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Anyone use Powerwall (with Solar) specifically for TOU savings? San Diego, CA

wamochi

Member
Apr 23, 2014
414
123
San Diego, CA
wondering if anyone out there has installed a powerwall with an existing solar PV system specifically for TOU savings?

ever since SDGE moved peak times from 4-9pm, it's been increasingly difficult to keep my bills close to zero. when peak used to be 12-6pm, we often had near-zero bills for the year because we were getting paid peak rates for generation from 12-6. since generation is low after 4pm, we aren't getting that benefit anymore and my annual true up will be close to $1800 this year. and that is after signing up for the EV-TOU-5 cheap charging rate of 9c after midnight.

my thinking was, what if you installed a powerwall that is charged after midnight (just like your car), and then fully discharged from 4-9pm each day, charging your home during that time until it's used up, while any solar production after 4p is being sent to the grid at peak rates.

one installer I spoke to didn't seem to agree that it would be beneficial financially, but i did the math and it seems like it should pay for itself within 4-5 years if you look at the cost differential between peak and super off peak for the Summer/Winter rates.

just wanted to see if anyone else out there has done this? if you try to add Powerwall on tesla's website, it suggests at least 3 for my home, but i'm not interested in a backup solution, just a way to offset peak rates.
 

Randy Spencer

Supercharger Hunter
Mar 31, 2016
3,737
3,913
Alameda, CA
Why did you specify With Solar in your title when your question is about charging a PW overnight?

I have friends who do TOU with their PW in the sense that they use solar power during peak rates by storing it in their battery.

I know that if you get the federal rebate the agreement won't let you charge the PW from the grid (except as part of Storm Watch) so you would have to pay full price for the Tesla battery.

Charging from the grid and then using it is pretty easy, I think the only way the PW is set up to work is if there is demand in the house it will discharge to meet that demand until it is used up. But I don't know a way to have it just discharge into the grid.

There are grid-tie inverters that will just pump up to 1000 watts into your house that overflows into the grid if there isn't enough demand, but the Tesla gateway has current sensors that know your demand and controls the Power Wall to meet that if possible.
 
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wamochi

Member
Apr 23, 2014
414
123
San Diego, CA
thanks for the reply. the solar part was just to show that i'd be sending back our production from 4-9p at peak rates (instead of using it up in our home).

i guess what i'm trying to say is i don't want to pay for peak rates and wondering if PW helps you accomplish that. i don't want PW to feed the grid artificially from 4-9, just want it to help offset the peak usage in the house during those hours.

say for instance, we use 20kwh of power from 4-9pm with lights, appliances, A/C. if the PW discharges and provides 13kwh of that each day, then i'm only paying for 7kwh of peak rates @ 50c. then at midnight, PW can recharge itself at 9c from the grid?

demand is higher from 4-9p than it was at 12-6p. kids are generally home more during those hours and 4pm is when it starts getting really hot in the house from the built up heat in the attic space. in the winter, if no AC is running, we use far less. but if you can power appliances, lights and refrigerator on PW energy that costs 9c vs paying peak rates, doesn't that seem like a no brainer?

i spoke to one installer and he seemed to think that PW is really just for people who are interested in backup solutions for power outages, and that it makes no sense to install one just to deal with peak usage. so i'm trying to figure out if there are people out there who use it specifically to offset TOU peak rates given how much the cost differential is.
 

ChrisJ

Member
Apr 5, 2016
202
161
SF Bay Area (Napa Valley)
You will not be able to get the PW to charge from the grid during the night-time off-peak unless Tesla themselves set it to Storm Watch. What you're trying to do is not how Tesla designed the system.

In most scenarios your battery will start charging as soon as the sun comes up, regardless of what your house is using (pulls from the grid), and when it's reach enough stat of charge to tied you thru peak-usage (and surpassed your minimum standy) - then it will start selling excess to the grid if your house doesn't need it. Once peak rolls around it will then try and run your house entirely from the battery. Selling all your solar to the grid if possible to maximize savings.
 
