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Is 1 PowerWall better than No PowerWall?

holeydonut

Supporting Member
Jun 27, 2020
1,508
881
East Bay NorCal
Anecdote, my situation, 1PW is plenty.
I'm in Los Angeles, SCE as well.
I had 4.8KWh solar installed in 2018, I added 1PW and 1.8KWh of extra solar (West facing) last year.
I'm on the 4pm-9pm TOU plan which in the summer is 3x the off-peak cost.
Solar production is about 40KWh per day now.
Daily home usage is about 30KWh, so 1PW is not quite enough to make me fully self sufficient.
But with the advanced time-of-use scheduling, on a clear day the PW is fully charged by around noon, I'm feeding excess into the grid until 4pm, then both feeding back to the grid from 4pm to around 7pm, and running the house from the PW until 10pm or later

So I'm getting full benefit of the TOU rate arbitrage, and I have the peace of mind of the backup for the odd grid disconnections.
And any time that SCE raises the peak rates, it benefits me.

I expect this to be much better month-to-month that previously with no PowerWall.

Winter months are going to be a bit lean, but that needs more solar, not more PW.


Yes, I fully believe the value you get by being mostly agnostic to TOU shenanigans has an implied ROI even if the actually financial benefit is hard to quantify.

What you describe is what I originally wanted to achieve back in 2020; and I was surprised (errr "shocked" pun) to learn nobody I spoke with (even Tesla) was willing to install a partial home backup solution that would also have TOU benefit the way you described.

It wasn't until I got on TMC and @Vines was here to help assure that such a design was possible. Every PV+ESS quote I had received simply had a single battery providing a mini-backup for a small load center and absolutely no upstream energy sharing when the utility was online.
 
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tomuo

Member
Mar 15, 2021
19
7
Los Angeles, CA
Yes, I fully believe the value you get by being mostly agnostic to TOU shenanigans has an implied ROI even if the actually financial benefit is hard to quantify.

What you describe is what I originally wanted to achieve back in 2020; and I was surprised (errr "shocked" pun) to learn nobody I spoke with (even Tesla) was willing to install a partial home backup solution that would also have TOU benefit the way you described.

It wasn't until I got on TMC and @Vines was here to help assure that such a design was possible. Every PV+ESS quote I had received simply had a single battery providing a mini-backup for a small load center and absolutely no upstream energy sharing when the utility was online.
Exactly.
The only drawback to 1PW vs 2+ is that it's partial home (because max 5kW won't drive HVAC during an outage). But it's wired so I get all the benefit described.
The AC, outdoor sockets and Garage are on the main panel so not powered during a grid outage, but I have no restriction on feeding back to the grid normally.
With Tesla's latest Gateway and inverters they may have made system level decisions (to unify installation) that makes this different, especially given Elon's latest comments.

My sub panel has 2 inverters and the Powerwall all interconnected. That panel connects to the Tesla gateway as the secondary source.
The main panel has the unbacked-up loads and a 60Amp feed to the gateway as the primary source.
The gateway has a set of breakers for the backed up loads.
This is going to be different to some people, a whole house backup has no breakers there, only the transfer switch.

So for a grid disconnect, the 60Amp feed between the gateway and main panel opens and I lose the HVAC and garage, but everything else is powered from the breakers in the gateway.
With the grid connected, current flows freely to and from the main panel over that 60amp link, so battery and solar feed the whole house normally.
 

tomuo

Member
Mar 15, 2021
19
7
Los Angeles, CA
It was a good assumption. Natural gas lines cover the whole area and is so much less expansive than electricity. Just about every house here has natural gas.

ETA: We also have more natural gas than we know what to do with. We have an entire area in downtown LA where there is tar, oil, and natural gas bubbling out of the ground. They even put a museum there collecting ice age animal bones from these tar pits.
I'm thinking that the gas vs electric comparison is already breaking down away from gas.
Mainly because the base allowance of gas in socal is ridiculously low. It's so easy to go over it and trigger 20% 30% more cost per therm.
The pipelines for SoCal aren't so well connected, so even last year we got notices to "please use less".
I replaced my HVAC (gas+AC) with a heatpump, but I still hit the base gas allowance with just water heating and dryer (family of 4).
Thinking seriously about switching the water heater to a hybrid Electric, but since it would be in the Garage, I'd lose hot water during Grid outages.
 

holeydonut

Supporting Member
Jun 27, 2020
1,508
881
East Bay NorCal
Looks like the average price delta is about 27c. If a third of your usage is during peak times that would be about 6kw a day which should almost always be more than generation from a 8kw system
27c x 6 = $1.62 * 360 days a year = $583
At a 8k after tax credit installed cost your breakeven is almost 14 years while the powerwall is only warrantied for 10. Doesnt seem like it would be worthwhile if there is not value to backup.

