Elon, I love you... but the PowerWall isn't that great...... yet.

[Merrill]

I was thinking about the Tesla PowerWall a lot today, and a lot of that has been in perspective of my own solar project that many of you have been keeping an eye on. So, prepare for a bit of a ramble...

I think there is a large distinction that isn't readily made clear by the presentation and the current website for the PowerWall.

The PowerWall page currently says 10kWh for $3500, or $350/kWh. Sounds pretty decent considering it has integrated BMS, cooling, etc, right? However, the caveat makes this is a mostly useless product for basically everything last night's presentation was about.

• Caveat: The 10kWh unit is "For backup applications."

Let's focus on that for a moment. So it's basically a 2kW standby generator with 5 hours of fuel at the rate (10 hours at 1kW, 20 at 1/2 kW, etc). Well, that's probably good if you have a very small mandatory loads panel for your refrigerator and a few lights... but not much, if anything else. It's also only good if you get a lot of power outages and they generally last for short periods. Be without power for more than a day, tops, and a PowerWall will be a PowerlessWall. You could combine this with a half decent solar setup to extend this, but people looking at it for backup purposes (perhaps thinking they could replace a standby generator) are going to be pretty disappointed when their electrician tells then they'll need five or more of them to come close to what a standby genset will handle. Keep in mind, these units only put out 2kW. You can get a 2kW standalone gasoline generator for something like $200 nowadays...

So for backup power I think it has severely limited applications as a single unit. It would take several to equal a small standby generator and people without solar would have no way to top off the tank, so to speak, and would be dead in the water after the packs were depleted.

Now I know, Elon means for this to be used with renewables. Makes sense. This would be fine for someone who already has or is getting solar if they want to be able to run their refrigerator and a few lights while the grid is down. I think the backup version, however, has some pretty narrow use cases in the current market.

• Daily Cycling 7kWh Unit

So let's look at the real product. The unit that can be cycled daily for balancing your power use and shifting power use to off-peak and all of the other wonderful things that were mentioned in the presentation that have absolutely nothing to do with the $3500 unit.

The 7kWh unit is the one that can be cycled daily and be used for these things, and costs $3000 or about $429/kWh. $429/kWh is pretty expensive. To get that money back at average USA utility rates of $0.1229 per kWh you'd need to use that $429 kWh about 3500 times just break even on the unit cost. At a daily cycle that means load shifting 7kWh per day at 100% efficiency (site claims 92%) for almost 10 years just to be neutral with the cost of the 7kWh unit. But wait, there's more...

The PowerWall site specifically says that the unit does not include an inverter. (I ignored this for the previous backup use cases above because it's impractical even without adding this on top.)

If you have grid tied solar already you may already have a grid-interactive inverter that is capable of accepting power from the PowerWall and feeding it into the grid as you desire. Great. It'll still take you over 10 years, average, just to cover the unit cost.

• Grid Arbitrage

For those who do not, or are thinking about just shifting loads from on-peak to off-peak, you'll need an inverter. A "dumb" 2kW grid-interactive inverter isn't all that expensive, but it will lengthen your ROI time none the less. Get one that can actually be smart about how it handles this battery power and you could approach doubling that time.

This is all assuming the PowerWall unit has within itself the means to charge from the grid. If you need an AC->DC converter, then add in that cost (unknown at this time if the PowerWall has this).

• Real world numbers

Now, let's say you've gone through all of this and you're still ready to jump on board.

The EIA says that the average American residence consumes about 10,908 kWh worth of electricity per year. That comes out to about 30 kWh per day, or an average load of about 1.25kW 24 hours per day, 7 days per week. The load will obviously vary, but we'll use this as a base. Using this, we can determine that it is likely that during the on-peak, off-peak, and solar peak times of day that there will be enough time in any of them to charge or discharge this 7kWh battery pack fully in any of those three time frames for the average American.

Hypothetical: Let's say that the household has a large enough solar array to produce their yearly kWh usage over the course of a year so their net usage is 0 kWh. Nice! Well, now the utility company gets smart (like many are) and isn't paying this household the same per kWh feed in as they are charging for feed out. We'll say they're nice and only charging $0.10 more per kWh used than sold.

Well this household is in a good position to benefit from the Tesla PowerWall, right? Let's run some numbers.

Assuming a good solar day, the Tesla PowerWall would be fully discharged at the beginning of the day because night time usage would have exceeded 7kWh on average. During the day enough solar power is generated so that all household loads are powered by local generation, and an 7kWh of what would normally have been put into the grid would be generated to charge the Tesla PowerWall. For the sake of simplicity we'll ignore what may have been generated beyond that and just say it was self consumed (best case).

The sun goes down and it's go time. Good stuff. Tesla PowerWall uses it's power to offset loads........ up to 2kW. Anything beyond that will still need to buy power from the grid (HVAC, cooking, hot water, EV charging, etc). But, we'll still be able to offset 7kWh worth regardless over the next 3.5+ hours. Now, 7kWh of usage later we are back to being dependent on the grid. No problem... we just saved $0.70 today, right?

