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

h2ofun

Active Member
Aug 11, 2020
2,136
332
auburn, ca
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.
totally agree
 

Vines

Active Member
Jul 20, 2018
1,870
2,185
Silicon Valley, CA
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)



Yes, my understanding is that watt for watt solar glass is less efficient due to the lack of airflow, and perhaps the construction. There may be string and target voltage issues I am not aware of since I do not install it.

I don't have great data yet, but have heard solar glass to be about 60-80% of the efficiency per square of a high efficiency discrete module. I am still trying to get good data on just how efficient it is when installed .
 

jboy210

Supporting Member
Dec 2, 2016
4,952
3,027
Northern California
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)


Solar roofs collect less energy per area. So you need more area of active tiles. They also often put them on North, East, and West side of the roof to get to compensate for the increased space requirements. This is a compromise that solar roof owners need to consider and balance against other priorities.

We decided our house would look better with a solar roof. And we have active tiles on North and South roof planes. The system is rated at 12.75 kW but we rarely see more than 8.5 kW. But because much of the roof is active, we see solar starting very early in the morning in Spring, Summer, and Fall. And at times when the south roof is partially shaded by trees. Also, the amount of energy (kWh) collected in can be day is quite high (nearing 90 kWh). And with Powerwalls carrying the nightly load means will not have to buy any power from our utility from late Spring late Summer. We saw the June bill drop from near $550 to $50 while testing last summer.

We like the look of the roof and are glad we did not go with another tile roof plus solar. The price was the same. And again the roofing/solar company that was going to install the roof + panels went bankrupt.
 

wjgjr

Active Member
May 11, 2020
1,168
912
Silver Spring, MD
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).
So, doing the math, you have 13.0 kW installed, and are producing 13,000 kWh annually, meaning you are producing 1,000 kWh per installed W. Your neighbor has 9.6 kW and is producing 9,800 kWh annually, or 1021 kWh per installed W. So, I would say the efficiency of the systems is very similar, assuming shading and other conditions are similar.

As to why your neighbor is limited, I know that at some point the amount of solar can saturate the circuits in an area. I don't have much knowledge of all the factors in play, but maybe they have introduced limits due to the other solar in the area?
 

RKCRLR

Member
Apr 13, 2020
310
121
Garden Valley, CA
Personally, I see no good reason to cycle the powerwalls in your neighbor's situation. Each cycle takes a little life out of the battery. It doesn't sound like there is currently any financial advantage.

Unless the utility has a specific need for more electricity at night I don't see a "green" reason for using the powerwalls at night. And since there is about a 10% efficiency loss that makes it less green. And wearing out the battery sooner is also anti-green.

If there is a certain time period that the utility needs more power and it starts using more fossil fuels during that time period, then I could see using the powerwalls during that time period. But the 10% round trip efficiency loss needs to be taken into consideration. I.e., is it better to be using the solar for consumption and giving back to the utility instead of the 10% loss.
 
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jboy210

Supporting Member
Dec 2, 2016
4,952
3,027
Northern California
Personally, I see no good reason to cycle the powerwalls in your neighbor's situation. Each cycle takes a little life out of the battery. It doesn't sound like there is currently any financial advantage.
....
Even full it cycles. The battery drains a little bit just sitting there. Then it starts charging at an already high state of charge, which can be quite bad for battery life.
 

LoudMusic

Member
Jul 21, 2020
332
430
Arkansas
You use a lot of exclamation marks in your writing.

!!!

Tesla Powerwall 2 | EnergySage

Looks like the warrantied cycles is 3200. Assuming that's one full charge and discharge per day, that'd be just over 8 years 9 months. But not even knowing your location I'd guess there will be many days they have very small partial cycles, and very few days of full cycles. A spitballing guess would be closer to 20 years. And that's just to get to reduced capacity - not even a replacement point.

I have about 14kWh of lithium batteries on my boat along with 2kW of solar and when we're away from the dock that's exactly how they work. I anticipate they'll last at least 20 years. On typical days the batteries don't get below probably 60% and fill up at least once per week.
 

Electrph

Member
Aug 29, 2019
323
189
Central California
You use a lot of exclamation marks in your writing.

!!!

Tesla Powerwall 2 | EnergySage

Looks like the warrantied cycles is 3200. Assuming that's one full charge and discharge per day, that'd be just over 8 years 9 months. But not even knowing your location I'd guess there will be many days they have very small partial cycles, and very few days of full cycles. A spitballing guess would be closer to 20 years. And that's just to get to reduced capacity - not even a replacement point.

