Tesla Powerwall: Engineering Analysis

[nitsuj]

I'm looking for HVAC related math for you to include in the analysis required for answering "Is a Powerwall right for me?"

 

[WhatIsGravity]

Apologies for intruding on the discussion, and I haven't read this entire thread, but the basic idea of this product, with our energy sources and efficiencies, makes me think of the quote, 'you're either crazy, or an economist.' With a really good marketing department... and hoping for investors.

 

[OmCheeto]

I'm looking for HVAC related math for you to include in the analysis required for answering "Is a Powerwall right for me?"

There's not much to it for most people.
Mine is more complicated, as I heat primarily with baseboard electric.
I have a woodstove for emergencies, and hyper-cold snaps.

Step 1:
Determine if your utility offers "Time of Use" metering.
If they don't, then there's no use in even calculating it.

If they do
Step 2:
Simply determine your mandatory hourly electrical loads for each hour of the day.
If the mandatory hourly electrical loads are less than 10 kwh, then the system will probably pay for itself, in a period determined by your electrical rates.

I probably didn't have to do all of my thermodynamicky stuff, as my energy use can be interpolated pretty easily from my monthly utility bill.

For fun over the last few months, I've been transcribing my old electrical bills.
Between 1989 and 1995 you can see my winter energy use dropping every year.
This is because I discovered my first winter, that the house had virtually no insulation.
So each year, for 5 years, I removed the sheetrock from a single room, rewired, and insulated the exterior walls.
I also added insulation in the attic and crawlspace.
The jump in 1997 and 1998 were due to me taking in a roommate.
Some people like the house warm, when they're not the one paying the bill.
I kicked him out.

 

[nitsuj]

Apologies for intruding on the discussion, and I haven't read this entire thread, but the basic idea of this product, with our energy sources and efficiencies, makes me think of the quote, 'you're either crazy, or an economist.' With a really good marketing department... and hoping for investors.

I think a product that can "pay for it self" has the potential to save money and in turn is a "smart buy", more so if it can be financed and still be a positive ROI...that said I do agree with you. In that this isn't some great leap in tech.

Side note, we know the cost and used that as basis for payback, but the unit also requires installation & I believe most municipalities would have it so only licensed electrician can do electrical work. So there is that additional cost of Professional installation.

I watched the product presentation and it's context is "environmental concerns"...my concern is with the battery manufacturing...in particular if the "dirty work" is done in China due to low cost, in part due to little to no regulations with respect to the environment. I see this product as a net "drain" on resources holistically. That said it maybe a required step to get that battery production up to a point that cost is low enough that Tesla can compete with "median" priced cars.

 

[OmCheeto]

I think a product that can "pay for it self" has the potential to save money and in turn is a "smart buy", more so if it can be financed and still be a positive ROI...that said I do agree with you. In that this isn't some great leap in tech.

Not everything has to be a "leap". This is merely a transition from one energy source to another: Carbon to Sun

$3500: Powerwall unit
$_600: 6000 watt power inverter
$_500: installation
$1600: Water heating installation
---------------
$6200: Total​

My usage:
1000 kwh/month average = $126/month on-peak & $42/month off-peak = $84/month savings
$84/mo * 12mo/yr = $1000

So I estimate my ROI at about 6 years. Still not great, but better than nothing.
But it also means I get a $1000 savings per year for the last 4 years. Which, to me, corresponds to a $400 savings per year, over 10 years.
By the time the first unit is worn out, in the guaranteed first 10 years, I'll have to replace it.
But then you can subtract the costs of the inverter, installation, and water heating device.
So the ROI on the second unit drops back to the original 3.5 years, which corresponds, to my situation, to an extra $650 per year.
And then I will be dead, of old age. or

But the kids who buy my house can look forward to cheaper energy, because I decided to do it.
Kind of like my dad, buying all those solar electric panels back in the 90's, dropping dead, and leaving me with 4 of them.
You can learn a lot, by having a solar panel.

Side note, we know the cost and used that as basis for payback, but the unit also requires installation & I believe most municipalities would have it so only licensed electrician can do electrical work. So there is that additional cost of Professional installation.