$0.70/day payback on $3000 + inverter + install................. not looking so good for ROI at 12+ years.

• What would it take to be useful?

Now these are all just contrived examples. What would it really take for this to make sense?

To ROI in 7 years (a reasonable long term investment, beyond this it's best to invest elsewhere usually) the unit would have to save about $1.18/day @ $3000. This isn't counting installation and other equipment needed. At a daily cycle, 7kWh per day, those kWh would each need to save at least $0.168 after efficiency losses. This would need to happen every single day, 100% discharge, 100% charge, to payback in 7 years. And this is just to get your money back.

So, if you have solar and you get paid at least $0.17 less per kWh fed into the grid as you pay per kWh out of the grid, this could work for you. Unfortunately, this is a small subset of solar users so far. A lot of incentives work the opposite and pay more for solar input than charge output. Most just credit the same both ways (net metering) currently. If you're on time of use metering and your on peak is $0.17 more than off peak, and you already have the rest of the equipment needed to make this work for arbitrage, then this could work for you.

The above, in my research, appear to be a pretty small subset of residences. I'm not sure how this can appeal, financially, to users who don't already have the related and required equipment to make this useful.

• Going off-grid with PowerWall

"But wk, what about using these to go off grid like Elon said?"

First, let's get one thing straight. One of these units is not going to let any normal user go completely off grid. Not one person. Don't kid yourself. 7kWh per day is 23% of what the average American household uses. Double that for daytime solar generation and consumption and you're still at less than half of average. Sure, some people can make that lifestyle work, but that's not mass market. I'm not talking about the people who are willing to make large changes in usage to fit this product into their lives. I'm talking about the regular people who just want something that works for them, not the other way around.

Then, you have to factor that there are days where there is little to no sun. (My worst solar day since testing has been 4% of expected average output... which is really bad and might as well be nothing.) You're going to need a couple of days of reserve power. At half of the average American usage (15kWh) and two days of reserve (probably bare minimum for an off-grid use case) you're looking at 4 or 5 of these PowerWall units *minimum* at half of average usage. Moving back to the average USA residence and that's more like the maximum of nine units just to have normal use + a "two rainy days in a row" reserve. Oh, and nine of these is $27,000 and is almost as powerful as 100A grid service (usually the feed for small homes with non-electric HVAC and cooking/hot water).

Long story short, without major energy efficient life style changes most people would not be able to go 100% off grid even with nine PowerWalls. You could get really close, but at some point you'll run out.

• My setup

I have avoided comparisons to my own large off-grid solar project to keep this write up as general as possible. However, I feel that it's worth pointing out that my complete setup including everything (100+ solar panels, 8 inverters, installation, and 191kWh worth of Tesla batteries) is costing somewhere around $500/kWh total per kWh of energy storage. Extracting just the parts that the $3000 PowerWall does for that price, those parts cost about $250/kWh (mainly the batteries). Even if my setup were lead acid based (the "ugly" and "smelly" batteries that Elon compared to in his presentation), which it doesn't, the cost would still be significantly less per kWh than a complete system using PowerWalls.

Not only that, many solar installers already do these battery setups. They may not be in a sleek and fancy case like Tesla's, but your wallet will be better for it.

I have 191 kWh of storage (that's the same as ~28 of Tesla's 7kWh units......) because that's what it will take to get me through a string of a few rainy days at average or slightly below average usage and to stay off grid as long as possible. Grid electricity is only $0.10 per kWh where I am, so no amount of grid arbitrage would ever pay for itself here. Honestly, my project is beyond what I probably should have invested in it. Unfortunately I was already too far into it to not follow through once that became apparent.

• Summary, TLDR version

I'm still confused as to why the unit *doesn't* include a grid interactive inverter (similar to a small Outback Radian)... if it did, at the same price point, I may be writing something entirely different.


Anyway, the TLDR summary version: PowerWall sounds cool, it just doesn't really seem practical for the masses. It makes no sense for the average person to put one of these daily cycle units on their garage wall, especially considering the cost of the additional equipment needed to make it work as advertised. It sounds good at first glance, but if you really run the numbers for your situation.... it probably doesn't make as much sense as you might think.

Feel free to post some numbers for your situation (on-peak/off-peak rates and times, solar buy sell prices/etc) and I'll do some crunching for you if you like in the thread and we'll see how far off I am with all of this.

I hope my dissenting opinion on the PowerWall doesn't bring about immediate hatred. I think that most people who have followed my projects here and elsewhere would know that at least somewhat know what I'm talking about and I wouldn't take such an opinion blindly.

(Keep in mind this is about the PowerWall residential unit. I do believe the commercial unit (PowerPack) has huge potential.)