I have about 14kWh of lithium batteries on my boat along with 2kW of solar and when we're away from the dock that's exactly how they work. I anticipate they'll last at least 20 years. On typical days the batteries don't get below probably 60% and fill up at least once per week.
I believe as pointed out earlier in thread warranty based on cycles only applies to batteries not charged by solar .. which are the minority from what I've read. So strictly 70% capacity at 10 year mark
unless i skimmed thread too fast and neighbor does not have solar .. then thats on me
 
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LoudMusic

Member
Jul 21, 2020
332
430
Arkansas
I believe as pointed out earlier in thread warranty based on cycles only applies to batteries not charged by solar .. which are the minority from what I've read. So strictly 70% capacity at 10 year mark
unless i skimmed thread too fast and neighbor does not have solar .. then thats on me

Is it somehow different charging by solar than grid in relation to battery cycles?
 

Electrph

Member
Aug 29, 2019
323
189
Central California
Is it somehow different charging by solar than grid in relation to battery cycles?
my guess is that without solar tesla anticipates more cycling per year (i know thats a weak obvious answer) rational for that .. i would say for example people shifting tou without solar would cycle more? but that would not always be case vs solar . i guess answer is i don't know .. i'm sure others here have more detailed scenarios
 

wjgjr

Active Member
May 11, 2020
1,168
912
Silver Spring, MD
my guess is that without solar tesla anticipates more cycling per year (i know thats a weak obvious answer) rational for that .. i would say for example people shifting tou without solar would cycle more? but that would not always be case vs solar . i guess answer is i don't know .. i'm sure others here have more detailed scenarios
I think that is correct as much of the answer. As I recall, the warranty gets you close to one full charge/discharge per day (especially if the capacity does decline towards 70% over 10 years) so it doesn't necessarily exclude TOU shifting of (close to) one charge/discharge cycle daily, regardless of how the PWs are charged. But it does potentially limit the warranty for uses that cycle the batteries more often.

And I would guess the other part is Tesla didn't want to complicate warranty limits when paired with solar. The 10-year limit is clear, and it is likely that very few solar customers will hit the cycling numbers anyway given the realities of when solar is generated and how load shifting and/or self-powered settings work.
 
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Zythryn

Model Y custom Warming Stripes wrap.
Mar 18, 2009
2,169
1,189
Minnesota
Personally, I see no good reason to cycle the powerwalls in your neighbor's situation. Each cycle takes a little life out of the battery. It doesn't sound like there is currently any financial advantage.

Rather than “I see no good reason”, I take it you meant ‘I see no financial reason’.
I myself like minimizing my reliance on the grid. As such, I use battery power overnight in the summer. In the winter, I do have most of the PowerWalls reserved for backup as power is critical.

There are valid reasons, other than financial.
 

pdx_m3s

Active Member
May 18, 2019
1,297
1,123
Portland, OR
It sounds like your neighbor just enjoys spending money and the latest tech gadgetry. Powerwalls rarely pencil-out from a financial standpoint, especially in this situation.
 
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jboy210

Supporting Member
Dec 2, 2016
4,952
3,027
Northern California
It sounds like your neighbor just enjoys spending money and the latest tech gadgetry. Powerwalls rarely pencil-out from a financial standpoint, especially in this situation.
The neighbor may want to ensure that they can use solar to power their house during an outage. Without the TEG and Powerwalls a home will not be able to use the solar power the panels are generating if the grid goes down. That was one of the reasons we added powerwalls to our install. We work from home all the time and can miss a lot of work hours if the grid is down. The 3-4 days of 2019 PSPS cost us considerable income. That income would have paid a nice chunk of the Powerwall installation.
 

h2ofun

Active Member
Aug 11, 2020
2,136
332
auburn, ca
The neighbor may want to ensure that they can use solar to power their house during an outage. Without the TEG and Powerwalls a home will not be able to use the solar power the panels are generating if the grid goes down. That was one of the reasons we added powerwalls to our install. We work from home all the time and can miss a lot of work hours if the grid is down. The 3-4 days of 2019 PSPS cost us considerable income. That income would have paid a nice chunk of the Powerwall installation.
how about a generator?
 

pdx_m3s

Active Member
May 18, 2019
1,297
1,123
Portland, OR
The neighbor may want to ensure that they can use solar to power their house during an outage. Without the TEG and Powerwalls a home will not be able to use the solar power the panels are generating if the grid goes down. That was one of the reasons we added powerwalls to our install. We work from home all the time and can miss a lot of work hours if the grid is down. The 3-4 days of 2019 PSPS cost us considerable income. That income would have paid a nice chunk of the Powerwall installation.

If backup is the concern, then (1) Powerwalls really aren't the best/most economical option and (2) Draining them overnight isn't a good idea. It would make more sense to set a pretty high reserve percentage, which would leave capacity for outages, and also reducing the cell degradation. We all know lithium ion cells degrade with each charge/discharge cycle.
 

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