When I rewired my house, back in the early 90's, homeowners were allowed to do their own wiring, provided that they lived in their amateurishly wired, dead traps. Fortunately for me, I was an electrician in the USN for 4 years, and didn't wire a death trap. (knocks on head)
If you can't afford the rates of a licensed electrician, you can always seek out someone like myself, to supervise everything.
I'm pretty sure, the installation rate will be much cheaper, if there are a pair of wires hanging out of the wall.

Silly safety side note: A friend of mine, about 30 years ago, asked me to install a 220 volt outlet, at his parents house, so he could make beer.
Upon inspection of the existing wiring, I started cursing, at the amateurish (ie: Om; "I'm surprised that your house didn't burn down years ago! Holy ****! ... ") job someone had done when wiring their electric range.
I discovered later, that my friend's father, who was standing there, when I blurted that out, was the one who had done the wiring.

I watched the product presentation and it's context is "environmental concerns"...my concern is with the battery manufacturing...in particular if the "dirty work" is done in China due to low cost, in part due to little to no regulations with respect to the environment. I see this product as a net "drain" on resources holistically. That said it maybe a required step to get that battery production up to a point that cost is low enough that Tesla can compete with "median" priced cars.

Um... Tesla is building their first battery factory in Nevada, and the second is planned for Japan. And lithium comes predominately from South America. Where are you coming up with China?

 

[nitsuj]

Um... Tesla is building their first battery factory in Nevada, and the second is planned for Japan. And lithium comes predominately from South America. Where are you coming up with China?

I did say "if they are made in China..." referring to the batteries themselves, not just the source of an input. The Tesla factory is still being built.

This link talks about China's growth in Lithium battery supply. I'd haven't found a source showing which country manufactures what % of total production. I'd venture a guess it's China. Though am unsure where Tesla batteries are made...again hopefully not China.

I am looking forward to see what impact the new plant has on lithium battery market. And props for having it the US!. Hmmm. I hope this means battery cells from scratch and not just "pack assembly".

That same site had this important piece of info I was missing "The mainstream Japanese energy storage systems cost approximately US$600 per kWh. Tesla’s batteries cost almost 60% lower than usual products, making them extremely price competitive in the battery market."

That is quite a price difference from competitors...I don't see how they'll actually beat China in that respect though, must be quite a factory their building in Nevada lol.

 

[OmCheeto]

...Though am unsure where Tesla batteries are made...

According to Elon Musk, in a video I'm currently watching, the Powerwall is being manufactured in their Fremont California plant, and the first packs are to be shipped in the next 3 to 4 months. These comments were made starting at 11:10.

I am looking forward to see what impact the new plant has on lithium battery market. And props for having it the US!. Hmmm. I hope this means battery cells from scratch and not just "pack assembly".

That same site had this important piece of info I was missing "The mainstream Japanese energy storage systems cost approximately US$600 per kWh. Tesla’s batteries cost almost 60% lower than usual products, making them extremely price competitive in the battery market."

That is quite a price difference from competitors...I don't see how they'll actually beat China in that respect though, must be quite a factory their building in Nevada lol.

Some interesting comments from the video starting at 16:15:

There will need to be many gigafactories in the future.
I do want to emphasize that this is not something that we think Tesla is going to do alone.
We think that there's going to be a need for many other companies building Gigafactory class operations of their own, and we hope they do.
And the Tesla policy of open sourcing patents will continue for the Gigafactory, and for the Powerpack, and for all these other things.

It sounds to me as though Elon thinks the market is big enough for more players than just Tesla.
So, just like you can choose between buying a Mercedes or Ford automobile, you'll be able to pick between a Bosch or Tesla battery.

All This Battery Hype Should Make Tesla Investors Nervous (Forbes)
5/12/2015
...
There’s competition for devices like the Powerwall, too. It didn’t get much attention, but a few weeks ago LG Chem also announced its entry into the home battery market, through a JV with Eguana Technologies. LG says it has sold 4,000 grid-tied battery systems into the European market over the past 18 months.

And several years ago German appliances giant Bosch started offering homeowners an integrated solar storage device combining lithium-ion batteries and a DC-AC inverter. It’s an attractive looking box the size of a refrigerator. As best as I can tell it costs upwards of $20,000 for a roughly 7 kwh system — before tax incentives.
...

 

[Highspeed]

The cars look good and perform well, the batteries are interesting, but are we really taking the numbers seriously from a company that has never made a dime in profit? Or did that change?

