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Tesla Will Sell Home Batteries

The electric vehicle manufacturer plans to offer two versions of a lithium-ion battery for home use. Solar installer Sungevity announces its own plan.

Posted on May 5 2015 by Scott Gibson

UPDATED on May 7, 2015

Later this year, Tesla Motors will begin selling two versions of a lithium-ion battery to homeowners who either want to store power generated by their photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) systems or store utility electricity purchased when rates are low.

The lithium-ion batteries for home use, called Powerwall, is part of a rollout announced by Tesla CEO Elon Musk. The product line also includes batteries for businesses and utilities, all derived from the batteries that power Tesla's Model S electric vehicle.

Batteries will be connected to the internet, allowing them to be managed remotely by Tesla, according to an article in The New York Times.

Powerwall batteries have three potential uses, according to Tesla's website:

  • Backup power for times when the grid is down.
  • Storing electricity purchased at low, overnight-rates for use during the evening when peak rates typically apply, what the company calls "load shifting."
  • Storing power generated by PV systems for use at a different time of day.

Batteries will be available in two configurations: a 7 kilowatt-hour version for $3,000, and a 10 kWh model for $3,500. They will be able to provide 2 kW of continuous power and 3.3 kW of peak power, and up to 450 volts. Two or more batteries can be paired for houses that need more storage capacity. The batteries will carry a 10-year warranty.

Sungevity also offers batteries for homeowners

Tesla isn't alone. The Oakland, California, based solar company Sungevity has announced that it will begin offering a home battery system manufactured by Sonnenbatterie, a German company, the San Francisco Chronicle reported.

Sungevity's chief product officer, Peter Graf, made the announcement two days before Tesla, but said it was not so much an attempt to upstage Musk's announcement but a response to customers who have been been clamoring for home storage.

Even so, the details of the Sungevity offer are yet to be worked out. Graf didn't disclose any technical details on the batteries, and offered no official pricing, although Greentech Media said it had been told by Sungevity that the system would cost less than $10,000.

Graf told The Chronicle that Sungevity and Sonnenbatterie were still discussing whether to sell the systems, lease them, or both. The home battery system will apparently become available in the second half of the year.

Sungevity does business in more than a dozen states, including Arizona, Massachusetts, New Jersey, Vermont, New York, and California.

Less expensive than lead-acid battery option

Tesla's lithium-ion battery appears to be a cheaper option for battery backup than the lead-acid batteries that have traditionally been used for off-grid photovoltaic systems, says Fortunat Mueller, co-founder and co-owner of ReVision Energy in Portland, Maine.

It would take six EnergyCell 200H batteries to form a 7 kWh bank and eight batteries to make a 10 kWh bank, given certain assumptions about the battery discharge rate and depth of discharge.

Each of those 12-volt batteries costs $545, so the 7 kWh sealed lead acid battery system could be set up for $4,200 and the 10 kWh bank for about $5,500, he said, if you include the cost of a rack.

Mueller added that cheaper batteries are on the market, but they aren't maintenance-free.

"But all in all, if Tesla's quoted prices are the retail price for a fully capable battery with a real 7 or 10 kWh of useable energy storage in a heavy cycling application, then they definitely have an attractive price point and will substantially improve what is available in the market today," he said in an email. "We're eager to actually get our hands on one and see what they can do in the real world."

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  1. Tesla Motors

May 5, 2015 2:00 PM ET

It's more than a backup battery
by Dana Dorsett

To unlock the value of the battery, having the utility use it to stabilize the grid (and being paid for those services) is much more valuable than self-storage or load-shifting.

But if the utilities won't play that game and put up roadblocks or abusive connection fees, unplugging becomes a real option in places like Hawaii (and in a state near you in less than a decade.):

May 5, 2015 3:08 PM ET

Can it replace a generator?
by stephen sheehy

My new, almost finished house has a grid connected pv system. I had planned to add a whole house generator for when the power goes out, as it often does. But I'm intrigued about the Tesla battery system.

Pros: quiet, attractive, no maintenance, no propane tank or expense.
Cons: Limited storage. $7,000 for 20kwh, about what the generator would cost.

But the biggest question I have is:
When the grid goes out, will my pv system keep the battery charged, such that I'm not limited to the 20 kwh, but can in effect be living off the grid and still run whatever i need, subject to how much sunlight is available? I would be limited in my electricity useage, but for a few days i could manage. I would need to have enough power to run a minisplit, water pump, a few lights, water heater, etc. But could limit the use of any of them at any particular time. I'd love to skip the generator, which is noisy, ugly and costs about $3/hour to run. On the other hand, not having any power for a few days is not an acceptable alternative.