Thanks for the detailed analysis, before I give you my stats for my solar system wonder if you can tell me if you store energy in the Powerwall would I then lose the peak generation credit from my tou 12pm to 6pm because for me this is the only reason my 4.5kW system works for me. If I do not get that credit because it went into storage the Powerwall would not make sense.

 

[Owner]

Great post and thread.

I'm out of state with the Tesla and contemplating this on the side. I have come to most of the same agreements as you did. I will post my info eventually but probably not in a couple of weeks.

 

[miimura]

I just ran the numbers for simple grid arbitrage for PG&E's EV rate schedule. The $/kWh is the difference between Peak and Off-Peak in this rate schedule.
7kWh/day * $0.328/kWh * 184 Summer Days = $422.32 per Summer Rate period
7kWh/day * $0.191/kWh * 181 Winter Days = $241.96 per Winter Rate period
So, if you assume the system cost is $5,000 with no solar or tax credit or anything, the payback is about 7.5 years. This is getting close to the reasonable payback period that WK mentioned in the first post. I would never spend the money just for arbitrage, I would design the system so that it was more useful, but this at least helps pay for it.

 

[liuping]

Thanks for the detailed analysis, before I give you my stats for my solar system wonder if you can tell me if you store energy in the Powerwall would I then lose the peak generation credit from my tou 12pm to 6pm because for me this is the only reason my 4.5kW system works for me. If I do not get that credit because it went into storage the Powerwall would not make sense.

This is a real problem in California. With TOU rates in San Diego, we get a credit of $.47 /kWh during peak, and pay $.22 for off peak and $.18 of super off peak.

The only way power wall would save any money for us is to use the battery during peak rate times (noon-6) so we get more credits and charging it off peek. But that is exactly what the grid does NOT need.

There is already a issue during with sunny days with too much power going to the grid. Using Powerwall we would be putting more on the grid during that time.

If we do the "right thing" for the grid, we'd charger during the noon - 2pm peak production (when everyone is dumping solar on the grid) and use the powerful during the evening spike that is outside the peak rates. But that would mean giving up the $.47 credit for the power used to charge the powerwall and using then using it during the $.22 rate time, effectively costing us $$.25 per kWh.

 

[Owner]

My thoughts on arbitrage:

1. Trying to play arbitrage for say $500 - $1K a year ?
2. The hassle with the system no matter how smooth will take effort.
3. Pay back of 7-10 years with a 10 year warranty?
4. PG&E is a monopoly folks.
5. The rates are not under your control and can change under your feet.

Doing this instead of playing poker?

Doing this to help the earth, well that might make sense!

 

[miimura]

This is a real problem in California. With TOU rates in San Diego, we get a credit of $.47 /kWh during peak, and pay $.22 for off peak and $.18 of super off peak.

The only way power wall would save any money for us is to use the battery during peak rate times (noon-6) so we get more credits and charging it off peek. But that is exactly what the grid does NOT need.

There is already a issue during with sunny days with too much power going to the grid. Using Powerwall we would be putting more on the grid during that time.

If we do the "right thing" for the grid, we'd charger during the noon - 2pm peak production (when everyone is dumping solar on the grid) and use the powerful during the evening spike that is outside the peak rates. But that would mean giving up the $.47 credit for the power used to charge the powerwall and using then using it during the $.22 rate time, effectively costing us $$.25 per kWh.

If the "right thing" is true, then the rates need to migrate their hours to reflect this. PG&E is on the way to this - Peak is 2pm-9pm on the E-9 and EV rate plans. That partially addresses the "duck curve".

 

[wk057]

I'll try to throw out some notes and responses.

yes... and no, but mostly yes I agree with you.

Firstly you missed another big negative of going off grid... Summer versus winter.
You aren't going to store energy from summer to use in winter (completely uneconomic).
So you need to size your solar to work for winter in general. This produces a great deal of extra energy in summer (assuming you don't live in the tropics). If you are doing that, why not sell the excess back to the grid for a profit?

However, having said all of this, nobody is "average", noone has 2.3 kids (etc)......
The product isn't for everyone or even the average person (i.e. I fully agree with you) ... almost no product is... but there are a significant number of people who aren't average that this would work for. If 1 in 10 US families are appropriate for this right now, the gigafactory will be busy for quite some time. With increasing solar penetration, the use cases will increase and this can stop solar killing the grid.