 

[nitsuj]

The cars look good and perform well, the batteries are interesting, but are we really taking the numbers seriously from a company that has never made a dime in profit? Or did that change?

I don't think it has, I am so confused and interested in this company. How can he give away patents? Those are assets; presumably valuable. I don't even know Musk's position in the company. I just don't get it. Is his particular talent "vision" or "dreaming"?

Ah he is CEO, but more inportantly when it comes to deciding open source patents (giving away assets)...he's the chairperson.

Anyways there goes hope of Tesla being an "OEM" supplier of patented parts to other companies for electric cars.

Heard Musk say "quarter million vehicles is about what is needed for economies of scale."

 

[nitsuj]

According to Elon Musk, in a video I'm currently watching, the Powerwall is being manufactured in their Fremont California plant, and the first packs are to be shipped in the next 3 to 4 months. These comments were made starting at 11:10.

Yea I saw that part, and looked up the Fremont plant. The Energy Trend website made a clear distinction in pack assembly (multiple cells) and cell manufacturing. Single cell is often a direct sale (electronic devices like phones) and often for multiple cells it is a different company that assembles the pack. So for me that raised the question about Fremont, if cells are made there, or are the packs just assembled there.

Some interesting comments from the video starting at 16:15:

It was interesting, so is Musk! "This is something Humanity is capable of doing." (2 billion "powerwall" type packs for renewable energy) That sounds like a pretty long term goal! lol

It sounds to me as though Elon thinks the market is big enough for more players than just Tesla.
So, just like you can choose between buying a Mercedes or Ford automobile, you'll be able to pick between a Bosch or Tesla battery.

That makes sense to me!

 

[OmCheeto]

Sometimes, I get this paranoid feeling, that smart people, steal ideas from our forum:

Electric power is almost worthless unless you can turn it on and off at will.

This is true. I once traded one of my 50 watt solar panels for a friend's surplus air conditioner. Although I've been using the air conditioner for 3 years, I've yet to deliver the solar panel. I told them it would be useless unless they purchased a deep cycle battery, as the panel would just be a silly wall ornament without one.

...

...
To be more precise, the sun does not shine on photovoltaic cells at night (local night for the photovoltaics). At least not in any relevant amount. And clouds give a similar problem.

Hence the deep cycle battery sitting in my living room.

ps. It's really only there to keep the crack heads from stealing it out of my boat. I cycle it lightly through the winter month by powering my xmas lights.

Yup. I've had a mini-me, DIY Powerwall, in my living room, for I don't know how many years. Maybe 10.
It's only a 1 kwh unit, and I can't remember the last time I actually needed it.
The electrical grid never went down for more than a few seconds at my house this last winter, even though we had some very nasty wind storms.

And the power to keep the battery alive comes from the grid, and not my solar panels, as, well, you know, the neighbors tall trees...

 

[OmCheeto]

There's not much to it for most people.
Mine is more complicated, as I heat primarily with baseboard electric.
...

On the other hand, for other people, it's just as complicated, only in a different way.
My current situation, retired, does not require me to know the specific heat capacity of my house.
Though two years ago, and for working class people, it does.

Unfortunately, all of the thermodynamic testing I did was 25 years ago, on an ancient PC, which is now dead.
hmmm...
I think I'll redo all the tests today.
My notes suck.

Anyways, while you are waiting:

I'm looking for HVAC related math for you to include in the analysis required for answering "Is a Powerwall right for me?"

maths
R = ΔT / Q dot_________________(thermal resistance)
U = 1/R =(Q dot)/ΔT____________(thermal conductivity)
Q dot = U ΔT___________________(heat loss)
C_house = Q delta T______________(house heat capacity)

It's best to do these thermodynamics experiments in the winter, and to check the weather report before you do them.
The maths is more accurate when you have a higher delta T.
Unfortunately for me, although the outside temperature is 52°F, it's supposed to reach 70° by 2 pm. (currently 10:46 am here)
This may be a "bust" experiment, and I'm already into it for 50 cents worth of electricity!

Argh!

 

[nitsuj]

... I'm already into it for 50 cents worth of electricity!

Argh!

Worse yet, that 0.50$ of electricity is or will be heat, that escapes and may contribute to evaporation, and rain down somewhere over Ontario/Quebec Canada,.And eventually the gravitational potential of that evaporated water will be converted back to electricity hydro-electrically...and do you think you're gunna get your share of that pie? No way hosay.

