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Why is my neighbor using his power walls daily (will they last long)?

fbitz777

Member
Apr 6, 2016
421
549
Wexford, PA
Ok this is an argument I am having with my neighbor who just got his solar roof installed:

9.6KW solar roof, 2 power walls.
House is all electric (Heat pumps with resistive heat of up to 20Kw when deep cold)
Net metering but electrical rate is same all the time so no games to play with peak vs off peak.

So his theory is to keep charging the 2 power walls with excess solar production and discharge them at night!
Power outages are not his concerns and 2 power walls would probably only power a few hours of A/C or heating.

Seems to me:
-The power walls will not last that long (due to number of cycles) or are they warrantied for as long as the solar roof?

-Isn't that inherently inefficient? charging/discharging power walls is probably only 80 percent efficient hence that is costing him money!

-If I am right how could this be explained clearly to him?


Again he claims to have learned that practice from the Tesla installers. Note that I have solar panels next door but no power walls (we have so few outages).
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
8,108
8,971
Riverside Co. CA
To answer the thread title... because that is the way they are designed to be used. There is absolutely zero "wrong" with discharging the powerwalls daily, in fact that is the way most people in california use them, which is the largest market for them.

In fact, tesla FULLY supports this, with usage modes in their app that are designed to specifically discharge daily during peak electric rates.
 

fbitz777

Member
Apr 6, 2016
421
549
Wexford, PA
To answer the thread title... because that is the way they are designed to be used. There is absolutely zero "wrong" with discharging the powerwalls daily, in fact that is the way most people in california use them, which is the largest market for them.

In fact, tesla FULLY supports this, with usage modes in their app that are designed to specifically discharge daily during peak electric rates.

YES BUT in our case we do NOT have peak electric rates! Hence you can't tell me how Tesla would advise my neighbor to loose 20 percent on his power wall production!

Does Tesla allow CA users to charge power wall from the grid during off-peak hours (I thought this was not "allowed")

BTW Power walls are guaranteed for 10 years.
 
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jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
8,108
8,971
Riverside Co. CA
YES BUT in our case we do NOT have peak electric rates! Hence you can't tell me how Tesla would advise my neighbor to loose 20 percent on his power wall production!

BTW Power walls are guaranteed for 10 years.

Not exactly, they are guaranteed to retain 70% of their capacity at the 10 year mark. Many people (for some reason I dont quite understand) boil this down to "it has a 10 year warranty" as if it could have 50% left at 10 years but because its "still running" wouldnt be covered.

I believe the losses are 10% not 20%, as well, but the product is specifically designed to cycle so there is no issue with your neighbor wanting to reduce their usage of the grid. Its not like batteries sitting at 99 to 100% is "great" for them either.

There is no issue here at all.
 

jrweiss98020

Tessa's Tesla
Jan 9, 2020
421
297
Edmonds, WA
To each his own...

When my system was first installed, I used the PWs overnight. However, I kept the reserve at 50-70% to guard against outages and prevent deep discharge cycles. They worked fine in that mode.

I then read more about the round-trip efficiency for charge/discharge, and decided to use Backup Only. However, my production and usage are such that I may have to give away electricity in the spring net-metering reset, so I may go back to self-powered once production ramps up in the spring.
 

Will792

Member
Oct 9, 2018
12
8
NY
It is not that simple to figure out the best way to use PowerWalls. It depends on owners priorities (maximize possible lifetime of PWs, reduce feed from the grid, optimize economics with TOU rates and soon) and options with electric service provider. Tesla gives 10 years warranty and certain number of MWh stored/discharged capacity. If LIIo battery is not fully discharged in each cycle it greatly increases number of lifetime cycles.

I doubt that 2 PWs would be able to provide enough current to even one heat pump with electric backup heat. Each PW can provide 20A and backup heat coil is commonly 30-50A, which is separate from compressor line.

I am not fully convinced that Tesla installers are aware of LIIo batteries peculiarities, related to effect of depth of discharge to degradation of battery. It also depends on owners expectations to get more than Tesla warrantable 10 years from PowerWall batteries.

PW round trip efficiency is 90% btw. Usable capacity of each PW is 13.5 so if you reserve 50% you use 13.5 KWh in 2 PWs system, with 1.35 KWh lost in storage/discharge cycle.

In theory PW would last longer if you use self-powered mode with 70% reserved for backup since it will increase amount of time PW batteries are charged at 70%-80% level, best for LIIo batteries longevity. I am somewhat surprised that Tesla does not give an option to charge PW batteries to less than 100%, similar to this option for Tesla cars.
 
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jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
8,108
8,971
Riverside Co. CA
It is not that simple to figure out the best way to use PowerWalls. It depends on owners priorities (maximize possible lifetime of PWs, reduce feed from the grid, optimize economics with TOU rates and soon) and options with electric service provider. Tesla gives 10 years warranty and certain number of MWh stored/discharged capacity. If LIIo battery is not fully discharged in each cycle it greatly increases number of lifetime cycles.

I doubt that 2 PWs would be able to provide enough current to even one heat pump with electric backup heat. Each PW can provide 20A and backup heat coil is commonly 30-50A, which is separate from compressor line.