For the 7kwh box...
1) Hawaii. If your electricity costs $0.50/kwh and your excess solar can't go to the grid then the economics are great. Install solar and batteries for your average usage and reduce your utility bill 90%.
2) CA. Incremental electricity price for most people is $0.30-$0.40/kwh. California has many people. CA may push everyone to time-of-use. Much of CA has time of use differential of $0.20/kWh. Do the numbers look any different for that?
3) Other places with expensive electricity...
4) Many people have solar... Solarcity has already announced this as an option for new systems and would be installed next to the existing inverter. This drops incremental installation and inverter cost to almost nothing.
5) off-grid... while this doesn't help you to cut the cord, it will be the method of choice if you build remotely and the utility wants to charge you $100k to connect then instead spend $60k on batteries and $30k on solar. [unless another brand of batteries is cheaper and better?] This may even spark more remote building as some things become viable.
6) The threat of cord cutters, and the threat of small users. Regardless of whether a utility has net metering, this gives everyone the chance to be a small user. This pushes infrastructure costs to everyone else. Utilities are pushing back by trying to charge small users more. If they do that enough, cord cutting can start, threatening to snowball into having the same infrastructure costs over less customers. Overall, this can change the attitude of utilities... they can't make solar a profit center for themselves, or the users go away. Then solar and users win.
For the 10kwh box:
1) An off-grid can supplement some 7kwh boxes with some 10kwh rainy-day boxes (cycling once per week).
For backup:
1) I just had my backup generator serviced after it failed in the last outage. Almost cost $3k. Can we factor maitanence in? Do the numbers improve?
2) I'm ok with 2kW if this allows me to use the full power of my solar panels as well while the sun shines. With some smarts in there I could still run my a/c while the sun shines, which I can't on my generator.

For the utility box (also used by many large businesses).
When the "duck" curve gets worse, as it will, this box can save the solar industry (e.g. Hawaii now, CA in 2 years).
Overall wholesale electricity rates become more smooth and more predictable.

I hadn't thought about using the 10kWh units as the rainy day backup, so maybe that's a good call. But they would still be cycled on non-rainy just cloudy days more than a typical backup solution. A cloudy day for me will easily drop me to 50% production.

As for comparison to a backup/standy generator... there is no comparison IMO. The least expensive standby unit I can pick up from Home Depot is $1900 and puts out 7kW (the same as 3.5 PowerWalls) for as long as I provide fuel (cheap). As a backup solution for anything but just the refrigerator the PowerWall is a pretty terrible solution.

There are certainly some places where the ROI might make more sense (Hawaii like you mention and maybe some parts of California), and there are certainly a select few who could stay under 2kW usage and 7kWh/day...... but these are not normal use cases.

Ok, I went back and did a lot of data lookups. For this analysis I crunched 2 months of power usage data (Juli and January) and I looked at our daily usage pattern. The bottom line is that the Powerwall would allow us to go off-grid for about 8-9 months of the year resulting in significant cost savings.

I also looked at our daytime vs. nighttime consumption and found that we never exceed 7 kWh of nighttime usage, typical number is about 4 kWh. So the Powerwall has the capacity to shift enough solar power from midday to the night. Basically we can cover all of our power needs with solar because of the Powerwall.

And now for the numbers. First up daytime power consumption vs nighttime. In this context daytime means sunrise to sunset.

Representing summer:

5th of May: daytime: 6.48 kWh, nighttime: 3.18 kWh.

And in the winter corner:

5th of January: daytime: 6.37 kWh, nighttime: 6.28 kWh.

I chose these days as examples because they were worst-case within that particular month. It is clear that we would never exceed the amount of power that the Powerwall can provide. Also I was unable to find a single instance of our house drawing more than 1,05 kWh of power at any one hour. So the 2 kW specification + 3kW peak is well within what would be needed.

I used a small 4.32 kWp solar-array as an example. This is the kind of array that would fit on our small roof. We could probably fit a 6 kWp array but that would just make all the numbers even better .

Next up is savings pr. month. In the summer would be around $160 in electricity, 30-40% of that comes from the powerwall shifting the power resulting in powerwall savings of about $56 pr month in summer. In the wintertime the savings are $8 pr month. On average we are looking at around $33 pr month in savings from powershifting. Multiply up to a year and we have about $400 pr year in savings from the powerwall alone and it pays for itself in ~8 years. Basically it will be under warranty the whole time and if you buy the extended warranty you should be good for another 10 years.

This is all based on current prices for electricity and the price is expected to go up over the next 10 years and the payout from selling power to the grid is expected to fall about 50-60%. At that point the powerwall becomes a complete no-brainer here.

I simulated expected changes to power-pricing and the monthly saving went up to $50 per month for the powerwall , now it would pay for itself in 5 years.

In closing here are some stats:

January:
Total power used: 362 kWh.
Solar power generated: 72.9 kWh.
Max daily usage: 17
Min daily usage: 7

July:
Total power used: 265 kWh
Solar power generated: 576 kWh
Max daily usage: 14 kWh
Min daily usage: 4 kWh.

Now I'm off to figure what is so power hungry in our house, 17 kWh is just embarrassing. Watch out, something is about to get turned off for good.

PS. If anybody wants a copy of my spreadsheet let me know. Most of it is in danish, you have been warned

I corrected my post to correctly reflect that we can use about 2-3 kW at peak loads. That includes stove and waterboiler. The house i heated by "remote heating" which means that we use surplus water from the local power plant for heating. This is probably a little unique to Denmark, but it provides efficient heat and warm water.