 

[OmCheeto]

Worse yet, that 0.50$ of electricity is or will be heat, that escapes and may contribute to evaporation, and rain down somewhere over Ontario/Quebec Canada,.And eventually the gravitational potential of that evaporated water will be converted back to electricity hydro-electrically...and do you think you're gunna get your share of that pie? No way hosay.

I knew I should have terminated the experiment after I checked the weather report.
The sun burst through the clouds only an hour after I started... Stupid Sun!

Though I was able to get a new, very approximate, heat capacity number: 7000 btu/°F (minus the cumulative heat loss. see below)
I suspected I'd collected too much junk in my house over the last 26 years...
I dumped about 52,000 btus into my house from T=0 to T=3 hours, raising the temperature from 61.4° to a maximum of 71.8°F, which stabilized at 68.7°F @ T=5.5 hours.
Hence, my new heat capacity.

Inside and outside temperatures ended up being the same at T=7 hours, @ 68.7°F
Attic and crawl space temperatures were both around 57.5°F at the beginning of the experiment. I didn't bother checking them after that.

edited for new experiment:

Outside temperature is 50.7°F ( 10.4 °C )
Specific heat capacity of my house is 5000 Btu/°F ( 2.6 kwh / °C )
R-value is 13 [edit: °F ft^2 hr / BTU] ( (2.3 m^2 °C) / watt )
Conductive surface area is 1900 ft^2 ( 176 m^2 ) [edit: I left out the floor area for some reason. It should be 2760 ft^2 (256 m^2). Probably because, thermodynamically, the crawl space is weird.]
...

Total thermal loss from T=0 to T=7 hours (equilibrium) = 2.4 kwh = 8200 btu

Reinserting that into my first equation:

43,000 btu
61.4 Tc
68.7 Th
7.3 delta T
yields: 5890 btu/°F (heat capacity of one OmCheeto house)

Good grief! I'm less than 10% off, after 25 years! Yay!

ps. This experiment cost me around $1.40, and took about a day. I had nothing better to do. Well, ok, I did have something better to do, but I was able to do it whilst experimenting.
pps. As to what this has to do with the Tesla Powerwall? hmmm... I would explain, but it's time for my nap.

edit: I forgot my obligatory nerdly graph...

Data was collected every 30 minutes, from 9:40 am to 7:40 pm.
Raw data available upon request.

 

[insightful]

Just to be a nag, ROI is percent return. Payback period is in years:

http://en.wikipedia.org/wiki/Payback_period

 

[nitsuj]

Just to be a nag, ROI is percent return. Payback period is in years:

http://en.wikipedia.org/wiki/Payback_period

Thanks! I didn't know there was a term for it.

 

[OmCheeto]

Just to be a nag, ROI is percent return. Payback period is in years:

http://en.wikipedia.org/wiki/Payback_period

Thanks! That might explain why I'm broke. I don't know anything about finance.

And I just realized, that I have two systems I'm working with.
The Powerwall, and the hot water system.
The hot water system could actually be implemented without a Powerwall, so that should be calculated separately.

Starting from scratch:

Powerwall               
cost (c)       $4,600       
On peak rate   $0.12581        $/kwh
Off peak rate  $0.04195        $/kwh
Difference     $0.08386        $/kwh
use                10         kwh/day
time             3,652         days
gain (g)        $3,063       
ROI = (g-c)/c   -33.4%

hmmm... Did I do something wrong? Why would anyone buy one of these Powerwalls?
In my defense, for being so far off previously, I did mention that this was worse than figuring out taxes.

pps. As to what this has to do with the Tesla Powerwall?

Obviously it shows that a single Powerwall can't heat my house, as the peak heating power use was 5.6 kw. A single unit is limited to 2 kw.
Though that was for heating it up, at an average temperature (inside vs outside) differential of 12°F (6.7°C).
Were I just trying to maintain such a differential, I would only require 500 watts.
In the winter, with an inside temperature of 65°F (18.3°C) and 32°F (0°C) outside, it jumps to 1400 watts.

But it's somewhat a mute point, as no matter how many of these things I have, they will always lose money.

On a happy side-note though, my thermal system has an ROI of 380%, over 10 years.

