Tesla announces "Powerwall" batteries

[Amy@altE]

Joe Homeowner should use the Grid as their battery.

Amen brother.

[Amy@altE]

First, I am a novice when it comes to designing solar systems and battery banks so please bear with me as I am in the initial phases of designing my totally off the grid home. Nonetheless from my understanding, basic electrical calculations are as follows:

Watts = Volts x Amps
Amps = Watts / Volts
kWh = (Amp x Volts) / 1000

Amps = Amount of current in a circuit
Volts = Strength of current flow
Watts = Total electrical power per second

For this post I'm going to make an assumption that I'll be using 20 kWh per day for calculations sake. From that I can calculate amp hours used per day per input volts.

20,000 / 120v = 167 Amps / day (Normal Utility Voltage)
20,000 / 48v = 417 Amps / day (48VDC Battery Bank)
20,000 / 350v = 57 Amps / day (PowerWall)

Obviously you're going to need an inverter capable of handling a 350V input voltage to use the PowerWall. I've seen them but their pricey (starting around 10K). In addition you'll need a charge controller capable of handling an input voltage of 350v as well - have not run across any of those for home use but I'm sure they're available. (i.e. I would think that utility scale solar farms have controllers capable of handling high voltages). I would also think that you would have to wire your solar array to provide the higher input voltage required of the PowerWall.

Another point of confusion (for me) is on the consumption side of things. I would think based on the calc's at the beginning of this post that amps used by a particular appliance falls as voltage increases.

For example, let's say you have an appliance rated at 1000 watts - that would seem to indicate the following:

Watts / Volts = Amps =

1000 / 48 = 20.8 amps with a 48V Battery bank
1000 / 120 = 8.3 amps at normal household voltage
1000 / 350 = 2.85 amps with a PowerWall

So based on my understanding you would need at least 3 PowerWalls to supply 20kWh per day and that's at almost 100% DOD (which I don't think is feasible even for a LiON battery). Additionally you have no days of autonomy with the scenario above and the real kicker is that I would think you're probably going to shell out close to 25K before you even get to the solar array side of things.

For the sake of comparison a conventional FLA or AGM 48V battery bank would cost about 9K for 1200 amp hours or roughly 58kWh (1200 x 48) / 1000 = 57.6). So if I have this correct, from just the battery bank perspective you're shelling out 9K for 58 kWh as opposed to 9K for three PowerWalls at 21kWh.

As I said above I'm still a novice so if anyone finds fault with my calc's and assumptions please feel free to reply.

Thanks in advance

You are making the common mistake of confusing watts and watt hours. So for your equations above, you need to add time into the math.
Watts = Volts x Amps
Amps = Watts / Volts
kW = (Amp x Volts) / 1000

Watt hours = Watts x hours
kWh = Wh x 1000


For your 1000W appliance; how long is it running? 1 hour a day?

Watts x hours / Volts = Amp hours =

1000Wh / 48V = 20.8 amp hours with a 48V Battery bank
1000Wh / 120V = 8.3 amp hours at normal household voltage
1000Wh / 350V = 2.85 amp hours with a PowerWall

But to normalize the voltage across the technologies, people have been using watt hours or kWh to describe the batteries, so voltage is irrelevant. Notice the watt hours in your examples are the same, all 1000Wh (or 1kWh). So to run that 1000W appliance for 1 hour, you need 1kWh, so you can run it for 10 hours with the 10kWh PowerWall (at 100% DoD, which you wouldn't do)

[SunEagle]

They can, how much are you willing to pay ?? If RE is causing the problem then PV owners should be willing to pay for it, Right ?? They'll be glad to take your money.

If PV owners are part of the cause for grid power fluctuations or can benefit from POCO stored energy then they should take some responsibility for paying to have it place.

I am sure the POCO will raise their rates to cover the cost of a battery system due to weather the RE system belongs to them or private individuals.

I believe CA has already put through a mandate for certain POCO's to have XX MWatts of energy storage in place by 2020. So their customers will be paying for batteries even if they don't have PV systems.

Of course the amount of the tariff increase may be conditional by how much competition the POCO has from privately owned RE systems. They would like to have total control of all power generation. It is in their blood.

[Willy T]

Maybe people like Tesla will come up with something that is very cost effective on a much larger scale used by the POCO at specific grid points to smooth out the variable voltage fluctuations and outages caused by RE. .

They can, how much are you willing to pay ?? If RE is causing the problem then PV owners should be willing to pay for it, Right ?? They'll be glad to take your money.

[SunEagle]

....... but for Joe Homeowner, AGM.

Joe Homeowner should use the Grid as their battery.

Maybe people like Tesla will come up with something that is very cost effective on a much larger scale used by the POCO at specific grid points to smooth out the variable voltage fluctuations and outages caused by RE. If that is perfected then Joe Homeowner shouldn't need to worry about having his own energy storage system but can still install some type of emergency power gen set that powers more than a few lights and fans.

[Amy@altE]

I prefer comparing cycle counts. Outback at 5 hour rate (only one I see on their datasheet) says 5700 cycles at 20% DoD and 1800 at 50% DoD. That's 15 years down to 5 years. People shouldn't be designing their system for 50% daily discharge, if they are, they get what they pay for. So I'll even cut this in half for a 10 year life. I know certain royalty will strongly disagree with me. But even if the AGM has half the life of the PowerWall, at half the price, available now, and works with standard equipment, I'm still not sold on Tesla. Yes, technically the flooded may still the best cost over life of system, but most homeowners will not take care of their batteries, so it will not meet its potential lifespan. So for the right person, by all means, flooded, but for Joe Homeowner, AGM.

