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How many Powerwalls should I get?

ElectricRed

Member
Nov 11, 2013
43
26
Bay Area
I finally decided to get a Powerwall after the recent power outage. We had a generator for the fridge and freezer so everything turned out ok, but with a newborn and a 3-year old, the stress of losing power was too great. Tesla gave me a quote for 2 Powerwalls at $18,305 and one at $11,704 before ITC, and I intend to sign the contract ASAP so I can still claim the 30% ITC.

We have a 3.25kW PV system on EV-A rate and generates on average 14kW per day. About 60 days out of a year it generates more than 20kW. EVs are on a separate EV-B meter.

Currently we're at -646kW with PG&E, and my estimate is at TrueUp we'll be at about -300kW. We get our power from Peninsula Clean Power and they cash me out every year at the actual rate + 1cent bonus per kW.

If we lose power, in a worse case scenario we'll be using 9kW a day if we don't conserve at all. If I factor in 50% growth for the future, it will be 13kW. If we aggressively conserve we can bring it down to 6kW/9kW.

My questions:

1. Since we're already producing more than we use, is one Powerwall sufficient in our case?

2. If I set the Powerwall to "Load Shift" mode, I assume the Powerwall will charge using off-peak/partial-peak rates and discharge during peak-hour rates, so the amount of power sent to the grid should be similar, but more of them will be at the peak-hour rate. Is this correct?

3. If I get two Powerwalls, will the system be smart enough to charge the Powerwalls over several days?

4. Any reason I should get two Powerwalls in my scenario?

Thanks
 

ajdelange

Active Member
Dec 10, 2018
1,077
540
Virginia/Quebec
First, get straight on the definitions of kW and kWh. Then figure out your average and peak demands. Size the powerwalls by the average demand and how long you want to be covered in the case of an outage. If your average demand is 1 kW that's 24 kWh/day and a single (13 kWh?) powerwall will carry you about 13 hrs. If that's enough, then one powerwall will do. Keep in mind that you can, in the case of outage, probably shed some loads and thus run longer.
 
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Jun 22, 2017
527
337
Bay Area, California
1. One PW works. Just backup the house panel only. This will cover refrigeration easily. Besides, if you run out of power, solar will charge the next day and charge enough to cool the fridge/freezer down. Repeat.
2. The mode is called Advanced-->Time Based Control. You have "Balanced" and "Cost Savings." The key difference is Balanced discharged the battery while you are asleep when rates are low. I like "Cost Savings."
3. Yes, it can charge over several days. In fact, you can be sly about this. Discharge M-Th on "Cost Savings" and Fri-Sun on "Balanced". Too bad switching modes are manual. This lets you drain the batteries over the weekdays and charge back over the weekends. Do this to save an extra $1 per week. This is just for fun.
4. Two PW will allow you to be less concerned with figuring out how to not backup your oven, or anything using more than 30A of power (e.g. oven, electric dryer, etc.). You have two EVs, eh? I kept a spare EV charge station backed up just in case. The main EVCS is standalone.
 

ElectricRed

Member
Nov 11, 2013
43
26
Bay Area
Sorry, all the numbers should be kWh.

Yes we have two EVs. The second one is actually a plugin Hybrid, so we can still use gas. We have an extra 120v EVSE we can use in case of emergency too.
 

gpez

Member
Apr 25, 2019
689
567
USA
In case more data is helpful: I have an 8.6kW PV system in the Seattle area that produces just over 40kWh during our long sunny summer days and 0 during our cloudy and short winter days. Our utility does not do ToU pricing so the Powerwall is strictly for backup during outages. Similar to you we have two young kids and my goal was to be able to keep the house lit and warm while keeping the food cold. We decided on 1 Powerwall and not back up the A/C, oven, dryer, or 240v EV charger because those weren't critical in an outage for us (we could always charge the EV on 120v if absolutely needed like you already noted).

Before our EV (Chevy Bolt) purchase our average home use was 24kWh/day, our lights are almost all LED and the appliances are new and fairly high efficiency. Like you I figured in an outage we could reduce that consumption by at least half to 12kWh/day, probably more if we were just looking at fridge+freezer and minimal lights. That means the Powerwall will last us a day without any recharging from the sun. For short (hours long) winter weather related outages that's plenty and since the furnace, hot water, and stove are all natural gas we could make do. For something more extreme, like a moderate or sever earthquake, we'd be looking at days or maybe weeks long outages for both electricity and natural gas, making a standby generator useless. During the non winter months our solar would be entirely sufficient. Long outages in the winter would be a challenge so I invested in a quality 1200w pure sine wave inverter for the Volt. By my math if it is fully charged and fueled it could provide 112kWh which after conversion losses is still over a week at 12kWh/day which also doesn't include any refills on the gas or charging off of excess solar. I investigated what it would take to "grid tie" the Volt to the house but based on this thread it just made more economical sense to get a regular inverter and a large extension cord should there be some sort of extreme event.

My suggestion would be to run the numbers on your different outage scenarios and see what makes sense!
 
Jun 22, 2017
527
337
Bay Area, California
Sorry, all the numbers should be kWh.

