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2 vs 3 Powerwalls

Drlev

Member
Oct 22, 2017
173
159
Harrisburg, PA
My 16.3 kw Solar + 2 Powerwall installation is pending and almost done with permitting. With Tesla’s more favorable financing now, I’m considering adding a 3rd Powerwall. I know my home’s AC compressor is compatible with 2 Powerwalls. Is there any clear reason for me to add a 3rd Powerwall to the install? Our daily electricity usage varies between 60-90 kwh this summer depending on our AC use and how much my car needs to charge. We have a gas furnace so electricity usage will be lower in the winter, though we also have an electric dryer.

Also, our utility will be starting some fairly high time-of-use rates, especially between 4-8 pm, some time later this year.

Happy to get any input people can give. Thanks!
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
10,444
11,793
Riverside Co. CA
Please explain for a technical mook like me.
Each powerwall can only take in 5kW of power at once. That means your system as designed can only take in 10kW of power. Not a problem when you have grid, because excess goes to grid. In a power outage, if your solar can produce more than your powerwalls can take in, the power has no where to go, and the powerwalls can turn off "the system" (your power) to protect themselves from too much power.

If your 16kW system is designed such that it will never have peek production above 10kW (due to multiple directions of panels on roof, etc) then probably not an issue, but its likely your system will generate max output of more than 10kW at some point.

I personally wouldnt have a system that size with 2 powerwalls, because teslas recommendation (as @jrweiss98020 said) is a max of roughly 7kW per PW. Those are just guidelines and each install / home is different, but 16kW calls for 3 powerwalls, to me (or none).
 

holeydonut

Supporting Member
Supporting Member
Jun 27, 2020
2,123
1,505
East Bay NorCal
14.kW > 10kW, so, in relation to what I said, you are confirming that you should have 3 powerwalls, not 2.



I guess it also helps to know which way his panels are orientated. Since it could be possible the peak AC output of his inverters is capped at 9.6 kW or something.

But I agree that in general 3 > 2 ... and since I very much dislike power company monopolies, more powerwall + solar is more better.
 

warjcowski

Member
Apr 22, 2021
12
6
Bay Area, CA
Your daily power usage would also be a good reason for a 3rd PW. In an off-grid scenario, 2 powerwalls might not get you through the night. And as everyone has said, it might be difficult for the inverter to keep them changed if excess solar can't be exported so it'll shut solar off.
 
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arnolddeleon

Supporting Member
Supporting Member
Jul 21, 2012
895
955
SF Bay Area
Each powerwall can only take in 5kW of power at once. That means your system as designed can only take in 10kW of power. Not a problem when you have grid, because excess goes to grid. In a power outage, if your solar can produce more than your powerwalls can take in, the power has no where to go, and the powerwalls can turn off "the system" (your power) to protect themselves from too much power.

If your 16kW system is designed such that it will never have peek production above 10kW (due to multiple directions of panels on roof, etc) then probably not an issue, but its likely your system will generate max output of more than 10kW at some point.

I personally wouldnt have a system that size with 2 powerwalls, because teslas recommendation (as @jrweiss98020 said) is a max of roughly 7kW per PW. Those are just guidelines and each install / home is different, but 16kW calls for 3 powerwalls, to me (or none).
The important information that @Drlev didn't provide is what their goals are with the Powerwalls and solar.

In the grid out case, it is quite likely the house has loads so even though the Powerwalls can only absorb 10 kW the rest of the generation probably has a place to go. This is also a brand new PV install. This also means the inverters will be able to modulate their output based on the frequency shifting of the Powerwalls to match what they can absorb. So they likely won't encounter the extremely unlikely situation where the batteries are depleted and the system can't charge.

In short, I think the primary question is how much energy capacity do they need?

If their goal is just "peak shaving" the 4-8 pm peak then two Powerwalls might be enough. This depends on how much they use during the peak hours (noting that production will help a little bit). It will also depend on where they set their reserve. If their goal is maximum savings the dollars might be better spent on more PV (if they are not roof limited) or sticking with the original configuration. The specific utility net metering (if available) tariffs also plays a big role. In simple terms can they offset peak consumptions with off peak generation? Are there limits on excess energy sent to the grid? Excess credits?

On the other hand if they want to be "self powered" then the answer is pretty obvious their going to want the 3rd Powerwall. This doesn't give the best return on investment if you are just talking about dollars in your pocket. But it can be very satisfying. I'm now able to run in this mode after adding a 3rd Powerwall and I'm loving it. If you are @holeydonut this probably approaches infinite utility.

TL;DR; I would buy the 3rd Powerwall if I could afford it
 

Electrph

Member
Aug 29, 2019
456
308
Central California
Great answers here already … I’m only chiming in cause I have 2 pw‘s and considered a 3rd at time of install … I have smaller solar so I cannot exceed the 10kw of my 2 pw’s during outage as you possibly can … BUT
if you are even considering and can afford a 3rd get it … I look at that extra space on my garage wall regularly knowing how well a 3rd would have fit there … now tesla will not sell powerwall w/o solar and even if they did would have been much more economical to get the 3rd back then than a la cart now
 
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reddy

Member
Jan 26, 2013
984
1,441
Amarillo, TX
As far as two PWs shutting down your inverter when off grid, that would only happen if you were not using much electricity, at a peak time, so that your production minus home usage at that moment was over 10kw.