Your usage is so low I don't know how you function. lol. When I'm not home my house "idles" at 30-40kWh/day. Granted it's a large home, but I have no hope of being anywhere near as low as the usage you supplied and neither would the vast majority of Americans or others who could actually afford a PowerWall.

Also, even with your use case... an 8+ year ROI still makes little sense as an investment, and you didn't factor in installation or the other needed equipment. Again, this thing does basically nothing by itself.

I was thinking about getting this and didn't know what the difference between the 7 and 10kWh versions was so awesome post. Thanks.

For summer the cost goes to a little over 40 cents in late afternoon to 20 cents from 6-9PM the back to 6 cents at night. Winter rates are between 5-8 cents all day. I don't have solar but may someday.

Would some combo of a backup battery (no generator on site) with a few 7kWh packs make any sense?

In the Winter this would make zero sense for you. In the summer, yeah I could see some grid arbitrage going on to bring 7kWh of usage per unit over the $0.06 rate vs the $0.40 rate, assuming you normally use 7kWh+ in the $0.40 area. However, you'll need an inverter to do this. Even assuming the inverter costs $0 and you can do this for 180 days per year, you'd be able to shift at most 1260 kWh per year at a savings of up to $428.40 per year. Factor in realistic efficiency losses and you're looking at closer to ~$360/yr. So still at a minimum 7 year break even with 100% efficiency (lol) a free inverter (I want to know where you get this one) and free installation (well, I can appreciate DIY). Still wouldn't make sense. If your rates had a $0.34 delta all year long, then maybe it would be closer to making sense.

Adding in solar, you'd already be shaving peak usage while reducing the overall cost. Solar *without* the PowerWall would seem to make the most sense in your situation.

there is another key difference with Tesla solution compared to other battery based offerings.

Li Ion - lifetime should be 10yrs+
Lead Acid batteries I believe only have half this lifetime (correct me if I am wrong here)

Lead acid isn't the only show in town. Lithium based alternatives are somewhat reasonable nowadays also.

Then there are second hand batteries becoming more and more available, including salvaged Model S batteries, Volt batteries, Leaf batteries, etc which still have long lives ahead of them in a stationary storage use case and are even cheaper than lead acid per kWh at times.

In Minnesota, Xcel customers, can get on their time of day plan, basically 2 cents at night. Normal customers are about 12 cents 24/7/365. Time of day subscribers pay about 16 cents 9a-9p. If you can save 10 cents per kwh, and use 1000kwh/month, you're looking at $100/month in savings.

I calculated a 4.5%, 5 year loan for $8,000 for two Powerwalls plus install, 93.22/month.

So, for the first 5 years you are ahead ($7/month/12month/5years) $420.

Assume you get another 5 years after that, 10 year warranty, with no loan payment and still saving $100 month. (5*12*100) = $6,000.

$6,420 ahead after 10 years. If the batteries are still doing their job it only gets better.

I do realize that a 1000kwh/month user will have a hard time staying at 14kwh 9am/9pm, but they could probably make it happen. Weekends and holidays are at the 2 cent rate 24 hours.

You guys tell me if I have made errors, because this looks compelling to me...I should say that I am a bit of a Powerwall skeptic.

OK, so, if you use 1000kWh per month, that's pretty close the the national average (a little over). With a $0.10 delta between time of use rates (the exact example I used in the first post, actually) you're looking at a savings of *at most* 7kWh per day, or $0.70 per day per PowerWall (14 kWh per day, or $1.40 per day with two). That comes out to $21.29/mo savings per month per PowerWall on average ($42.58/mo savings for two). This is at 100% efficiency, which is impossible... so keep that in mind because these are super best case numbers. So if you could get two installed at $93/mo, you're behind about $50/mo. By the time you pay the loan in 5 years, you'll be behind about $3000 vs just not having done anything at all. Then you start turning a small profit by saving the $42/mo and would still take another six years to break even. Add about 15% to all of these times to take into account realistic efficiency losses.

Basically, you can't shift 1000kWh per month with a couple of PowerWalls. You'd need about five of them to do that, at which point you'd be WAY beyond a reasonable ROI.

Thanks for the detailed analysis, before I give you my stats for my solar system wonder if you can tell me if you store energy in the Powerwall would I then lose the peak generation credit from my tou 12pm to 6pm because for me this is the only reason my 4.5kW system works for me. If I do not get that credit because it went into storage the Powerwall would not make sense.

You would lose it, yes. If the utility is paying you the on-peak rate for pumping on solar.... stick with that. You have no incentive to self-consume (what the PowerWall would enable) whatsoever.

I just ran the numbers for simple grid arbitrage for PG&E's EV rate schedule. The $/kWh is the difference between Peak and Off-Peak in this rate schedule.
7kWh/day * $0.328/kWh * 184 Summer Days = $422.32 per Summer Rate period
7kWh/day * $0.191/kWh * 181 Winter Days = $241.96 per Winter Rate period
So, if you assume the system cost is $5,000 with no solar or tax credit or anything, the payback is about 7.5 years. This is getting close to the reasonable payback period that WK mentioned in the first post. I would never spend the money just for arbitrage, I would design the system so that it was more useful, but this at least helps pay for it.