May 5, 2015 3:22 PM ET

Edited May 5, 2015 3:24 PM ET.

Response to Stephen Sheehy
by Martin Holladay

For $3,500, you get 10 kWh. In most areas of the U.S., that's about $1.20 of electricity.

For $7,000, you can store about $2.40 of electricity.

When you've used up that much electricity, you had better hope that the sun is shining.

Whether or not this is enough to get you through a power outage depends what you expect. The peak power output of these batteries is 2,000 watts. If you are using electricity at that rate, the $3,500 battery will be dead in 5 hours. The $7,000 battery will be dead in 10 hours.

If you use electricity sparingly, and the sun is shining, you can make these things work for much longer.

Needless to say, a gas-powered generator is much more versatile in an emergency.

May 5, 2015 8:43 PM ET

who are they kidding
by Hobbit _

"Batteries will be connected to the internet, allowing them to be
managed remotely by Tesla" ...

There's the other showstopper, right there.

I'm having trouble deciding who has the most blatant disregard for
subscriber privacy these days ... Google, Facebook, or Tesla.


May 5, 2015 9:10 PM ET

One thing that is absent from
by Jason Hyde, Peterborough 6A

One thing that is absent from the article, and discussion, is that the $3000/3500 price is for just the batteries. You need to install it, and buy the inverter.

Still, the cost/kw is coming down, which is a good thing no matter what your application.


May 6, 2015 6:18 AM ET

Response to Jason Hyde
by Martin Holladay

Probably most GBA readers -- at least those who own a car with a 12-volt DC power system -- realize that you can't connect your home appliances (which use AC) directly to a battery (which produces DC) and expect the appliances to work. You're right, of course -- you need an inverter to make AC from DC.

One of the three possible uses of the battery listed in the Tesla press release -- "Storing power generated by PV systems for use at a different time of day" -- is an application that won't require users to purchase an inverter, since almost everyone who has a PV system already owns an inverter.

May 6, 2015 8:23 AM ET

Lack of details
by Daniel Young

We can speculate on how the battery wil be used, and how much it costs. But in reality Tesla has not released enough information yet for anyone to do much with. There is no cycle life data yet on the battery, no details on the battery interface, no clear indication that the 10kWh and 7kWh rating are true, usable kWh (other batteries state larger kWh capacities, but then you can only actually use 50-70% of that and get the expected life from them).

Also, no inverter company has a product for this battery yet (unless tesla will include a 48Vdc interface, which they have not yet indicated). SolarEdge and Fronius are going to be partners in this venture, so we've been told, but they won't have products out till Q3 2015 at the earliest as far as I can tell based on their press releases. And there is no real information on how they will work with the batteries either. They may allow you to collect daytime sun and release it at night, they may allow demand management, they may even allow backup power (I'm really curious is solar edge will, as they have never had a grid-independent inverter before). Time will tell.

The Tesla battery may not actually be a game changer on costs, at $350-429/kWh it's more expensive than any lead battery still, but that does not account for possible cycle life benefits and usable kWh differences. It may be a game changer if the interface with grid-tie inverters allows for easy load shifting and demand response though. It goes back to us not yet having enough detail.

We can all speculate on this, but unfortunately that's all we can do until Tesla, or some inverter manufacturers, start giving some substantive information on the products. I have clients already telling me they "want that battery". Makes me sad, they really just can't know if it is the right move for them yet. I don't even know yet and this is my job! The product won't even be available in a practical system for 3-6months.

May 6, 2015 9:13 AM ET

Response to Daniel Young
by Martin Holladay

You wrote, "There is no cycle life data yet on the battery."

True -- but Tesla is saying, "Powerwall comes in 10 kWh weekly cycle and 7 kWh daily cycle models. Both are guaranteed for ten years." The first sentence is ambiguous -- I assume this means that Tesla has engineered the 10 kWh model to be optimized for once-a-week cycling, while the 7 kWh model has been optimized for daily cycling.

I'd want to read the fine print on the warranty before I bought one, but a battery that can be cycled daily for 10 years, guaranteed, is a good battery.