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ucmndd

Well-Known Member
Mar 10, 2016
7,289
14,252
California
I am having two powerwalls installed specifically for this purpose, anticipating that I will be moved to a less favorable PG&E plan next year with a later peak period like you describe. The backup functionality will be nice too, but it's a secondary concern.

As others have mentioned, you can't charge the powerwalls from the grid overnight as you're describing - they can only be charged by your solar production. That said, you can still realize a benefit. The powerwalls are programmed with your TOU periods. During peak solar production earlier in the day, your powerwalls store the excess solar energy instead of selling it back to your utility. Then when your peak period hits from 4-9, you draw down the powerwalls instead of using energy from the grid. Same concept, provided your solar array is large enough to produce more than you're using during the day.
 

mblakele

FSD Beta (99)
Mar 7, 2016
1,791
6,033
SF Bay Area
I am having two powerwalls installed specifically for this purpose, anticipating that I will be moved to a less favorable PG&E plan next year with a later peak period like you describe. The backup functionality will be nice too, but it's a secondary concern.

Could you share your ROI calculation for those two powerwalls?
 

animorph

Active Member
Apr 1, 2016
2,161
1,584
Scottsdale, AZ
I think a small PV+PW system would be able to supply most of my peak rate energy usage, with a 3p - 8p peak period. The bummer would be if at anytime during a billing period I had to draw from the grid at peak rates (bad weather or whatever) I'd incur the demand charge. It might take a bit of work and over specification to ensure I never used peak energy.
 

ucmndd

Well-Known Member
Mar 10, 2016
7,289
14,252
California
Could you share your ROI calculation for those two powerwalls?

I must admit, I didn't actually do a particularly detailed one because my cost for the two powerwalls is $0 (I live in a high fire zone and have a confirmed SGIP Equity Resiliency rebate that covers the entire cost of installation). So the ROI in my case was more than good enough to not bother too much with the math. ;)
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
11,464
13,358
Riverside Co. CA
wondering if anyone out there has installed a powerwall with an existing solar PV system specifically for TOU savings?

ever since SDGE moved peak times from 4-9pm, it's been increasingly difficult to keep my bills close to zero. when peak used to be 12-6pm, we often had near-zero bills for the year because we were getting paid peak rates for generation from 12-6. since generation is low after 4pm, we aren't getting that benefit anymore and my annual true up will be close to $1800 this year. and that is after signing up for the EV-TOU-5 cheap charging rate of 9c after midnight.

my thinking was, what if you installed a powerwall that is charged after midnight (just like your car), and then fully discharged from 4-9pm each day, charging your home during that time until it's used up, while any solar production after 4p is being sent to the grid at peak rates.

one installer I spoke to didn't seem to agree that it would be beneficial financially, but i did the math and it seems like it should pay for itself within 4-5 years if you look at the cost differential between peak and super off peak for the Summer/Winter rates.

just wanted to see if anyone else out there has done this? if you try to add Powerwall on tesla's website, it suggests at least 3 for my home, but i'm not interested in a backup solution, just a way to offset peak rates.

People do it to time shift peak rates, but as mentioned a couple of times up thread you need to redo your calculations, because the powerwall will only be charged from your generated solar. It will NOT be charged "overnight from cheap TOU power" as that isnt allowed in the US when one has solar.

"isnt allowed" meaning tesla itself does not allow it to be setup that way. the only time it charges from the grid is during stormwatch which is when tesla triggers that mode due to either weather issues, or possibility of power safety shutoffs.

So, your calculations have to include charging your powerwall with your solar only. If your PV isnt enough to both run your home and charge your powerwalls enough to get through the 4-9 peak time, then it wont work for you... but there are many in california who buy powerwalls for BOTH the "not use peak energy" AND "backup" reasons. I dont know if it pencils out if there is no value to you for the backup portion though.
 

ucmndd

Well-Known Member
Mar 10, 2016
7,289
14,252
California
I dont know if it pencils out if there is no value to you for the backup portion though.
I imagine the ROI horizon is very long. I also strongly doubt it makes sense from a pure economics standpoint. You need to assign some value to the backup functionality or emotional value to sticking it to the power company.
 

