There's rightly been a lot of hype about electric cars in the past years; however, this is accompanied by what I see as irresponsible and misleading labeling (as are most consumer goods). When I was at the post office a few weeks ago, I saw a Nissan Leaf with the words "Zero Emissions" written on the side. In Washington State, this is nearly true because most of our power comes from hydro and nuclear. However, a sizable portion also comes from coal and natural gas combustion.



As you can see, the vast majority of energy consumed in the United States does not come from renewable sources. When you charge an electric car, emissions are almost certainly being produced. Now, if you live in a region where a sizable portion of your electricity comes from renewable sources, an electric car may indeed produce fewer net GHG emissions than a gas powered car. This article from Mother Jones discusses a study that found that the environmental damages from electric cars are higher than those of gas driven cars in some areas.


             Gas                                     Electric
Calculated environmental damages by type of vehicle. (I am skeptical of the accuracy of this chart.)

My point is that while electric cars may produce fewer emissions than regular cars in some regions, they are by no means "Zero Emissions." Labeling products in this way distracts from the actual problem, and can even worsen it. If you have an electric car that is less expensive to drive than a gas powered car, you may be inclined to drive it more than you otherwise would. Since it is labeled as "Zero Emissions" you feel that you have done no harm while you have actually produced more pollutants in some distant place.
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The Tesla Powerwall is also bothersome to me in several ways.

First of all, there is simply no way that owning a Powerwall will save you money. PG&E peak electricity rates are $0.05/kWh greater than off peak rates. For the sake of this calculation, we will assume that the Powerwall is 100% efficient (which it of course is not). If the Powerwall stores 6.4 kWh of energy in one charge and costs $3,000, it will take $3000/(($0.05/kWh)*6.4kWh)=9375 cycles to break even. The problem is that the battery is only good for 1,000 to 1,500 cycles. If you throw in the fact that 8% of the energy is lost in the charging and discharging process, you would only save $327 on electricity over the course of 1,500 cycles (with the summer PG&E rates). In the end, your purchase will have saved you $327-$3000=$-2673.

Another problem is the use of this misleading diagram:


While it may be true that there is larger residential demand in the morning and evening, that demand is relatively insignificant compared to the overall peak demand. There is a reason why some electric companies charge peak rates during the middle of the day. This is what actual demand looks like:



It seems to me that the peak solar an peak demand line up much more closely than what Tesla claims.

Most grid-tie systems are between 94%-96% efficient, compared to the 92% efficiency of the Powerwall. By connecting a residential PV solar system to the grid, more of the energy produced will actually be consumed (albeit not by the homeowner).

Moreover, using a Tesla Powerwall is not an environmentally friendly choice. No matter how you slice it, using a Powerwall will result in the connected appliances indirectly consuming >108.6% of electricity they normally would. Even if the electricity used was produced during off-peak times, there will still be more GHG emissions produced.

Finally, there are many issues surrounding the sourcing of lithium and other metals used in the batteries. Producing billions more lithium-ion batteries is not going to solve our environmental problems.

https://en.wikipedia.org/wiki/Tesla_Powerwall
http://insideevs.com/solarcity-reveals-installed-pricing-for-tesla-powerwall/
https://en.wikipedia.org/wiki/Grid-tie_inverter
http://www.digitaltrends.com/cars/hold-smugness-tesla-might-just-worse-environment-know/

Where else do you see products or movements that distract from real problems by making people feel falsely good?

Please let me know if you find any discrepancies in my reasoning.

Semantics. EV's are marketed as "Zero Emission Vehicles," or ZEV; These vehicles produce zero emissions while driving compared to their combustion cousins. Secondly, there are tons of things out there that show how owning an EV can save you money. Which, admittedly, isn't your gripe about EVs. Your gripe is about how they are more damaging to the Earth than combustion vehicles; in that the means to produce the energy to charge an EV is more damaging to Earth.



Of course not. We have deep pocketed lobbyists who want to keep their business afloat and profitable. Allowing clean energy to advance would impact that. And sadly, we have politicians who will accept "campaign donations" from lobbyists in exchange for favorable laws.



Of course. But that's not how marketing works. You buy a Clothes Dryer because it'll dry your clothes faster, you don't see any promotional material about how that speed impacts the environment by increased energy usage. On the topic of cars, we don't go to the gas station and ask about the efficiency of the process that brought the gas here just like we don't look at the impact the power plants have which produce our electricity.

It's the invisible cost of business. The thing consumers overlook and corporations abuse. A majority of us don't care how things arrive to us as long as it's available. It could arrive on a monster truck or a purely solar powered, aerodynamic 18-wheeler. As long as the product the consumer wants is available and cheaper than somewhere else, they don't care. And truth be told, that monster truck is probably cheaper to operate than that hypothetical big rig, so the product delivered by the monster truck will be cheaper.