Ok so you're assuming the blended average summer/winter rate differential between peak and non-peak. I can accept $0.27.

I agree that about 1/3 daily usage occurs during peak (even more-so for people with big AC's and electric kitchens). But I do not accept that 6 kWh is one third of an average household daily usage.

The average household in the USA uses about 10,649 kWh per year (877 kWh per month or 29 kWh per day). And using the CPUC's logic that the average solar customer is a moneybags rich person who is sticking it to the disadvantaged population at large, a typical home that is consider PV+ESS will use more.

So, using this EIA average... change your math to 10 kWh per day used at peak x $0.27 average rate differential x 365 days a year = $985.50 per annum. Or almost $10k over a 10 year useful life. And that's a conservative estimate using today's figures. I believe no chance in hell that $0.27 will hold for 10 years. I expect that to hit like $0.50.

Anyway, the peace of mind that one or two Powerwalls buys is not in the pure financials. It is in the comfort knowing you can run your AC at 5:30pm because it's hot and you don't care what the TOU rate is.
 
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tomuo

Member
Mar 15, 2021
19
7
Los Angeles, CA
Ok so you're assuming the blended average summer/winter rate differential between peak and non-peak. I can accept $0.27.

I agree that about 1/3 daily usage occurs during peak (even more-so for people with big AC's and electric kitchens). But I do not accept that 6 kWh is one third of an average household daily usage.

The average household in the USA uses about 10,649 kWh per year (877 kWh per month or 29 kWh per day). And using the CPUC's logic that the average solar customer is a moneybags rich person who is sticking it to the disadvantaged population at large, a typical home that is consider PV+ESS will use more.

So, using this EIA average... change your math to 10 kWh per day used at peak x $0.27 average rate differential x 365 days a year = $985.50 per annum. Or almost $10k over a 10 year useful life. And that's a conservative estimate using today's figures. I believe no chance in hell that $0.27 will hold for 10 years. I expect that to hit like $0.50.

Anyway, the peace of mind that one or two Powerwalls buys is not in the pure financials. It is in the comfort knowing you can run your AC at 5:30pm because it's hot and you don't care what the TOU rate is.
I have a different calculation arriving at the same result (numbers from Tesla app, which seems a bit buggy for TOU filtering)

For April (which includes poor weather, and only 3 weeks of proper Powerwall operation), during 4-9pm peak time I used 207kWh, imported none, and exported 124kWh.
If I had no PW, I would have still used the 207kWh, but offset by the 124kHz local production, so imported a total of 83kWh
So the difference between no PW and 1 PW is 124+83 = 207kWh. That's at the peak rate, which for D-Prime averages to 41 cents all year.
So I'm saving $85/month or >$1000 year just looking at 3 weeks of usage in April.
The ROI is measurable, piece-of-mind off the chart.
 
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tomuo

Member
Mar 15, 2021
19
7
Los Angeles, CA
I have a different calculation arriving at the same result (numbers from Tesla app, which seems a bit buggy for TOU filtering)

For April (which includes poor weather, and only 3 weeks of proper Powerwall operation), during 4-9pm peak time I used 207kWh, imported none, and exported 124kWh.
If I had no PW, I would have still used the 207kWh, but offset by the 124kHz local production, so imported a total of 83kWh
So the difference between no PW and 1 PW is 124+83 = 207kWh. That's at the peak rate, which for D-Prime averages to 41 cents all year.
So I'm saving $85/month or >$1000 year just looking at 3 weeks of usage in April.
The ROI is measurable, piece-of-mind off the chart.
Correction - forgot to add in the cost to charge the PW, it charges from excess solar that would have been fed back to the grid at off-peak rates in the case of no PW. So need to subtract 124kWh * 18 cents.
Still at nearly $1000 / yr. I'll come back in a month and redo this calculation with numbers from May.
 

tomuo

Member
Mar 15, 2021
19
7
Los Angeles, CA
Correction - forgot to add in the cost to charge the PW, it charges from excess solar that would have been fed back to the grid at off-peak rates in the case of no PW. So need to subtract 124kWh * 18 cents.
Still at nearly $1000 / yr. I'll come back in a month and redo this calculation with numbers from May.
Now let's consider the ROI on an additional PW past the first.
Assuming that the 1st PW covers all of that peak usage and allows me to arbitrage that cost difference, the only cost benefit to the 2nd+ PW is the difference between saving the excess solar in the grid vs saving it locally.
But we already know that saving it locally has a 10% penalty on the battery round trip, so in theory saving it to the grid is actually better cost wise.
But then, we have the 2 cents per kWh non bypassable fee, which at 18c for off peak power, is higher than 10%.