HankLloydRight

No Roads
Supporting Member
Jan 18, 2014
13,118
11,400
Connecticut
I had this exact same discussion in another thread which I’ll post here tomorrow.

What you're trying to do is not how Tesla designed the system.

It is possible to charge from the grid and the system is designed this way because it’s allowed and works this way in every other country (and during Storm Watch in the US). In the US Tesla just disables it because the ITC didn’t allow it. But if you didn’t pay for the PWs, or don’t take the ITC for them, Tesla has no reason to bar you from charging from the grid if your electric utility allows it. One person here already got Tesla to enable it with a letter from their utility.

Since both my PWs were refererral awards, I can’t take the ITC, and I plan on getting my utility to agree to allowing off peak battery charging from the grid.
 

ChrisJ

Member
Apr 5, 2016
202
161
SF Bay Area (Napa Valley)
Very interesting. I've not claimed Federal ITC yet, so I'll have to do the math if it makes sense for me to do it. I wonder how difficult it would be to get PG&E to cooperate with a letter.

I had this exact same discussion in another thread which I’ll post here tomorrow.



It is possible to charge from the grid and the system is designed this way because it’s allowed and works this way in every other country (and during Storm Watch in the US). In the US Tesla just disables it because the ITC didn’t allow it. But if you didn’t pay for the PWs, or don’t take the ITC for them, Tesla has no reason to bar you from charging from the grid if your electric utility allows it. One person here already got Tesla to enable it with a letter from their utility.

Since both my PWs were refererral awards, I can’t take the ITC, and I plan on getting my utility to agree to allowing off peak battery charging from the grid.
 

HankLloydRight

No Roads
Supporting Member
Jan 18, 2014
13,118
11,400
Connecticut
Last edited:
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Dave EV

Active Member
Jun 23, 2009
1,921
1,626
San Diego
During peak solar production earlier in the day, your powerwalls store the excess solar energy instead of selling it back to your utility.
Just a clarification - I believe (please correct me if I'm wrong as I don't have a Powerwall - yet) that it's possible to have the Powerwalls charge using ALL your solar energy and not just excess solar energy.

Time Based Control is designed for this exact scenario where you want to maximize ROI. Reading the docs, though, it does mention charging off-peak using "excess solar", but it seems to me that it would be you'd want to use all solar to charge the Powerwall during off-peak and peak hours.

Also, one additional thing to keep in mind is that at least in winter months in SDG&E, the spread between off-peak and peak rates are minimal and not worth shifting, cost wise. It is worth shifting from Super Off-Peak on the weekends and holidays to Off or On-Peak and don't forget that March/April from 10am-2pm are Super Off-Peak, too.

EV-TOU-2 Summer / Winter
On-Peak $0.552 / $0.305
Off-Peak $0.338 / $0.298
Super Off-Peak $0.193 / $0.194


EV-TOU-5 Summer / Winter
On-Peak $0.503 / $0.257
Off-Peak $0.289 / $0.249
Super Off-Peak $0.086 / $0.086
 

miimura

Well-Known Member
Aug 21, 2013
6,695
6,538
Los Altos, CA
Just a clarification - I believe (please correct me if I'm wrong as I don't have a Powerwall - yet) that it's possible to have the Powerwalls charge using ALL your solar energy and not just excess solar energy.

Time Based Control is designed for this exact scenario where you want to maximize ROI. Reading the docs, though, it does mention charging off-peak using "excess solar", but it seems to me that it would be you'd want to use all solar to charge the Powerwall during off-peak and peak hours.
The Powerwall logic is adaptive. If it thinks you will have enough solar generation to fill the batteries before Peak starts with only Surplus Solar, that is what it will do. If it is running behind and won't make it, it will switch dynamically to charge from All Solar. The reverse is also common - start in the morning charging from All Solar and then when it becomes evident that it can finish with only Surplus Solar, it will make that change.
 
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