That being said, I have an 18 kWh solar with max inverter power of 15.2 kw, and I have 4 Powerwalls. I need that many in order to maintain my overnight electric consumption. Even with 4, I end up giving away free electricity to my local utility. It’s been great avoiding the highest TOU rates for us from 1p to 7p.

I was off grid for 30 hours once after a major windstorm knocked out most of the local power lines, and when the PWs got full , it shut down my inverters until the battery level dropped a little. Here you can see the inverters turning off and on when the PWs get full.
F64DCBC5-5BD9-4BC0-A661-156E21F6CC56.jpeg
 
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Southpasfan

Member
Jun 2, 2019
462
587
Pasadena
Get the third. It’s not even a close question if you can afford it, which you can or you wouldn’t be asking. For that matter, get 4. I have three and four would be nice. Basically If, as you said, you use 90kwh during hot months, at least 30 of that 90 will be after the sun goes down. Two can’t handle 30 with no reserve- with a 20 percent reserve two PWs are close to 20 available. With three you can get through most nights
 

roblab

Active Member
Jul 15, 2008
3,664
2,734
Angwin (Napa Valley) CA
My 16.3 kw Solar + 2 Powerwall installation is pending and almost done with permitting. With Tesla’s more favorable financing now, I’m considering adding a 3rd Powerwall. I know my home’s AC compressor is compatible with 2 Powerwalls. Is there any clear reason for me to add a 3rd Powerwall to the install? Our daily electricity usage varies between 60-90 kwh this summer depending on our AC use and how much my car needs to charge. We have a gas furnace so electricity usage will be lower in the winter, though we also have an electric dryer.

Also, our utility will be starting some fairly high time-of-use rates, especially between 4-8 pm, some time later this year.

Happy to get any input people can give. Thanks!
Depends on why you're doing it. My wife thinks our 3 power walls are overkill with my 11 kW solar, but I figure I might as well pack away all the solar power I can. With PG&E, we never know if, when, or how long our power will be out. I sell extra power to the grid, so all is not lost, but the PWs plus installation are expensive, a cost one would likely not recoup in selling to PG&E. It's got to be for your own satisfaction in having power when all your neighbors' houses are dark.
 

Dukeybootie

Member
Nov 12, 2015
224
139
United States
Another vote from me to get the third. We added our third 10 months after the first two were installed and I am so glad we did. For the flexibility, peace of mind, increased storage capacity, longer potential off-grid or emergency backup runs, and maximum freedom from the negative effects of SCE plan pricing changes, it has been the right move for us 100%.

Conversely, I doubt you will find anyone who regrets adding that last PW to their system (budget permitting).
 
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Sporterman

New Member
Jul 18, 2021
2
3
Atascadero, CA
My 16.3 kw Solar + 2 Powerwall installation is pending and almost done with permitting. With Tesla’s more favorable financing now, I’m considering adding a 3rd Powerwall. I know my home’s AC compressor is compatible with 2 Powerwalls. Is there any clear reason for me to add a 3rd Powerwall to the install? Our daily electricity usage varies between 60-90 kwh this summer depending on our AC use and how much my car needs to charge. We have a gas furnace so electricity usage will be lower in the winter, though we also have an electric dryer.

Also, our utility will be starting some fairly high time-of-use rates, especially between 4-8 pm, some time later this year.

Happy to get any input people can give. Thanks!
We have a 16K system with 4 powerwalls with usage of 60 to 100kwh per day. To beat the time of use charges I made sure my powerwalls could support the house usage during the peak usage time window. You can use your electric meter to gauge if 2 is enough. The other factor I would consider is how reliable your utility is and how long your typical outages are. Hope that helps!
 
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navydoc001

Member
Jul 8, 2021
20
15
Loganville, PA
well said .. I have read of many and count myself as one who had ragrets not getting that extra contemplated pw. ... rarely hear anyone say .. "we just dont need that one extra powerwall why did we get that?"
In that boat right now. Our system was installed about a month ago. The roof is not yet done, but the system works. 11.46 kW with 2 Powerwalls. We were hit with the price increase, but Tesla is reimbursing us for the second Powerwall. Now that the system has been working, we should have added a third Powerwall. I have talked to my advisor and due to permitting and solar sizing, we will have to wait for PTO to do a system add-on. Reading about that has me concerned.
 

h2ofun

Active Member
Aug 11, 2020
2,937
564
auburn, ca
In that boat right now. Our system was installed about a month ago. The roof is not yet done, but the system works. 11.46 kW with 2 Powerwalls. We were hit with the price increase, but Tesla is reimbursing us for the second Powerwall. Now that the system has been working, we should have added a third Powerwall. I have talked to my advisor and due to permitting and solar sizing, we will have to wait for PTO to do a system add-on. Reading about that has me concerned.
Yep, so many try to save a penny, and later, .. I did the same, could have had 7 PW's but said but I could save 20 grand. I will regret this forever.
 

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