18 cents becomes 18/.9 via Powerwall = 20
18 cents becomes 18+2 via Grid = 20
So assuming 500kWh of off-peak usage, I would save absolutely nothing with the 2nd Powerwall.
Zero ROI on the additional PW, only good-feelings.
 

ccrowder

Member
Mar 25, 2021
8
8
San Diego
And any time that SCE raises the peak rates, it benefits me.
I think this is one key point that folks tend to forget about when calculating ROI. When I look at how SDG&E have raised their prices over the last 5 years who knows just how high it will be in another 5 years. I was doing ok with a 5kW system and no powerwalls before since peak times were 11a-5p (6p?).. but now that we are out of our 5 year grandfathering and on the new peak hours it sucks. Putting excess onto the grid at ~0.30/kW but having to use in the evening at 0.60/kW is the reverse of what we did before. Cannot wait for my additional 4kW system and 2 powerwalls. Even with 2 it will pay itself off before the warranty is up on the powerwalls at current rates... likely sooner given past trends in rate hikes.
 
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Mar 15, 2021
172
64
California
Also keep in mind that some utility companies offer TOU plans that you can only get with a battery backup. SCE for example has their TOU prime plan which you cannot get with solar only. You must have a battery, EV, heat pump, or well pump. The prime plan lowers the off peak price 10 cents and increases the peak price. This makes the battery even more of a long term investment savings as any grid power you use during off peak cost you less and with a battery you won't have to use the grid at all during peak time
 

Southpasfan

Member
Jun 2, 2019
422
535
Pasadena
I'm not a fan, analytically, of using "Return on Investment," or ROI, when what people are actually talking about is different thing, which is "price of system divided by cost per year without system" or PSDCWS, or something.

PSDCWS is what solar salesman used to sell systems when either they didn't understand finance or they were pretty sure their customers didn't understand it either. They would take a $30,000 system and tell someone with an average $500 per month bill that "the system would pay for itself" in five years.

Even if that were a useful way of looking at it (I don't think it is), it leaves out the entire discussion of this thread, which is that in reality with a solar-only system one's bill does not go to zero anyway. And not only does it not go to zero depending on the utility TOU might end up with some pretty steep charges.

I have only seen a couple of examples of actual ROI, which would be something like this. (1) system costs $30,000 (after all rebates/credits), (2) system produces, say 20,000kwh per year, (3) 20,000 kwh per year would otherwise cost $6,900 per year for 20 years, or $138,000. Thus, since the $138k is post tax payment (for individuals) "investing" in a $30,000 solar system is the equivalent of a tax exempt bond paying 7.6% tax free.

That's a pretty good investment.

I am a fan, however, of PWs, not only for the fact that with them you don't have to worry about what the utility does with TOU pricing, but for all the other reasons which are hard to put a dollar figure on, such as (1) back up, (2) you get to use all the green power you produce, (3) you get to go completely off grid for long periods of time, (4) its a more sustainable personal power plant, by far, than solar only, which is really a subsidized part of the grid.
 
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h2ofun

Active Member
Aug 11, 2020
2,449
418
auburn, ca
I'm not a fan, analytically, of using "Return on Investment," or ROI, when what people are actually talking about is different thing, which is "price of system divided by cost per year without system" or PSDCWS, or something.

PSDCWS is what solar salesman used to sell systems when either they didn't understand finance or they were pretty sure their customers didn't understand it either. They would take a $30,000 system and tell someone with an average $500 per month bill that "the system would pay for itself" in five years.

Even if that were a useful way of looking at it (I don't think it is), it leaves out the entire discussion of this thread, which is that in reality with a solar-only system one's bill does not go to zero anyway. And not only does it not go to zero depending on the utility TOU might end up with some pretty steep charges.

I have only seen a couple of examples of actual ROI, which would be something like this. (1) system costs $30,000 (after all rebates/credits), (2) system produces, say 20,000kwh per year, (3) 20,000 kwh per year would otherwise cost $6,900 per year for 20 years, or $138,000. Thus, since the $138k is post tax payment (for individuals) "investing" in a $30,000 solar system is the equivalent of a tax exempt bond paying 7.6% tax free.