I am not fully convinced that Tesla installers are aware of LIIo batteries peculiarities, related to effect of depth of discharge to degradation of battery. It also depends on owners expectations to get more than Tesla warrantable 10 years from PowerWall batteries.

PW round trip efficiency is 90% btw. Usable capacity of each PW is 13.5 so if you reserve 50% you use 13.5 KWh in 2 PWs system, with 1.35 KWh lost in storage/discharge cycle.

In theory PW would last longer if you use self-powered mode with 70% reserved for backup since it will increase amount of time PW batteries are charged at 70%-80% level, best for LIIo batteries longevity. I am somewhat surprised that Tesla does not give an option to charge PW batteries to less than 100%, similar to this option for Tesla cars.

The "certain number of MH stored" part of the warranty is only in place if NOT charged from solar. If charged from solar, the warranty is 10 years unlimited charging.

https://www.tesla.com/sites/default/files/pdfs/powerwall/powerwall_2_ac_warranty_us_1-4.pdf

Also, each PW can provide 30amp, not 20.
 

Will792

Member
Oct 9, 2018
12
8
NY
Also, each PW can provide 30amp, not 20.
From PW2 manual:
Real Power, max continuous 5 kW (charge and discharge)
Real Power, peak (10 s, off-grid/backup) 7 kW (charge and discharge)
Apparent Power, max continuous 5.8 kVA (charge and discharge)
Apparent Power, peak (10 s, off-grid/backup) 7.2 kVA (charge and discharge)
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
8,108
8,971
Riverside Co. CA
From PW2 manual:
Real Power, max continuous 5 kW (charge and discharge)
Real Power, peak (10 s, off-grid/backup) 7 kW (charge and discharge)
Apparent Power, max continuous 5.8 kVA (charge and discharge)
Apparent Power, peak (10 s, off-grid/backup) 7.2 kVA (charge and discharge)

Look at this data sheet: (you left out the part of those specs that say "overcurrent protection device, 30 amp" and "maximum output fault current 32 amp"

https://www.tesla.com/sites/default.../Powerwall 2_AC_Datasheet_en_northamerica.pdf
 

power.saver

Supporting Member
Mar 4, 2018
506
511
Arcadia, CA
5kW / 240V = 20.8A

But since load is continuous, breaker has to be 125% which would be 26A. That size isn't available, so round up to next size, 30A.
 

fbitz777

Member
Apr 6, 2016
421
549
Wexford, PA
Ok looks like my neighbor's system is undersized....I saw close to 100A on his heat coils (only when deep cold) Otherwise it's on a 3 ton Heat Pump.

What bugs me the most is that his system is only 9.6kw and slated to produce 9800 Kwh a year. That's not even half his early consumption.
And it a fairly large house (2000sqft roof) with nice exposure. Is solar roof less efficient? I get 13Kw on 40 solar panels (600sqft)


 

fbitz777

Member
Apr 6, 2016
421
549
Wexford, PA
You worry about your neighbor a lot?
Yes! I talk to him every day and I referred him. For me it's a learning experience of Solar Panels vs Solar Roof and we keep comparing production curves. Next I guess he will move to Tesla car based on my fairly good experience with mine.
 

dareed1

Member
Jan 15, 2021
63
60
Belmont, CA
I'm a California boy, so my understanding of the PA PUC rules could easily be wrong, but it looks to me that residential customers may opt for TOU under Rider K. It only applies during June, July, and August, but if this Rider is chosen, then the customer pays 1.41 times the normal rate during peak, and 0.78X during off peak. Time shifting could save some money if the house requires substantial power in the evening. A 3 ton heat pump, if it is modern, will probably pull 3kW if running flat out, but it probably averages much less than that.

Another reason to use energy from the batteries is surcharges, which may be levied against the power drawn from the grid as opposed to the net usage from the grid. May or may not apply where you are. In PA, if excess power is generated over the course of a year, the amount rebated is retail price minus the distribution costs. It isn't clear to me what is exactly meant by "retail price", but the distribution cost is 4.4 cents/kWh.

Solar roofs and solar panels have similar efficiencies. Only a fraction of the tiles in a solar roof generate PV. Tesla generally will recommend a system which will generate less power than the house is likely to use on an annual basis, because that approach typically has the best ROI.

On the subject of ROI, it is difficult to make a case for Powerwalls on that basis alone. I have two because I expect frequent planned power outages here in summer and fall. We have a heat pump, so any outage in the winter would be inconvenient without battery backup. Did your neighbor buy them for protection against power outages?
 
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fbitz777

Member
Apr 6, 2016
421
549
Wexford, PA
I'm a California boy, so my understanding of the PA PUC rules could easily be wrong, but it looks to me that residential customers may opt for TOU under Rider K. It only applies during June, July, and August, but if this Rider is chosen, then the customer pays 1.41 times the normal rate during peak, and 0.78X during off peak. Time shifting could save some money if the house requires substantial power in the evening. A 3 ton heat pump, if it is modern, will probably pull 3kW if running flat out, but it probably averages much less than that.