More down to Earth, let's look at the real impact to owning an electric car.

$0.13 - Cost per kilowatt hour in 2015 averaged across the USA. (Source)

2,500sq/ft - Average house size in 2015 across America. (Source)
I used only houses because it's easier to guesstimate the difference between owning an electric car and not.

2667.33kW/h - Average usage of a 2,500sq/ft house in Dallas, TX. (Source)
This data set was from 2008 but it shouldn't matter too much since we just want their average usage. I chose a narrow scope for this range since usage could vary between Texas and Washington in the summer months. Keeping this scope narrow should provide a better real world example.

$320.04 - "Average" bill for an average home in Texas in 2015.
This isn't really an average, since it's from a short period. But my data is only going to get more localized from here.

Now, let's add in an electric car. I'll use the Nissan Leaf from your first paragraph. It looks like the biggest battery that's available is 30kWh but that wasn't offered until 2016. 24kWh was offered in 2015, so we'll use that. It has a range of 84 miles.

I'll do a big no-no. I'm going to pull in my own data from a typical month and my last 30 days.

Typical Month, 320 miles or $40 in gas:


Last 30 Days, 760 miles or $93 in gas:


So, since I've changed this from an average to a single data point, let's find out if owning an EV is a money-wise decision. What I'm going to do here is divide the range of the 24kWh battery by my two distances then multiple the answer by the kWh to determine how many additional kWh this car would add to my home energy use.

320 miles would be 3.81 charges with the 24kWh Leaf, an additional 91.44kWh per month.
760 miles would be 9.04 charges with the 24kWh Leaf, an additional 216.96kWh for that month.

91.44kWh is an additional $11 a month.
216.96kWh is an additional $26 a month.

But Alex, how does this determine if it's better for the environment? Well, I'm under the basis that my car, a 2008 2.5L VW Golf, produces a certain amount of emissions and that the power plant producing the energy for my home, while producing more emissions, is more efficient.

I'm not exactly sure how to go about determining how much more efficient it is but I'll get to that in part 2, once I figure out how to calculate. My assumption is that owning an EV will be more Green if the power plant is more than 3.5x efficient at producing energy than my cars' combustion engine.



Let's stop right here. Let's now check out the Powerwall page on Tesla.


I just want to preface this by saying that my computers Autocorrect is routinely replacing Powerwall with Powerball. I'm correcting it back but I apologize if I miss one or more.

The Powerwall is suppose to accompany solar panels. If the solar panels aren't producing enough power to charge the Powerwall then yeah, it'll pull from the grid. Please, factor in the cost of solar into the ownership of a Powerwall. I don't mean the cost of the system, I mean the cost of energy converted from the sun. If the system produces 20kWh during peak solar hours when we aren't home, then that's something like $2.50 that we save into the Powerwall.

So, using my numbers above for a typical 30 day month, our hypothetical house uses 88.9kWh a day. I personally think this is way to high or I got my math/numbers wrong above. Anyways, the Powerball provides a 6.4kWh storage. Granted, the Powerball is not a one time cycle. It can be used a little bit and charged a little bit. Let's break down our daily kWh usage a bit more. We use 3.7kWh every hour.

How may solar panels do we need to produce enough? I'm not exactly sure how to best calculate this but I assure you the cost of ownership of a Powerball will not be a negative number at the end of 1,500 cycles.



Whoa, what. Are we reading that first image the same way?

The yellow line is not the peak power we use, that would be the blue hills. The yellow line is the amount of energy from the Sun that is hitting Earth. Peak solar hours are usually midday. Peak usage hours are typically when people get up and when they get home. Peak rates do not happen in the middle of the day. They happen when those people come home from work and demand power so they can cook dinner, watch TV, start the heater/AC, etc etc. The power company needs to provide power to meet the demand and thus, charge more.

Additionally, there's definitely a difference between a residential peak and a business peak as we see in your second graph. It makes sense that residences don't pay peak prices during the business peak hours, and visa versa.

That second graph is the demand for power. The solar peak is not on this graph. However, I see where you're coming from but you're still wrong. The Tesla graph doesn't have times, it does have the day and night cycle so it's kinda easy to guess the times but irregardless, it's a generalization. The Powerball is for a single residence. That graph is for a single residence, not a neighborhood or a city like your second graph. Not everyone get's home at the same time which is why there's a gradual climb to the peak in that second graph.



What's your source on this? How do appliances "know" when to draw more than 100% power?



I think the biggest discrepancy was the math regarding the Powerwall cost of usage, which I covered.