That's a pretty good investment.

I am a fan, however, of PWs, not only for the fact that with them you don't have to worry about what the utility does with TOU pricing, but for all the other reasons which are hard to put a dollar figure on, such as (1) back up, (2) you get to use all the green power you produce, (3) you get to go completely off grid for long periods of time, (4) its a more sustainable personal power plant, by far, than solar only, which is really a subsidized part of the grid.
With my solar only, and a 99% electric house, my True up bill is going to be zero, technically more generation, so it can be done.
 

ccrowder

Member
Mar 25, 2021
8
8
San Diego
have only seen a couple of examples of actual ROI, which would be something like this. (1) system costs $30,000 (after all rebates/credits), (2) system produces, say 20,000kwh per year, (3) 20,000 kwh per year would otherwise cost $6,900 per year for 20 years, or $138,000. Thus, since the $138k is post tax payment (for individuals) "investing" in a $30,000 solar system is the equivalent of a tax exempt bond paying 7.6% tax free.
That works out to only $0.34 a kW.. Wish SDGE was that cheap in the summer evenings when solar does not produce as much =)
Yes.. winter is slightly less at around 30 cents a kW but peak summer rates are 60cents a kW when you get home and want to turn on the AC. But yes.. it may be possible to suck it up and pay for grid power while investing that money elsewhere for a good return to cover the difference.. but SDGE has not showed any slowdown in their rate hikes.. who knows what another 5 years will bring.
 

Southpasfan

Member
Jun 2, 2019
422
535
Pasadena
With my solar only, and a 99% electric house, my True up bill is going to be zero, technically more generation, so it can be done.
h20,

I wasn't saying it couldn't be done. For many utilities the minimum charge is not much and it all works, like yours.

What I am saying is that Powewalls are the only way to use all the electricity you produce. Whether one should "care" about that is a function of (1) the economics of NEM pricing, which is the rabbit hole this thread and others goes down, and (2) how a person feels about backup, grid independence, and green energy, which are things that are hard to put a dollar amount on.

With Tesla's current pricing, you can get a system and PWs for about 16 cents per kwh. You can get solar alone for less, maybe 10 cents per kwh, but then you may have some additional NEM TOU calculations to do.

For me, 16 cents is less than I was going to pay anyway, end of analysis. I don't really look at someone who elects panels only to have gotten the same product. With panels only you are as dependent on the utility as ever. You will lower your costs though.

With panels and PWs you can be pretty close to 100% green and independent of the grid.

Making it through peak period is attractive and saves money. Making it through the whole day for months at a time has an incremental cost, but for many its cheaper than the status quo anyway.
 
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h2ofun

Active Member
Aug 11, 2020
2,449
418
auburn, ca
h20,

I wasn't saying it couldn't be done. For many utilities the minimum charge is not much and it all works, like yours.

What I am saying is that Powewalls are the only way to use all the electricity you produce. Whether one should "care" about that is a function of (1) the economics of NEM pricing, which is the rabbit hole this thread and others goes down, and (2) how a person feels about backup, grid independence, and green energy, which are things that are hard to put a dollar amount on.

With Tesla's current pricing, you can get a system and PWs for about 16 cents per kwh. You can get solar alone for less, maybe 10 cents per kwh, but then you may have some additional NEM TOU calculations to do.

For me, 16 cents is less than I was going to pay anyway, end of analysis. I don't really look at someone who elects panels only to have gotten the same product. With panels only you are as dependent on the utility as ever. You will lower your costs though.

With panels and PWs you can be pretty close to 100% green and independent of the grid.

Making it through peak period is attractive and saves money. Making it through the whole day for months at a time has an incremental cost, but for many its cheaper than the status quo anyway.
Agreed. With my PW's now, I am 95% off the grid. I send back all my solar during peak, and after about noon when the PW's, are 100% charged. Once I turn on the AC, that will change. And winter, do not want to think about it.

We all elect and justify spending our money in different ways! I currently have 15K solar on the roof. Looking at maybe adding 15K more, if I can get approved. TOTALLY A STUPID, non ROI emotional possibility. If not for NEM3, would never have considered.