May 6, 2015 9:48 AM ET

Response to Martin
by Daniel Young

I agree Martin. If it can cycle daily for 10yrs, It would be a good battery, better than most out there. I tend to take warranties with a grain of salt, as i have had to replace entire PV systems due to premature failure of warrantied components. (luckily this was done at the component manufacturers expense.)

For me though, when comparing batteries to offer a client, the cycle life info i look at is the curve of DOD (depth of discharge) vs cycle life, then taking maintenance and other costs into account. It tells a more detailed story on the expected life time costs of the battery. So Tesla gave some information on cycle life, but not what i'd call detailed from a system designers point of view. We're not talking about all the variables though; size, lack of a need for a battery box, lack of a need for a venting system, weight, etc. Those would also have an affect on what battery is best for a given client.

The difference between the 10kWh and 7kWh battery actually points to a DOD vs cycle life curve. I speculate the battery is exactly the same in both, but the electronics are different, such that the 7kWh battery limits the DOD in order to ensure a 10yr life even with daily cycles (and i bet the engineers/statisticians did some extra math to assume some % of people will only cycle maybe 5kWh/day or something like that, that lets them offer the 7kWh with a 10yr warranty with a certain expected % of warranty claims). The 10kWh battery probably just lets itself go down to 90+% DOD, because it's only expected to have to do that a few hundred times in it's life, given a "weekly" cycle. Again, just speculation.

May 6, 2015 4:00 PM ET

Battery vs generator? or both!
by ven sonata

Stephen Sheehy, The single 7kwh battery seems ideal for you during short power outages. But what about long ones? Here is how I do it, completely off grid. A generator as a single source of power is extremely inefficient, noisy, and very expensive per kwh. I know, mine is a 12 kw kubota diesel! So, what you do is run it into a battery. The generator runs for maybe an hour and you are done, the battery, especially this lithium battery, accepts the charge at about 96% efficiency. (Lead acid does not, it will not accept more than a trickle after 85% full, and that is a nuisance to run a generator for 5 hours to finish a charge) The lithium battery does not require full charge, it is happy to continually cycle below 100%, it is a breakthrough for this application. Now when your solar PV is basking in sunlight the battery eats and you can also use electricity at the same time. On a heavy snowy cloudy day, run your generator for an hour or two into your battery and also feel free to blast your heating and your microwave and your vacuum etc etc. it is very efficient to do that. Then turn off the genset for the night and let your battery carry you. What generator? A junker. 5kw tops, cheapo. You just aren't going to use it very much, it will have a trivial few hours per year. your battery on the other hand will get nicely cycled which is the whole point after all. These Tesla batteries at 7kwh will sustain at least 5000 cycles. Just try to use that up!
By the way this is way I live. My generator hasn't come on since January 21, and right now I have 20 people in residence, the battery bank hit 100% full by 10AM through the 12 kw PV array and will stay there until 6pm. By 7 am the battery will still be at 83% full and starting to gain through morning sun. That is only 6 kwh use through 12 hours night with 20 people. And yes, we have two water boilers going for coffee, a 300 ft well pump, microwaves, all led lighting, computers etc etc. My total generator run time for the year is 40 hours. $100 diesel.
You have heat pump etc, which is high demand, but no problem for combo of generator and battery. That generator should not cost more than $1000, it just doesn't need to be either powerful or quality.

May 6, 2015 7:23 PM ET

The specs on the Tesla battery
by ven sonata

Lots of us are trying to look this battery over with a magnifying glass. Although we don't have one on hand a few conjectures can be made. I suggest the closest battery which lists all the specs that we want to know is on the Fronius inverter site. Download the beautiful PDF and see two items: the hybrid inverter and the 9kwh lifepo4 battery which accompanies it. The battery is stylish and sleek and the specs say " at 7.2 kwh" the cycle life is 6000, with 80% remaining capacity! The Tesla batteries are likely identical. The 10 kwh battery just has electronics to discharge to 70% giving, I presume. at least 6000 cycles. Elon Musk said " the stationary battery is different chemistry than the car battery", meaning, I think, state of the art lifepo4, the same as in all sensible stationary lithium batteries. Apparently then, the 10 kwh battery can be discharge to 100% more than 500 times to get "once a week for 10 years". These warranties are extremely conservative, since it is possible to extend them another 10 years. Sonnen battery gives a 20 year warranty on their lifepo4 Sony battery and claim 10,000 cycles to 100%! (That must also mean it is "really" a 12 kwh battery "actually" cycling to 80%). The Tesla battery like the Fronius battery is "internally isolated" above 400 volts DC, allowing for smaller wire sizes, but requiring a DC to DC inverter built in. The Fronius inverter will deal with voltage as high as 1000 volts DC! That sounds crazy but they must know what they are doing.
I want to know if I can adapt this Tesla 7kwh battery to my already existing inverters. That is the question on every off-gridders mind. Stay tuned.