But this all brings up an interesting question, given that I haven't boogered my maths again.
Why were $800 million dollars of these ordered in the first week?

Is it viable for people with solar panels? Do Peak vs Off-Peak rates vary more elsewhere?
I'll have to spend some more time on this.

 

[Meloetta]

idk if you guys are aware of this, the output is actually 2 kw, and as for the 800m$ in sales, lot of good marketing, personal brand goes a long way

 

[OmCheeto]

...
But this all brings up an interesting question, given that I haven't boogered my maths again.
Why were $800 million dollars of these ordered in the first week?

Is it viable for people with solar panels? Do Peak vs Off-Peak rates vary more elsewhere?
I'll have to spend some more time on this.

Apparently it is viable.

Why this winery is using a bunch of Tesla batteries (Fortune)
by Katie Fehrenbacher JUNE 26, 2015, 4:29 PM EDT
...
Across the Jackson family wineries, solar panels and Tesla batteries are expected to lower the company’s electricity bill by nearly 40% in 2016, which is a savings of about $2 million.
...

$2,000,000 will buy a nice little yacht.

The Jackson family spent about $10 million installing a collective 6.5 megawatts of solar panels across their wineries...

I wonder if this included tax incentives. That is freaking dirt cheap.

Tesla’s algorithms use data like historical temperatures, energy use, time of day, electricity rates, and many other factors. Over time, Tesla’s algorithms learn what the winery’s power needs are and figures out the best way to use the storage to save energy and money.

Sweet! I love algorithms. And a "learning" algorithm? I've never heard of such a thing. Double sweet!

 

[Kafzilla]

A side question, could one hook one of these up with a generator and charge with the unused power?

 

[anorlunda]

A side question, could one hook one of these up with a generator and charge with the unused power?

Sure. I imagine that batteries in combination with on site generation are very attractive.

Remember that not all locations have net metering where you sell excess power back to the utility, nor are all users on the grid. On-site energy storsge is most attractive to those users.

I think the solar/storage combination is attractive. Once you have a solar installation, you already bought the inverter, mandatory grid interface (if any), and installation labor. The incremental cost to add additional panels is small. Combine that with the cost of adding battery storage, and the economics are compelling.

Where net metering is available, the grid acts as your storage battery at no charge. That is why I think net metering is not sustainable.

 

[shanaya]

i have question that how can we increase the output voltage 90vdc to 200v if we are having the input voltage 220v ac?

 

[anorlunda]

The DC to AC inverter will boost the voltage. It has an internal transformer.

 

[mheslep]

By the way, the world's biggest utility battery installation is in Fairbanks Alaska.

That facility was build with Ni-Cads in 2006 (45 MW, 4 MWh). The new largest in N. America is li-ion based and eight times larger, http://www.greentechmedia.com/articles/read/The-Biggest-Battery-in-North-America-Gets-Unveiled-By-SCE-Today in California.

 

[mheslep]

They are required to pay whatever it takes to buy all the energy needed to satisfy the demand, not matter what the price.

Also, in many places utilities are required by the state to buy a quota of certain types of power, specifically renewable power (aka RPS).

 

[anorlunda]

That facility was build with Ni-Cads in 2006 (45 MW, 4 MWh). The new largest in N. America is li-ion based and eight times larger, 32 MWh, in California.

Thank you mheslep, I was not aware of the California installation. In another recent thread, Batteries for utility storage, I found news of a battery facility under construction in Austrailia with 50 MW. 300 MWh . Maybe we'll see more wide spread use of utility batteries in the future.

I'm curious about choice of chemistry. For utility applicaitons, volumetric or mass energy density sounds unimportant. Their lifetime measured in cycles may be similar. Is it simply $/MWH that determines the choice?

Also, in many places utilities are required by the state to buy a quota of certain types of power, specifically renewable power (aka RPS).

But batteries are not an energy source, just energy storage. Do purchases of battery power count as renewable? It sounds complex, because you have to figure the source of energy used to charge the battery.

I would also add, that the location of the generation is sometimes also a requirement. For example, Manhattan Island in NYC, where some fraction of reserves must be on-island.

 

[mheslep]

Is it simply $/MWH that determines the choice?

amortized capital cost of storage $/MWh, efficiency, and $/MW, i.e. cost for storage, cost of wasted generated energy, and cost of rate of charge/discharge. The last may or may not add to the total cost.