The one real plus of not getting batteries is one is not forced onto EV2-a TOU. Its nuts
 

SaveOurPlanet

Member
Jan 6, 2018
237
140
San Francisco
I do not weigh ROI on things in my life, I think people should just do whatever make them feel good. As a matter of fact, the only ROI that counts in my life is my portfolio, that enables me to not think of ROI on anything else and enjoy life. I will never ask myself what is the ROI on first growth Bordeaux, Black label Armani suits. Gucci loafers, Ferragamo Oxfords or Michelin starred restaurants. If I can pay then just enjoy, if I cannot pay then there are always something else that would make me just as happy. So just do whatever makes you happy and try not to count pennies, life is too short. BTW, I am happy with my 2 PWs and never tried to figure out what is their ROI, not worth my time.
 

holeydonut

Supporting Member
Jun 27, 2020
1,508
881
East Bay NorCal
I do not weigh ROI on things in my life, I think people should just do whatever make them feel good. As a matter of fact, the only ROI that counts in my life is my portfolio, that enables me to not think of ROI on anything else and enjoy life. I will never ask myself what is the ROI on first growth Bordeaux, Black label Armani suits. Gucci loafers, Ferragamo Oxfords or Michelin starred restaurants. If I can pay then just enjoy, if I cannot pay then there are always something else that would make me just as happy. So just do whatever makes you happy and try not to count pennies, life is too short. BTW, I am happy with my 2 PWs and never tried to figure out what is their ROI, not worth my time.


Yeah... so you're the reason Assembly Bill 1139 was written. These lawmakers want to make all of us pay more for the privilege of having solar since you have enough money to not care.

Lorena Gonzalez is going to quote you in her next speech... (yes, this is sarcasm).
 
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SaveOurPlanet

Member
Jan 6, 2018
237
140
San Francisco
That's the thing, solar is not for the privileged, actually it is for everyone that cares about the environment. I believe some states have community solar program that actually benefit the under-privileged. As a matter of fact, I've been telling friends and relatives that even if they do the month to month solar rental with Tesla they will save money over PG&E and not a dime of upfront money. I was trying to make a point of not looking at PW in terms of ROI. Oh well.
 
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Southpasfan

Member
Jun 2, 2019
422
535
Pasadena
Yeah... so you're the reason Assembly Bill 1139 was written. These lawmakers want to make all of us pay more for the privilege of having solar since you have enough money to not care.

Lorena Gonzalez is going to quote you in her next speech... (yes, this is sarcasm).
Isn't the good news that if Tesla can manage to supply PWs to existing customers that the PWs will be cost efficient?

I actually read the bill. I am not sure if anyone pointed out the following -- sure, its accurate to say that customers who install rooftop solar only are "subsidized" by getting peak credits which, since the burden has to go somewhere, falls on those without personal solar.

However, its just as accurate to say that those with personal solar subsidize the zero effort which all ratepayers without personal solar are putting into transforming the overall grid from whatever percentage green to 100% green. There is a chart in the bill which, I somehow can't believe ($3 billion per year in peak costs not paid by those with personal solar?, it looks like three thousand in millions, anyway) but assuming its true what about all the green energy pumped back into the grid by personal solar owners? All of that is a "subsidy" to ratepayers who are doing nothing towards shifting to green energy.

So what is more important? Lower bills or more clean energy? I'm not blaming anyone who either rents or can't afford solar (although anyone can afford solar unless your credit rating prevents it) but that group is doing exactly zero to move us to sustainable energy, other than maybe voting for it.

In any event, Powerwalls are the answer.

And by the f-ing way, why is power so ridiculously expensive on peak anyway? Its not that way in LADWP. Sure, the cost of electricity goes up, but not as much as the rates. Its just a higher tariff to discourage use.

Since PWs are the functional equivalent of simply not using the energy, its the best solution.
 

Southpasfan

Member
Jun 2, 2019
422
535
Pasadena
Ok, this is where it lands. I even read the underlying report on which the relatively sloppy bill summary was based.

I have not idea where this ends up, however, the value of solar during the middle of the day is not going to get more valuable.

Given how poorly the investor owned utilities behave, NEM credits are only going to get less generous (regardless of this bill) and peak energy needs are what they are.

If I did the math right, though, about 3, 000 PWs, used daily, would eliminate the need for about one peaker plant. The Vaca Dixon plant t puts out about 10 megawatts at 6 pm and you would need 3,000 pws at 3kwh per to equal that.

Maybe someone ought to point out that you could just forget about us wealthy solar adopters and give free powerwalls to CARE clients, who could charge them when rate are low and then not pay peak rates anyway.

Which, come to think of it, is what we pretty much proved in this discussion! :)
 

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