May 6, 2015 7:52 PM ET

12kWh seems likely
by Tim C

The weight is consistent with putting a 12kWh battery under the hood as well.

May 7, 2015 11:30 AM ET

Edited May 7, 2015 11:41 AM ET.

The true value of the battery
by Dana Dorsett

The value is pretty low as a backup generator. The value as a time of use shifter or demand-response charges reducer can be pretty high in some markets.

There is currently only one utility in the US assessing demand charges residential scale PV owners as a means of paying for the grid, but it's a common charge for commercial ratepayers, and often by far the LARGEST part of the bill. A demand charge is assessed based on highest power used in any half-hour or hour during the billing interval, and is independent of the total energy use. That is a fairer method of charging for grid use than a fixed amount based on energy use, since it's the peak draw, not average that drives the infrastructure capacity requirements, and it's the peak draws that reduce grid-equipment lifespans. As small scale PV becomes ubiquitous demand charges may become a primary method of paying for the grid for residential customers (perhaps all, not just PV owners.) When this comes to pass (and it's coming soon in many areas) the ability to limit the peak draw from the grid with a local smart-battery will have far more economic value to the homeowner than as backup power.

The value to the GRID OPERATOR when aggregated with a fleet of batteries to be used for frequency & voltage control is HUGE, since it can get rid of high cost low capacity-factor spinning reserve peaker generators for providing those services. If & when state & local regulators allow that to happen and the battery owner is COMPENSATED for those ancillary services, that's when the remote control by Tesla/SolarCity becomes highly beneficial to the battery owner, the grid operator, and by extension, other ratepayers. (And a huge detriment to the owners of low capacity peaker power generators.)

Ancillary grid services it the big-picture big-impact use for this product. The home-backup and time of use shifting, and even demand charge avoidance value is peanuts by comparison, and that's why SolarCity wants to install a battery with every system in the coming years, and split the ancillary services revenue streams with the homeowners in exchange for managing it.

Having the battery visible to Tesla has additional value to the homeowner in that Tesla would be able to automatically analyze and detect performance or hazard issues remotely and deal with it. Who the hell wants to be a battery baby-sitter, anyway? And who even knows HOW to maintain and diagnose a lithium ion battery pack with smart controls, or wants to learn that stuff?

With the terms of use properly spelled out, remote control of the battery by third parties isn't a problem, and could be a real asset.

My biz-partners pipsqueak internet connected natural gas fired cogenerator is managed in a similar way- he never has to check the hours it's been running or analyze the output looking for maintenance issues. When the total engine-hours are approaching oil-change or tune up time, or if misfires or other issues are detected (hasn't happened yet) he gets an email prompt.

In markets like Australia where home PV power exported to the grid is basically a donation to the utility, and the millisecond by millisecond draw of the house from the grid pays full retail, the Tesla battery pack is already pretty much at-parity or better with grid retail electricity for those who already have rooftop PV (which is a double-digit percentage in that country!) simply by limiting the amount exported to or drawn from the grid:

That fact will limit the amount of continued gold-plating of the grid resources in Australia, which were incentivized by the regulatory structures to keep expanding grid capacity even long after it was clear that power use was no longer increasing, and in fact shrinking. That over-capacity of the wrong grid infrastructure has led to some of the highest electricity prices in the world, which in, turn has fueled the rapid expansion of rooftop PV. If PV were net-metered at retail as commonly done in the US, the utilities would all be bankrupt by now (and that's still a real possibility, especially for the Western Australia grid region.)

May 7, 2015 1:09 PM ET

RMI's take on it is easier to read than mine
by Dana Dorsett


May 10, 2015 5:29 AM ET

"Grid Backup"
by Kevin Dickson, MSME

My Denver supplier has anticipated this market, using LiFePO4:

The grid is fine for backup.

After monitoring rate schedules vs. policy for 35 years, I've given up hope for any common sense in the sector.

May 10, 2015 6:43 AM ET

Response to Kevin Dickson
by Martin Holladay

This is indeed an interesting development. The most intriguing aspect of this marketing approach is that "no approval is required by your utility. We won’t touch the meter at all."

