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To Powerwall or Not to Powerwall?

wjgjr

Active Member
May 11, 2020
1,349
1,061
Silver Spring, MD
I see. I seem to be getting conflicting information from Tesla on this:

Powerwall Time-Based Control | Tesla Support says in balance mode (available with Solar only) that it does perform off peak charging. This seems to imply load shifting is possible. Maybe I am reading this wrong ?

Yet this page Combining Systems | Powerwall Support explicitly says you cannot charge from grid when using solar, supporting your assertion.

The balanced mode states that "Powerwall will charge from excess solar during off-peak and shoulder (if required)." This is consistent with what @getakey stated that you "cannot charge the PW at night during low rate. It can only charge from Solar. You can use PW to power house during Peak from charging during morning from solar."

The issue is that the peak/shoulder time can run into or past sunset, meaning there is no evening off-peak time for PW charging. So, you may find that your PWs are partly (or mostly, depending on the system and your settings) drained from the end of peak until the morning, when your PWs can charge during the off-peak period from sunrise until you hit peak or shoulder.

In your specific case with 4-9 peak, that seems a likely scenario. Every morning, your PWs will charge so that by 4:00 they are full. Then, from 4:00 to 9:00 they will discharge during your peak. They will remain at that lower level until sunrise the next day, when they can begin re-charging from solar. What Tesla claims they do is figure out how much you use during peak and how much you will generate in order to optimize PW use, consistent with your selection of balanced or cost savings.
 
  • Informative
Reactions: heems

Southpasfan

Member
Jun 2, 2019
467
610
Pasadena
Here is the equation.

1. Figure out how much per kwh you are paying. That's not quite as simple as it sounds, since there are so many different ways utilities charge. In my case its $.25 avg.

2. Next up, figure out what the size of system you are considering will produce per year. This is not easy either, since everyone lives in different places and has different roofs, but PVwatts estimates are good. In my case the XL system I have on order will do 25k kwh per year.

3. You have to assume something as to time period, I have assumed a 20 year window. Over that period of time the system will produce 500,000 kwh. If I were to pay for those kwh, at 25 cents per kwh, it will cost at least $125,000 over the 20 years.

4. If a person had 1 to 1 net metering (I don't) as the cost of solar is well under, WELL under $125k, simply click "order."

5. In my case, solar and powerwalls are BOTH under $125K. However, this is where you might pause. The Powerwalls cost extra. Its still cheaper than buying the power, but Powerwalls do enable you do use all of the power your panels generate and are a hedge against Time of Use Billing, where the utility buys from you at a low rate and sells back to you at night at a higher rate. If the utility does that, the reality is you would not (without powerwalls) get the full benefit of the Kwh your system produces.

6. Finally, you have to put a value on actual independence from the utility, and actual back up, like our posters from Florida.
 

Vines

Active Member
Jul 20, 2018
2,170
2,654
Silicon Valley, CA
You cannot charge the PW at night during low rate. It can only charge from Solar. You can use PW to power house during Peak from charging during morning from solar

If the Powerwalls are connected as a backup source without Solar, then you can charge from the grid. However to tax credit or SGIP would apply.
 

jboy210

Supporting Member
Supporting Member
Dec 2, 2016
5,682
3,685
Northern California
Here is the equation.

1. Figure out how much per kwh you are paying. That's not quite as simple as it sounds, since there are so many different ways utilities charge. In my case its $.25 avg.

2. Next up, figure out what the size of system you are considering will produce per year. This is not easy either, since everyone lives in different places and has different roofs, but PVwatts estimates are good. In my case the XL system I have on order will do 25k kwh per year.

3. You have to assume something as to time period, I have assumed a 20 year window. Over that period of time the system will produce 500,000 kwh. If I were to pay for those kwh, at 25 cents per kwh, it will cost at least $125,000 over the 20 years.

4. If a person had 1 to 1 net metering (I don't) as the cost of solar is well under, WELL under $125k, simply click "order."

5. In my case, solar and powerwalls are BOTH under $125K. However, this is where you might pause. The Powerwalls cost extra. Its still cheaper than buying the power, but Powerwalls do enable you do use all of the power your panels generate and are a hedge against Time of Use Billing, where the utility buys from you at a low rate and sells back to you at night at a higher rate. If the utility does that, the reality is you would not (without powerwalls) get the full benefit of the Kwh your system produces.

6. Finally, you have to put a value on actual independence from the utility, and actual back up, like our posters from Florida.

One minor change. You can add the 26% (2020), or 22% (2021), tax credit into the mix and go even higher on price and still come out ahead.
 

getakey

Active Member
Jan 28, 2020
1,497
483
95762
The $.25 ave is going to be low. Most power consumption is AC during peak period. Best if you can look at years worth of bills
 

Southpasfan

Member
Jun 2, 2019
467
610
Pasadena
3 years of bills, average is under $.20.

In my case, 16.32 solar plus 3 powerwalls including finance charges over twenty years is .20 per kwh.

Solar alone would have been roughly half, or .10 per kwh (the total amount of power is the same whether you get powerwalls or not).

That's why the first step is figuring usage and the utilities rates. Someone who only needs a smaller system has a lower cost. Then the math also depends on estimated production.

That's the real kicker. Two 16.32 systems might have vastly different production results for different locations and different roofs. Yet, the system cost is the same.
 

getakey

Active Member
Jan 28, 2020
1,497
483
95762
I've done a reverse type of calc. I've taken my Investment on Day 0 as a negative number. Then took my annual savings based on real production numbers. Over 20 years, I have an 18% rate of return and thats with no risk compared to other investments. Plus, if rates go up, my return will be higher
 

Fragger

Member
Mar 26, 2020
246
36
Texas
Friends, thanks for the wonderful advice, once again off of this great board!

A lot of great points and I think based on most of the responses and the math we did, I think I'll put off the Powerwall for now...
 

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