Another reason to use energy from the batteries is surcharges, which may be levied against the power drawn from the grid as opposed to the net usage from the grid. May or may not apply where you are. In PA, if excess power is generated over the course of a year, the amount rebated is retail price minus the distribution costs. It isn't clear to me what is exactly meant by "retail price", but the distribution cost is 4.4 cents/kWh.

Solar roofs and solar panels have similar efficiencies. Only a fraction of the tiles in a solar roof generate PV. Tesla generally will recommend a system which will generate less power than the house is likely to use on an annual basis, because that approach typically has the best ROI.

On the subject of ROI, it is difficult to make a case for Powerwalls on that basis alone. I have two because I expect frequent planned power outages here in summer and fall. We have a heat pump, so any outage in the winter would be inconvenient without battery backup. Did your neighbor buy them for protection against power outages?
I'm a California boy, so my understanding of the PA PUC rules could easily be wrong, but it looks to me that residential customers may opt for TOU under Rider K. It only applies during June, July, and August, but if this Rider is chosen, then the customer pays 1.41 times the normal rate during peak, and 0.78X during off peak. Time shifting could save some money if the house requires substantial power in the evening. A 3 ton heat pump, if it is modern, will probably pull 3kW if running flat out, but it probably averages much less than that.

Another reason to use energy from the batteries is surcharges, which may be levied against the power drawn from the grid as opposed to the net usage from the grid. May or may not apply where you are. In PA, if excess power is generated over the course of a year, the amount rebated is retail price minus the distribution costs. It isn't clear to me what is exactly meant by "retail price", but the distribution cost is 4.4 cents/kWh.

Solar roofs and solar panels have similar efficiencies. Only a fraction of the tiles in a solar roof generate PV. Tesla generally will recommend a system which will generate less power than the house is likely to use on an annual basis, because that approach typically has the best ROI.

On the subject of ROI, it is difficult to make a case for Powerwalls on that basis alone. I have two because I expect frequent planned power outages here in summer and fall. We have a heat pump, so any outage in the winter would be inconvenient without battery backup. Did your neighbor buy them for protection against power outages?
Yes we do not (as there not indication in near future) TOU or even peak measuring from the utility company.
And no my neighbor reason for powerwalls was not power outages...we have almost none. He thought powerwalls were sexy and good for the environment; also $$$ were not really an issue and his roof needed to be replaced.
The only non-sense is really that he was somehow limited to 9.6kw output and claimed it was limited by power company. Makes no sense as I have 13kw and there are a 22kw and 44kw neighbors on the same power line.
 

wjgjr

Active Member
May 11, 2020
1,159
894
Silver Spring, MD
What bugs me the most is that his system is only 9.6kw and slated to produce 9800 Kwh a year. That's not even half his early consumption.
And it a fairly large house (2000sqft roof) with nice exposure. Is solar roof less efficient? I get 13Kw on 40 solar panels (600sqft)
There are two types of efficiency. In terms of the efficiency of turning the sun's energy into electricity, they are pretty much the same. The main issue is efficient use of limited roof space. The placement of PV modules in a solar roof is less dense, which generally means you cannot place as much capacity in one area (though since the shingles are smaller than panels, in some case that can mitigate this issue.) Depending on the size of the roof and the desired system output, this can mean placing more solar roof shingles in less desirable locations.

In this specific situation, there is not enough information to say what is going on. Why did the neighbor choose that size? Was it limited by budget, usable roof space, desired size, etc? And, what is the rated capacity (in Watts) of your panels? If they are 300W or more, then it seems like you are getting essentially the same production as your neighbor per installed capacity.
 

mswlogo

Well-Known Member
Aug 27, 2018
6,016
4,627
MA, NH
It depends on your Net metering plan. When you push excess Solar to the Grid and then pull it back later, that typically is not free. I lose 20% with my plan. Power Walls should be 92% efficient. So you save 8% doing your own storage. In my case Power Wall is not worth it. Especially if my Solar produces 20% more than I use annually I can afford that loss so Power Wall would gain me nothing. Money wise. Power Wall only makes sense if helps you shift grid load to off peak rates. And way over priced for backup IMHO.
 
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fbitz777

Member
Apr 6, 2016
421
549
Wexford, PA
There are two types of efficiency. In terms of the efficiency of turning the sun's energy into electricity, they are pretty much the same. The main issue is efficient use of limited roof space. The placement of PV modules in a solar roof is less dense, which generally means you cannot place as much capacity in one area (though since the shingles are smaller than panels, in some case that can mitigate this issue.) Depending on the size of the roof and the desired system output, this can mean placing more solar roof shingles in less desirable locations.

In this specific situation, there is not enough information to say what is going on. Why did the neighbor choose that size? Was it limited by budget, usable roof space, desired size, etc? And, what is the rated capacity (in Watts) of your panels? If they are 300W or more, then it seems like you are getting essentially the same production as your neighbor per installed capacity.
I will investigate as to why he was limited by power company....I have 40 of the 325W panels myself and produce about 80 percent of my consumption (I have gas heat).
 

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