Well, charging an EV may be more damaging in some cases, but it is also much better in many. A typical combustion engine has a thermal efficiency of around 20% (Source). A combined cycle natural gas plant has an efficiency of 54% (Source) with a transmission loss of 12% (Source). A Nissan Leaf has a charging efficiency of ~90%. So the thermal efficiency of the Nissan Leaf ends up at around 43%, which is about twice that of a combustion engine. Natural gas plants also produce fewer emissions per kWh than gasoline combustion.



In my opinion, coal is the only obstacle that is preventing EVs from being more environmentally friendly everywhere. However, diesel and gasoline still have the advantage when it comes to energy density and initial equipment cost.



I am mainly talking about the part that says "or when utility rates are low." Keep in mind that the Powerwall can only store 6.4kWh of energy, which equates to $0.80 with your price.

If the Powerwall battery is good for 1,500 6.4kWh cycles, that means that 1500*6.4kWh=9600kWh of electricity can pass through the Powerwall. If this electricity is from solar power that would otherwise go unused, then you are saving 9600kWh * $0.13/kWh = $1248 over the life of the battery. While this number is higher than what I calculated using the off-peak rate savings of 5 cents, it is still far less than $3,000.

With net metering, you can get the economic effect of the Powerwall, but with greater efficiency and without the need for energy storage.



I am well aware that the Tesla graphic indicates the supposed residential demand. My point is that most residences do not operate in a vacuum. Houses are part of a neighborhood, city, state, and larger interconnection. Instead of saving the generated solar for an individual home, it would make more sense to efficiently transmit it to other places where it can be used. When this happens, the homeowner is paid for the electricity they sell back to the grid. Grid tie systems are more efficient than battery storage and last much longer.

The Powerwall would be useful for completely off-grid applications, but the vast majority of homes where Powerwalls would be used are grid connected.


(92%)⁻¹=108.6%
A better wording may be "using a Powerwall results in 108.6% of the electricity being used directly by the appliance being consumed."



I don't think I understand what the discrepancy with my math is.

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The Powerwall page also notes that it requires installation and a compatible inverter, which could easily add $2000-$4000 to the total bill. It is probably possible to utilize the inverter from a solar system instead of a separate one, but people just looking to save on peak rates won't have that luxury.

Additionally, the Tesla Powerwall page indicates that the power is 3.3kW; however this is the peak power, not the continuous power. The continuous power rating is 2.0kW, so a Powerwall could run small appliances for several hours during an outage (Source). However, you can get a 12.4kW backup generator for $3,000. This could power your entire home indefinitely, including running an electric oven and dryer simultaneously. The backup generator also does not require an additional inverter.

I used that last bit wrongly. It wasn't so much a discrepancy as it was an oversight.

I didn't catch this last night but the Powerwall does not have a life-time cycle of 1,000 to 1,500. I'd love to see where you got this number but it may have been from a poorly phrased sentence. The source below, coupled with your post, almost had me filling in the holes myself and believing the Powerwall does only have a max of 1,500 cycles. I bolded the Powerwall and underlined the Powerpack.



So, if we're talking about the Powerwall, let's use those numbers. We still don't get near your number, of requiring 9,375 cycles to break even but we certainly get closer. Again, your numbers are purely based on off-peak rates. You don't take into account that a majority of consumer residences purchasing a Powerwall likely already have solar. Additionally, anyone who purchases a Powerwall with the intention of using it as a back-up has money to spend.

Seriously. Think about it. If you're that concerned about the power going out a household UPS is not a matter of cost. I am concerned about the power going out but I only protect my vitals: Computer, Monitor and, NAS/. I can replace the TV, my food in the fridge won't spoil if I don't open it, if I'm cooking something I can just fire up the BBQ.

By your calculation, it'll also never pay for itself either. The power goes out maybe 3-4 times a year for me. Mostly as briefly as a few minutes but every once in a while it's longer than an hour. I'd never recoup the cost of a Powerwall. Heck, I'll likely never recoup the cost of my $220 UPS via the energy it provides but it's probably paid for itself in other ways. I have it because I do work on my computer. If the power goes out while I'm working, I can get corrupted data and that means lost pay because I'll need to either pay to recover the data or reimburse my clients for data loss. So, having this UPS let's me save and shut down my systems. Much like someone who buys a Powerwall likely has some other reason for it if they don't have solar power.

Let's assume a 99.99% majority of consumer residences who purchase Powerwall have solar panels and want a battery back-up.

Since the sun is strongest during non-energy peak levels, let's go with my value above that $0.12 a kWh is the going rate. Using Google Sunroof I'm going to calculate just how many kWh my roof can generate and I'm going to artificially adjust my bill to match the previous number of $320 a month for an average electric bill in Dallas, Texas.



Looks like I'll need ~9kW a day to meet my energy demands if my bill is $300 a month. At least, that's what I think that means. I'm not in-charge of the electric bill and don't actually know if we use more or less. Maybe that's what I need to generate every hour? I don't know enough to do the math for this. Haha, sorry.