[DanKegel]

Sunking's advice was to always check the warranty, so let's see:

http://www.outbackpower.com/outback-products/energy-storage/item/energycell-re

says the warranty is 2 years, and does not cover "routine or daily discharge of more than 40% of capacity".

http://www.trojanbattery.com/pdf/TRJ...edWarranty.pdf says the warranty
is 3 years outright and 8 years prorated, and covers 50% DoD.

http://www.teslamotors.com/powerwall just says "10 years". Since they haven't claimed DoD yet, people are making guesses (I've seen 70%, 80%, and "it's really a 12 kWh battery", http://rameznaam.com/2015/04/30/tesl...-almost-there/).
For comparison, http://bosch-solar-storage.com/the-s...echnical-data/ says 5 years, 50% DoD for their lithium system.
If I had to make a wild guess, I'd assume their 7kWh battery is really a 12 kWh battery, warrantied to handle a daily discharge of 7kwh for 5 years, ramping down to some lower figure at 10 years.

If you include the known warranty terms from Trojan and Outback, and your own reasonable guess at Tesla's warranty terms, what are the resulting lifetime cost per kWh?

[Amy@altE]

For flooded, SunKing's favorite series: Trojan's IND9-6V, 464Ah @ 6V for $834. 4 in series for 24V (just to keep things in the same range) = 11.1 kWh for $3,336.

[Amy@altE]

Which AGM batteries are you talking about?

I randomly picked the Outback EnergyCell 200RE. 12V 178Ah C/20, designed for daily use with solar. 2136Wh for $425. 4 of them for 48V = 8.54kWh and costs $1700. Granted, without Tesla's life cycle data or DoD, you can't compare lifetime costs, but purely on price per Wh, AGM wins.

[DanKegel]

Who really wants to use a FLA battery system that would be high tech yet familiar on a WWII uboat?

Don't laugh at lead-acid just yet. There's some life left in that technology, it's way down its cost curve, and it's got a killer track record.

[DanKegel]

I did the math, the price per watt for just the 7kWh battery is about the same as Aquion's stack, and 2X as much as the Trojan Industrial line and some AGM batteries.

http://www.trojanbattery.com/pdf/Sma...rn_Handout.pdf does make their industrial FLA line sound pretty good (speaking as a non-battery-expert with just a smattering of EE background).

Which AGM batteries are you talking about?

But the litany of potential pitfalls at http://www.survivalunlimited.com/bat...ryblunders.htm makes me think that any consumer battery solution is going to need to include remote monitoring and periodic service by the installer as part of the deal. Do you agree, and what does this add to cost of real-world systems you've seen?

[bberry]

That is not accurate, Tesla may want you to believe that, but not factual. Even top of the line FLA and AGM cost less and proven to last. Secondly as learned yesterday the $3000 and $3500 price Tesla is completely bogus, as Tesla and Solar City now say you can buy the battery outright for $7000 and no discount for quantity. That jacks up the price to $1000 - Kwh or 5 to 6 times higher than top of the line FLA.
........

First, all rational people will go with the lease. Second, why would solarcity discount? They will not be able to get enough Powerwalls to meet demand for several years. Powerwalls won't be the only choice in several years.

Complaining that the price isn't $3500 is like complaining that it costs extra money to retrofit your car with a newly announced, inexpensive engine.

[bberry]

I still don't understand why people in Germany or Europe would decided to spend a lot of money on battery systems without thinking about getting any return on their investment or at least minimizing the cost.

It's a big world with a lot of different needs and wants. The "home appliance battery" is now half the cost it was before Tesla's announcement. That change increases demand. Do you have a good understanding of worldwide demand for a home battery with the characteristics of the Powerwall? I don't.

The home battery announcement isn't the important part anyway. It's just the relatable aspect that can be used by journalists. What Tesla and competitors can deliver to commercial and utility users is what matters.

The business plans of new batteries are justified by the potential size of the utility and commercial markets. That should be exciting, because it potentially improves residential solar. Especially off grid. Who really wants to use a FLA battery system that would be high tech yet familiar on a WWII uboat?

[Amy@altE]

That is not accurate, Tesla may want you to believe that, but not factual. Even top of the line FLA and AGM cost less and proven to last. Secondly as learned yesterday the $3000 and $3500 price Tesla is completely bogus, as Tesla and Solar City now say you can buy the battery outright for $7000 and no discount for quantity. That jacks up the price to $1000 - Kwh or 5 to 6 times higher than top of the line FLA.
.

The $7000 includes the inverter, obviously you'd need one regardless of which type of battery, and installation and maintenance. Figure at least $2k for the inverter, probably closer to $3k, and I can see where the $7k comes from. But I did the math, the price per watt for just the 7kWh battery is about the same as Aquion's stack, and 2X as much as the Trojan Industrial line and some AGM batteries.

[Willy T]

No one was more disappointed than I was. .

This is good for a laugh. Had you planned on installing one in your golf cart ??