# What is the BEST Powerwall setting for long-term?



## harrison987 (Jun 30, 2018)

Hey all...

I am curious what the best long-term powerwall settings are?

I have a solar system and 1 powerwall.

In the app, the setting of "time-based control" makes sense...as it focuses ALL solar energy towards the grid at peak times in order to get the most credits...AND...using battery at night. Essentially totally off-grid.

Next morning, the powerwall (which depletes to around 50% overnight) then charges up to 100% using solar energy (instead of sending to grid)...and the routine happens again as mentioned above.

BUT...doing this also means the battery is going through a charging cycle the next day, thus depleting the life of the battery (rated at 5000 cycles before it is considered "Dead"). So batteries will need to be replaced in about 12 years.

My solar company is telling me to leave in "back-up only"...as it will extend the battery life. BUT...this also means relying on the grid more often. Their point of view is to look at the grid as being a battery of its own. The more I send there...the more I can take. AND...if there is a power outage...I know my batteries will always be at 100%,

There seems to be pros and cons...and was curious what others are currently doing?

TIA

MIke


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## Bigriver (Jan 26, 2018)

harrison987 said:


> My solar company is telling me to leave in "back-up only"...as it will extend the battery life. BUT...this also means relying on the grid more often. Their point of view is to look at the grid as being a battery of its own. The more I send there...the more I can take. AND...if there is a power outage...I know my batteries will always be at 100%,


That is basically my approach, but only during the winter when our production is low. The winter solar production is so much below our needs that if I regularly used the powerwalls, I would be jeopardizing having adequate backup. If the season/weather condition is such that I don't think the powerwalls can fully recharge the next day (in addition to our household needs being supplied by solar), the powerwalls are in backup.

Late spring, summer and early fall we cycle the powerwalls and mostly stay away from using grid power. The grid only receives - yes, it is just a big battery as I rack up credits for the winter. However when I charge a car or if we have a crazy hot day with lots of a/c (too many of those days this year) I usually put the powerwalls in backup mode as it makes no sense to drain them (especially the car - why take the inefficiencies of moving energy from one battery to another?)

I have no time of use differences in the cost/benefit of grid power. Wish I did. That would be a fun variable to maximize.

Life of the powerwalls... I only remotely think about that. We bought them expecting a 10 year lifespan, and I expect that at that point there will be such advances in battery technology that 1) I will want it, and 2) the price will be much better. But, as my primary purpose of the powerwalls is for backup (and to make solar available if the grid is down), I could find myself reconsidering my powerwall usage if, for example, the lifespan could be doubled or tripled. I would be interested in the effects if they always sit in a backup mode. I don't know how much buffer they have at the top.... so if they sit at "100%" is this actually only 90%? Is there an adverse impact to never (or rarely) discharging them?


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## harrison987 (Jun 30, 2018)

Well...

From what I understand...the powerwall loses around 1% per day of charge...and charges automatically up to 100% again after around 3 days...so basically, that is considered another charging cycle. So it appears one cycle every 3 days, as opposed to one cycle every day (when relying on battery and such).

My solar company says that is it not worth it to be a net exporter. Basically...and if I send my extra solar energy to the battery (instead of the grid), and discharge via battery at night or during peak (instead of grid)...then I am not using the solar credits from Edison.

For example...if I sent 10kwh TO the grid...that gives me exactly 10kwh of energy I can take FROM the grid. It is a 1:1 exchange. 

However...over the course of a year...if I send MORE to the grid than what I take from the grid...the remaining gets sent to me as cash (at the end of the year), at a wholesale price. Which is around $0.03 per Kw. 

Miniscule value.

So my Solar company is insisting that I refrain from trying to be a net exporter. Having credits with the electric company is bad...as the are not worth the same as using them as electricity credits (cash credit vs. electricity credit). and on top...battery lasts longer.

For the past month, I have sent 168kwh TO the grid...but have only taken 46.3kwh from the grid. As such, I have 121kwh credit with the Edison. If I USE those credits, by using grid electricity before the year is up ...they are worth on the low side $0.17/kw ($121.53) - this is the lowest off-peak amount. If I do not use them (and Edisons sends me a credit back at the end of the year for NOT using them)...I only get a $3.63 credit, as they pay back wholesale ($0.03 on average)

So for sure it makes more sense to use up whatever credits I send TO the grid...as opposed to what I am sending to the Powerwall. Credits are worth more.