I'll ask the bill-payers about our electric bill and get back to you when I can justly do the math. But you're welcome to carry on the calculations if you'd like.



It sounds like it'll add an additional $1,500, the page says those who buy a 10kWh Powerwall will pay around $5k with installation and the unit was $3.5k by itself. Since the 6.4kWh unit is all that's being produced and assuming the installation figures are still valid, it'll be $4.5k with installation.



This still does not make sense to me. Where does the factorial -1 come from? Why are we using the efficiency of the Powerwall? How can the answer be larger than 92%? I'm no math wiz, I just need some one to explain like I'm five here.

Well, the Powerwall seems to give you back 92% of the energy it used to charge itself, meaning 8% go for nothing. To calculate how much the net usage is, you just do efficiency^-1 which is .92^-1 and equals to about 1.086.

That makes more sense, but you would still need to buy another battery pack after it wears out. I can't seem to find any information about the price of the battery pack, so it may or may not be worth it.



If you already have solar with a grid-tied net metering system, there is no incentive to purchase a powerwall, other than use as a back-up. A battery based backup has an advantage over a generator since it can take over the demand instantly - whereas by the time a generator starts up, equipment will have lost power.



Where did 99.99% come from? That is equivalent to 1 out of 10,000 not having solar panels.



! is the symbol for factorial, ⁻¹ represents the inverse. Since the Powerwall is 92% efficient, that means that 8% of the energy going into the Powerwall does not come out as usable energy. To find out how much we need to put in to get 100% out, we simply divide 1 by the efficiency. Hence: 1/0.92=1.087


I agree that there are valid uses for the Powerwall, but it seems inappropriate to market it as an environmentally friendly device that will save you money.

EDIT:



I was looking at that page, and the $5,000 price with installation is a 9 year lease from SolarCity. It says this exactly:



So, if we subtract $500 from the 10kW unit price, and another $500 for a smaller inverter, the cost still comes out to over $6,000 which is twice what I used in my calculations.

There is incentive. What do you do at night? It's unlikely your house is using all the electricity those solar panels are generating while you're at work during peak hours. I get it, with a grid-tied system that electricity is distributed to your neighbors and perhaps even sold back to the power company but, not every power company offers such a luxury.



I'm just assuming. It's hypothetical. I'm assuming that 99.99% of the people buying a Powerwall have solar panels since the marketing for Powerwall is very strongly tied with the Solar aspect. Not 99.99% of the population of wherever has solar.



Of course but not the end of the world. As below:



I disagree. I think it's entirely appropriate to market it as environmentally friendly. If you're producing more energy than you need during the day, why not store it for use at night? If your Powerwall is fully charged and you're still generating more power than you need then you can also utilize the grid-tie system, if applicable.

The system will pay for itself if you have Solar panels. Again, I still need to do the math but I'm positive the amount of money I'd save could help pay for a new Powerwall in 15 years. Heck, by the time I need to replace it in 15 years, the technology would have come so far along that buying a new one would be sensible. Tesla isn't going to stop and only make this one version for the next 30 years. They'll certainly release new models and versions as technology advances and improves. Tesla probably won't even be the only manufacturer of home battery packs in 15 years.

Depending on how fast solar panels pays off the Powerwall maybe I'd even upgrade to a newer model before the 15 years.





Thanks! That helped.

Isn't that the case for every battery then? I understand that it's a bit more vital with a UPS like the Powerwall but I don't see that as a big deal. Especially since most of the power going to the Powerwall should be from Solar in an ideal usage environment.

Grid-tie systems are very standard, and are part of nearly every existing solar installation. If you are not connected to the grid, you will definitely need battery storage. There are problems associated with net metering, but there are other approaches to grid-tie as well. My point is that spending the extra $6,000 for a Powerwall will never save you enough money in comparison to a simple grid tie system, even if you have to sell the extra power at a wholesale rate.


You should not store it if you do not have to, because it could be more efficiently used at the time of production. Storing the power when you are already connected to the grid reduces the overall system efficiency from what it could be.



I'm pretty sure that the solar panels would pay for themselves, but the Powerwall would not. You would be better off going with pure grid-tie, unless there are extenuating circumstances.



It is the case for every battery. The efficiency isn't that big of a deal per se, but that 8% of energy used for storing power could instead be consumed by other grid users, thereby reducing the need for energy from other sources.


The real need is for grid-scale energy storage that can provide extra capacity during periods when renewable energy is scarce. For example, the pump generating plant at Grand Coulee Dam pumps water up into a secondary reservoir when there is extra capacity, and uses the stored energy when demand is high. This can power thousands of homes, and ends up being much more cost effective for everyone.