" devices.
 

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