Yes we have two EVs. The second one is actually a plugin Hybrid, so we can still use gas. We have an extra 120v EVSE we can use in case of emergency too.
Excellent! In light of the recent PG&E outage, I recommend for just (1) one PW. At least with the bare minimum your house will match public perception that solar owners produce their own power, and therefore have power during outages. (2) With a 3.25kW solar system, a single PW will receive greater utilization for the entire year compared to a second PW which may not be cycled as often (<-- diminishing return). If you are interested into making thru a natural disaster (earthquake) in the winter, then 2 PW would be appropriate.
 

GenSao

Member
Aug 3, 2017
550
938
Pleasant Hill, CA
One Powerwall is all you need for critical loads.

Wht that said, do consider the second Powerwall as 2X capacity (27 kWh) and 2X power (10 kW) will only cost 56% more. Most, if not all, appliances and circuits can be backed up. AC may be included/excluded dependent on size. We went with two Powerwalls to include the AC and provide more flexibility and headroom.

As you have a generator (depending on setup), it may be possible to integrate it into your electrical system. Per the Tesla Powerwall FAQ:

Powerwall & Generators
Powerwall can be added to a system with a backup generator connected with an external Automatic Transfer Switch (ATS) or Manual Transfer Switch (MTS).

Powerwall is installed between the utility meter and the transfer switch and can charge from solar while the grid is operational. However, the Powerwall and generator are not directly integrated which means Powerwall does not charge from the generator.

In an outage, Powerwall responds immediately and provides backup power before the generator can detect the outage. The generator is turned on only when the Powerwall has low charge, or if loads exceed Powerwall's maximum output.

When grid power returns, the generator will turn off and Powerwall can again charge from solar. If Powerwall is installed with an MTS, manual operation of the switch is required to power the home with the generator.
 

ewoodrick

Well-Known Member
Apr 13, 2018
5,285
3,733
Buford, GA
If you want to protect yourself against long term power outages, Powerwalls are probably not your best option. Air Conditioners will tend to drain one quickly.
If you don't let the AC run, your refrigerators will also require more power, since the room is warmer.

A Powerwall really isn't much more than a online UPS. They are great for short duration and storage, but obnoxiously expensive for long term storage.
 

nwdiver

Well-Known Member
Feb 17, 2013
8,117
10,586
United States
If you want to protect yourself against long term power outages, Powerwalls are probably not your best option.

.... better than any other available option... which from what I understand of the way words work would make it the ‘best’... as long as it’s combined with solar which in the OP case it is.
 

miimura

Well-Known Member
Aug 21, 2013
6,365
6,071
Los Altos, CA
If you want to protect yourself against long term power outages, Powerwalls are probably not your best option. Air Conditioners will tend to drain one quickly.
If you don't let the AC run, your refrigerators will also require more power, since the room is warmer.

A Powerwall really isn't much more than a online UPS. They are great for short duration and storage, but obnoxiously expensive for long term storage.
If you properly size your solar system to offset your annual energy usage, you should have enough generation to run the A/C unless you have hot muggy days with dense cloud cover or require air conditioning through the night. The Powerwall batteries are mostly a buffer for your solar generation so that you can continue running the house through the night.
 
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ewoodrick

Well-Known Member
Apr 13, 2018
5,285
3,733
Buford, GA
If you properly size your solar system to offset your annual energy usage, you should have enough generation to run the A/C unless you have hot muggy days with dense cloud cover or require air conditioning through the night. The Powerwall batteries are mostly a buffer for your solar generation so that you can continue running the house through the night.
That's definitely not what I said. I said that it may be expensive. And in this case, I may not want to rely on sunshine with smoke clouds floating by.
 

miimura

Well-Known Member
Aug 21, 2013
6,365
6,071
Los Altos, CA
That's definitely not what I said. I said that it may be expensive. And in this case, I may not want to rely on sunshine with smoke clouds floating by.
You were talking about long term power outages. I maintain that, all things considered, solar plus Powerwall is the best solution for long term power outages. Generators are noisy and require fuel. During a prolonged outage, you can decide what is important and adjust your usage to stay within the energy you have available.

Any way you look at the problem, it is expensive to operate a complete household when the grid is out. If you only need a refrigerator and a laptop computer, then it's a lot simpler and cheaper.
 

jkoya

NA2 NSX
Nov 21, 2018
3,626
1,556
Northern CA

ElectricRed

Member
Nov 11, 2013
43
26
Bay Area
I pulled the trigger on two Powerwalls. $18k before the ITC. Two is a bit overkill for now, but with the ITC credit, the second one is only $4,600. I had previously gotten a quote for a Powerwall in June, so they already have a design on file, which lets me jump ahead of the install queue.

I saw in another thread somewhere that Tesla wants $3k labor + $500 permit to ADD a second Powerwall later. This plus the potential new fire code changes made me decide to get two now.

BTW, the 10% Elon discount only applies to Solar+PW. Also the referral program doesn't apply to Powerwall-only purchase.

Thanks everyone for the help!
 
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astrorob

stealth performance M3
Aug 27, 2014
474
110
oakland, ca
I pulled the trigger on two Powerwalls. $18k before the ITC. Two is a bit overkill for now, but with the ITC credit, the second one is only $4,600. I had previously gotten a quote for a Powerwall in June, so they already have a design on file, which lets me jump ahead of the install queue.

this is the installed price from tesla themselves? i've got 3rd parties quoting between $22 and $24k for 2 powerwalls right now, and $13k installed for one. might make sense to just risk only being able to install one if tesla's installed costs are so much lower.
 

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