That's a pretty decent difference, well above average. So you'd be looking at about $660 in savings per year at 100% conversion efficiency. The PowerWall itself claims 92% round trip efficiency, and the best inverters are in the low to mid-90s%. Lets say 95% for fun, so 0.92*0.95 = 0.874, times $660 = $576/yr realistic savings per PowerWall. 5.2 year ROI on the PowerWall itself not counting install and a suitable inverter and grid charger. More than likely even at these ridiculous TOU rate differences it would *still* push it beyond a 7 year break even.

Honestly, if it can't reach a reasonable ROI in this situation, which is virtually tailored for the PowerWall, than it *really* is not worthwhile for such things.

My thoughts on arbitrage:

1. Trying to play arbitrage for say $500 - $1K a year ?
2. The hassle with the system no matter how smooth will take effort.
3. Pay back of 7-10 years with a 10 year warranty?
4. PG&E is a monopoly folks.
5. The rates are not under your control and can change under your feet.

Doing this instead of playing poker?

Doing this to help the earth, well that might make sense!

Yeah, I'd rather play poker or stocks than just pay to help the grid. The gambling aspect of grid arbitrage is that the grid provider can change rates after you've invested.

The only way to isolate yourself from that risk would be to go completely off grid.... which as previously described isn't going to happen with a couple of PowerWalls.

- - - Updated - - -

I feel it's worth pointing out that, all of this said..... I'll probably still buy one. If for no other reason than to just tinker with it. I kind of want to tear one down....

 

[mwulff]

Your usage is so low I don't know how you function. lol. When I'm not home my house "idles" at 30-40kWh/day. Granted it's a large home, but I have no hope of being anywhere near as low as the usage you supplied and neither would the vast majority of Americans or others who could actually afford a PowerWall.

Also, even with your use case... an 8+ year ROI still makes little sense as an investment, and you didn't factor in installation or the other needed equipment. Again, this thing does basically nothing by itself.

And would you believe that we even have a fridge, a tv and two computers. I'll admit that we have a small house and we don't use electric heating and no air conditioning. Our house idles around 2 kWh per day when we are on vacation just for comparison, a little bit more in the wintertime.

I didn't factor in the other equipment as I would buy that with the solar installation and add it to the price there. But there is an added cost. There is also a lot of value to my wife and I in the "off-the-grid and on our own clean energy" factor, but that is not really something you can put a pricetag on.

 

[wk057]

There is also a lot of value to my wife and I in the "off-the-grid and on our own clean energy" factor, but that is not really something you can put a pricetag on.

As someone who has gone a bit overboard in such a solution already, this I can agree with.

 

[mwulff]

As someone who has gone a bit overboard in such a solution already, this I can agree with.

I love the work you did with your system.

 

[Evbwcaer]

Originally Posted by Evbwcaer
In Minnesota, Xcel customers, can get on their time of day plan, basically 2 cents at night. Normal customers are about 12 cents 24/7/365. Time of day subscribers pay about 16 cents 9a-9p. If you can save 10 cents per kwh, and use 1000kwh/month, you're looking at $100/month in savings.

I calculated a 4.5%, 5 year loan for $8,000 for two Powerwalls plus install, 93.22/month.

So, for the first 5 years you are ahead ($7/month/12month/5years) $420.

Assume you get another 5 years after that, 10 year warranty, with no loan payment and still saving $100 month. (5*12*100) = $6,000.

$6,420 ahead after 10 years. If the batteries are still doing their job it only gets better.

I do realize that a 1000kwh/month user will have a hard time staying at 14kwh 9am/9pm, but they could probably make it happen. Weekends and holidays are at the 2 cent rate 24 hours.

You guys tell me if I have made errors, because this looks compelling to me...I should say that I am a bit of a Powerwall skeptic.




OK, so, if you use 1000kWh per month, that's pretty close the the national average (a little over). With a $0.10 delta between time of use rates (the exact example I used in the first post, actually) you're looking at a savings of *at most* 7kWh per day, or $0.70 per day per PowerWall (14 kWh per day, or $1.40 per day with two). That comes out to $21.29/mo savings per month per PowerWall on average ($42.58/mo savings for two). This is at 100% efficiency, which is impossible... so keep that in mind because these are super best case numbers. So if you could get two installed at $93/mo, you're behind about $50/mo. By the time you pay the loan in 5 years, you'll be behind about $3000 vs just not having done anything at all. Then you start turning a small profit by saving the $42/mo and would still take another six years to break even. Add about 15% to all of these times to take into account realistic efficiency losses.

Basically, you can't shift 1000kWh per month with a couple of PowerWalls. You'd need about five of them to do that, at which point you'd be WAY beyond a reasonable ROI.

WK057, what you are missing here is that ALL of the energy used in this case is 2 cents/kwh, not just what the Powerwalls consume off peak, so you need to roughly double your savings estimate.