I only just got the system...so I have no clue what it will be like come the fall. I may have to play around with it. 

There is a good feeling of never having to use the grid for anything...but it appears by doing so, and using the battery as much as possible:

a) less credit with Edison for excess Kwh sent to the grid.
b) shorter battery life


mmmmmmmmmm....


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## Bigriver (Jan 26, 2018)

harrison987 said:


> .the powerwall loses around 1% per day of charge...and charges automatically up to 100% again after around 3 days...so basically, that is considered another charging cycle. So it appears one cycle every 3 days, as opposed to one cycle every day (when relying on battery and such).


I'm not fully following you here.... sounds like you are describing backup mode, but that wouldn't be considered a full cycle if you are referring to total lifespan cycles.



harrison987 said:


> My solar company says that is it not worth it to be a net exporter.


I think that is always the goal, or at least I know it is here. They will not allow the system to be sized substantially over your projected need. However, daily use or non-use of the powerwall is not going to have any notable effect on whether you are a net exporter on an annual basis. That is only driven by your total solar production and total house usage. Whether you are cycling daily with the powerwall or the grid doesn't really change that overall net energy consumption/supply. (Caveat - there is a small inefficiency of using the powerwall. On average I put in 1 kWh/day more than I take out out of my 2 powerwalls when not in backup. In backup, there is still some loss but it is much less.)



harrison987 said:


> However...over the course of a year...if I send MORE to the grid than what I take from the grid...the remaining gets sent to me as cash (at the end of the year), at a wholesale price. Which is around $0.03 per Kw.


Is your end of the year the end of the calendar year? My "end of year" is May 10 when they do a "true-up" reconciliation of any surplus. This is an ok time of year for me because it lets the excess I generate in the summer be used during the winter. In PA where I live, I massively overproduce in the summer and massively underproduced in the winter, relative to my household needs. In some parts of the country, the production and need curves are more aligned so the timing of the reconciliation would not be as critical.


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## harrison987 (Jun 30, 2018)

Hi!

According to Tesla...they said that the 5000 cycle life span is based on ANY discharge followed by a re-charge. So whether it drops 50%, and then charged to 100%...or if it drops to 95% and then you charge to 100%...either are considered a cycle.

Is this mis-information?

Mike


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## harrison987 (Jun 30, 2018)

also, if you cycle once a day...the warranty is only 8 years...not 10....  Once you hit 3200 cycles...warranty expires.


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## Bigriver (Jan 26, 2018)

I am only aware of a 10 year warranty, not dependent on the number of cycles. Warranted to still have 70% capacity at 10 years.
https://www.tesla.com/sites/default/files/pdfs/powerwall/Powerwall_2_AC_Warranty_USA_1-1.pdf


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## harrison987 (Jun 30, 2018)

There are two categories:

*Warranty:*

*Category #1:* Tesla warrant the Powerwall 2 DC, for 10 years and unlimited cycles when used for "solar self consumption / backup".
They include a footnote that defines this as:
"Storing solar energy generated by an onsite array, and using that stored solar energy (i) for daily self-consumption and/or (ii) for use as backup power."​But this definition does not include:

Charging the battery with off-peak electricity.
Charging the batteries with a generator.

*Category #2: * For all uses that do not fall into category #1 Tesla will warrant the Powerwall 2 DC, for 10 years OR until it provides 37,800 kilowatt-hours of stored electricity (approx 3,200 cycles).

I was reading it wrong, I think.


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## Bigriver (Jan 26, 2018)

It has been my perception that the "normal" installation of the powerwall has it charging only from solar. If it has the capacity to charge from anything other than solar, it is my understanding that it does not qualify for the federal residential energy tax credit (line 1 of form 5695).
https://www.tesla.com/support/energy/powerwall/learn/incentives
Thus the 10 year unlimited part of the warranty is what I expect to be applicable to most.


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