 

[wk057]

WK057, what you are missing here is that ALL of the energy used in this case is 2 cents/kwh, not just what the Powerwalls consume off peak, so you need to roughly double your savings estimate.

How do you figure? The PowerWall is only capable of shifting 7kWh per day no matter how you slice and dice it.

Edit: I did misread the numbers and used 12c on-peak instead of 16c on peak. Let me recalculate... still not a huge difference in any case.

$0.14/kWh delta @ 7 kWh/day = $0.98/day per PowerWall, or $29.80/mo per PowerWall. $59.60 for two still puts your loan at a loss of $33.62 per month on two PowerWalls.

I don't see where you're coming up with doubling those numbers...

 

[MassModel3]

My SolarCity install just went online in February. It wasn't until it was installed 6 weeks earlier that I realized that when grid power is down, solar goes offline. For me, having just the 7kW unit makes sense simply because it means that if we have a storm blow through and take down power for 3+ days, I still get to use my solar panels during the day, plus I get at least lights at night. Add to that owning a Tesla, I'd also be able to charge my primary means of transportation during long term power outages. For me, the ability to do that far outweighs the cost vs. ROI.

 

[wk057]

My SolarCity install just went online in February. It wasn't until it was installed 6 weeks earlier that I realized that when grid power is down, solar goes offline. For me, having just the 7kW unit makes sense simply because it means that if we have a storm blow through and take down power for 3+ days, I still get to use my solar panels during the day, plus I get at least lights at night. Add to that owning a Tesla, I'd also be able to charge my primary means of transportation during long term power outages. For me, the ability to do that far outweighs the cost vs. ROI.

It's definitely more marketable to people who already have grid tied solar, but a standby generator is still cheaper and more effective.


Back to my post from a couple back:

WK057, what you are missing here is that ALL of the energy used in this case is 2 cents/kwh, not just what the Powerwalls consume off peak, so you need to roughly double your savings estimate.

How do you figure? The PowerWall is only capable of shifting 7kWh per day no matter how you slice and dice it.

Edit: I did misread the numbers and used 12c on-peak instead of 16c on peak. Let me recalculate... still not a huge difference in any case.

$0.14/kWh delta @ 7 kWh/day = $0.98/day per PowerWall, or $29.80/mo per PowerWall. $59.60 for two still puts your loan at a loss of $33.62 per month on two PowerWalls.

I don't see where you're coming up with doubling those numbers...

I did up a quick spreadsheet about your situation: Two PowerWalls, 14kWh/day load shift to off-peak - Google Sheets

Not sure how it could get much clearer than that. Let me know if I missed something.

 

[deonb]

As for comparison to a backup/standy generator... there is no comparison IMO. The least expensive standby unit I can pick up from Home Depot is $1900 and puts out 7kW (the same as 3.5 PowerWalls) for as long as I provide fuel (cheap). As a backup solution for anything but just the refrigerator the PowerWall is a pretty terrible solution.

A generator isn't a UPS though. When you have a failure you're offline for a few seconds or minutes until the generator turns on. That means you still need UPS's for critical devices.

So if the PowerWall can be directly used in a UPS fashion (not sure if it could be), it will provide an advantage over a generator.

Otherwise, the cost of replacing my batteries for my UPS's every 3 years run around $250. So over 12 years, it adds up to $1000.

So in my case, it will still make me chose the PowerWall over a generator, specifically due to this reason. And then I have a 3.5 hour UPS instead of 45 minutes.


But that presupposes that the PowerWall is usable as a UPS... If it has more than a 4 m/s transfer time, it won't work.

And if the difference between the 7kWh and the 10kWh is that the 7kWh is effectively a No Break UPS, and the 10kWh is a Line Interactive UPS, even better.

 

[idoco]

My SolarCity install just went online in February. It wasn't until it was installed 6 weeks earlier that I realized that when grid power is down, solar goes offline. For me, having just the 7kW unit makes sense simply because it means that if we have a storm blow through and take down power for 3+ days, I still get to use my solar panels during the day, plus I get at least lights at night. Add to that owning a Tesla, I'd also be able to charge my primary means of transportation during long term power outages. For me, the ability to do that far outweighs the cost vs. ROI.

What you need is something like the a Sunny Island by SMA. Not only is it a charger (AC-->DC) and inverter (DC-'>AC). It also is a transfer switch that isolates the grid from your house when the power is down. The unit communicates with the solar and allows the solar to "stay up" when the grid goes down.

 

[wk057]

A generator isn't a UPS though. When you have a failure you're offline for a few seconds or minutes until the generator turns on. That means you still need UPS's for critical devices.

So if the PowerWall can be directly used in a UPS fashion (not sure if it could be), it will provide an advantage over a generator.

Otherwise, the cost of replacing my batteries for my UPS's every 3 years run around $250. So over 12 years, it adds up to $1000.

So in my case, it will still make me chose the PowerWall over a generator, specifically due to this reason. And then I have a 3.5 hour UPS instead of 45 minutes.


But that presupposes that the PowerWall is usable as a UPS... If it has more than a 4 m/s transfer time, it won't work.

And if the difference between the 7kWh and the 10kWh is that the 7kWh is effectively a No Break UPS, and the 10kWh is a Line Interactive UPS, even better.

Whether or not it can be used as a UPS has nothing to do with the PowerWall, really. It will depend on the not-included inverter being able to do so.

My Outback Radian has a UPS mode which works pretty well. It is virtually no break, but uses about 1kWh/day do maintain this vs a transfer delay, also. I don't think the powerwall can power that specific inverter (too high voltage output).

 

[Evbwcaer]

Ok Wk057, lets work this out.

Regular customer, 1000kwh/month at 12 cents equals $120/month.

10 year total is $14,400 regular customer

Customer with two Powerwalls, 1000kwh/month at 2 cents equals $20/month for electricity (Assume a 1000kwh user uses 33.33kwh/day and can use only 14kwh 9a-9p)

Something went wrong with my initial loan payment calculation...starting over and changing it to ten years.....$8,000 at 4% for 10 years equals $81/month.

(12*81*10) + (12*20*10)

10 year total is $12,120 for Powerwall payment and electricity

Again, WKo57, you're obviously a smart guy, I'd bet I'm wrong here but I don't see where. An assumption I am making is zero peak priced electricity purchased.

In reality, the numbers are probably too close to go with the Powerwalls. What if the utility changes their pricing structure, and the hassle of trying to stay at 14kwh in the day? I have only figured for a $1,000 in install costs.....I'm just not seeing it.

Assume it was a $$$ tie, are there meaningful environmental gains to be had?

 

[wk057]

Ok Wk057, lets work this out.

Regular customer, 1000kwh/month at 12 cents equals $120/month.

10 year total is $14,400 regular customer

Customer with two Powerwalls, 1000kwh/month at 2 cents equals $20/month for electricity (Assume a 1000kwh user uses 33.33kwh/day and can use only 14kwh 9a-9p)

Something went wrong with my initial loan payment calculation...starting over and changing it to ten years.....$8,000 at 4% for 10 years equals $81/month.

(12*81*10) + (12*20*10)

10 year total is $12,120 for Powerwall payment and electricity

Again, WKo57, you're obviously a smart guy, I'd bet I'm wrong here but I don't see where. An assumption I am making is zero peak priced electricity purchased.

In reality, the numbers are probably too close to go with the Powerwalls. What if the utility changes their pricing structure, and the hassle of trying to stay at 14kwh in the day? I have only figured for a $1,000 in install costs.....I'm just not seeing it.

Going to just run through the summaries real quick to make sure we're on the same page:

• 10 years, regular customer, non-ToU @ $0.12/kWh @ 1000 kWh per month = $14,400.
• 10 years, regular customer, ToU with same average load, @ $0.02 off peak, $0.16 on peak @ 1000kWh per month = $10,800.

So, assuming the load is constant (which is partly a flaw in this whole scenario, but...) means a savings of $3600 of 10 years without the PowerWall just by switching to ToU. (This is similar to how NJ's PSE&G ToU works out for a constant load). I think this is actually where you may be seeing your disparity because you're comparing a non-ToU customer rate with a ToU customer rate at a constant load.

Add in the cost of two powerwalls ($6000) and take off their savings over 10 years at 100% efficiency ($7154) add in $2000 for inverters and installation (very cheap), and add in a 0% APR loan:

​

• 10 years, customer with Two PowerWalls, ToU with same average load, @ $0.02 off peak, $0.16 on peak @ 1000kWh per month = $11,646.

So, still more to do this with the PowerWalls, even at 100% efficiency, after 10 years.

Edit: In reality this entire hypothetical is contrived in favor of the PowerWall, and it still fails. In reality your loads will exceed 14kWh during peak times, and be lower during off-peak (hence the terms) which would make the advantage of the whole thing cancel out entirely for the ToU savings, and make the PowerWall even more expensive in the long haul.

Assume it was a $$$ tie, are there meaningful environmental gains to be had?

Not sure. If you really think about it, doing this with the PowerWall you'd be shifting load AWAY from the day, which is when renewable sources are prominent, and over to other sources that function at night. So, my first guess would be that it would be worse for the environment, technically, to do grid arbitrage like this.

 

[deonb]

Whether or not it can be used as a UPS has nothing to do with the PowerWall, really. It will depend on the not-included inverter being able to do so.

My Outback Radian has a UPS mode which works pretty well. It is virtually no break, but uses about 1kWh/day do maintain this vs a transfer delay, also. I don't think the powerwall can power that specific inverter (too high voltage output).

It's not just the inverter. If the PowerWall battery needs to be heated before you can draw full power out of it (ala Model S), there is nothing that the inverter can do by itself to overcome that.