As utilities try to tack on fees to grid-tied owners of PV systems, the economics of this approach will become increasing attractive. Instead of paying (for example) $25 a month to the utility for the privilege of connecting your PV array to the grid, you can spend $20 a month to pay off a battery loan.

May 11, 2015 4:29 PM ET

Round trip efficiency?
by Jonathan Beers

Was the round trip efficiency of this battery included in the specs? For those charging a battery using grid electricity, how much more energy will be consumed due to cycling losses?

Jay Stein of E Source blogged about Tesla's battery announcement: "Utilities, Cheap Batteries Won’t Hurt You. You Have Much Worse Things to Worry About" It's a two-part blog:

May 18, 2015 6:56 PM ET

Response to Martin
by Kevin Dickson, MSME

Right, for 20 years we've believed that net metering is the perfect solution. But the recent utility and political pushback is inadvertently steering us toward batteries, an even better solution if their lifespan improves.

May 20, 2015 1:38 PM ET

Update to Grid Backup Info
by Mark Simmons

Please update our contact info from that old promo card- my email address is now The Colorado Energy Experts trade name is mostly gone by the wayside. We opened a large scale showroom in Denver, called SolarMart, knowing we can deliver Grid Backup. We've been open a year and business tripled from before the showroom. One in five systems sold have been Grid Backup or Off Grid.
Lithium iron phosphate batteries are becoming our mainstay, with 14 year life cycle. Lithium ion isn't quite the right chemistry but it works, with 8 year life. What I wrote on that promo card, about independence from your utility, is more true now that battery prices are coming down.
But the biggest thing is for the solar industry as a whole- WE no longer need the utilities. They cannot slow or stop us anymore. They will, however, mess with homeowners who go solar, adding surcharges for being low power users or other punitive fees.

May 20, 2015 3:30 PM ET

Tesla availability going forward.
by Dana Dorsett

Good luck finding a Tesla home battery in the US any time soon. The Australian market it going to vacuum up quite a bit of their production capacity even AFTER the giga-battery plant gets built. The market in Australia (and Germany) has a huge retrofit market, according to the analysts at Morgan Stanley & UBS:

When the investment banking sector believes it's economic, there will be a way to finance it. The economics in the US (outside of Hawaii or Block Island RI) are still pretty poor compared to Australia & Germany, where the existing numbers of economically retrofittable PV installations are high. There is an existing base of 1,100,000 economically retrofittable installations in Australia alone (about half of all PV installations in Australia) in a market where PV is still going up fast due to the very high cost of grid power. By the time the gigafactory is in full production that number will likely have doubled.

And SolarCity (the largest US installer of residential scale solar, at something like 50,000 installations per quarter, and doubling every year) is talking about installing a Tesla battery as standard equipment in the vast majority of new installations starting in 2017. They have an existing base of battery-ready PV installations already, but those numbers are small compared to what they think they'll be installing in 2017 (even after the federal income tax credit takes a big step down.)

It's a nice market to be in if they can be sold with any margin. The home battery market is clearly big enough to be split among competitors, as others cross the necessary price thresholds.

The big grid battery market is still likely to go to a technology other than lithium ion though. The clunky-heavy but very high cycle number very high current products like Ambri's molten metal batteries have a good shot at a big chunk of that market, since the anticipated lifecycle is so much longer than lithium technology, with very low maintenance (and no cooling) required.

Jun 12, 2015 1:11 PM ET

Capturing the full value of the battery for all.
by Dana Dorsett

Green Mountain Power (the largest electric utility in Vermont) is offering on-bill financing of Tesla batteries on the customers at attractive rates under the condition that GMP has some independent control over the battery.

"GMP will partner with customers to utilize the batteries during peak energy times to directly lower costs for customers by reducing transmission and capacity costs."

Unstated in their public statement, if they're controlling the battery they would also be able to use it for grid stabilization, voltage & frequency control, and peak load shaving, which is of benefit to ALL ratepayers, since the capitalization cost for the hardware is being primarily borne by the battery owner, and it would be providing those ancillary services at effectively zero cost- MUCH cheaper than providing those grid services with utility-owned assets (be it batteries, load dumps, and other fast-reacting hardware.)

In fact, there is a good rationale for all ratepayers to subsidize deployment of both distributed PV (for now) and behind the meter utility-controlled battery assets (pretty much forever), since the value to the other ratepayers is much higher than a simple net-metered at retail scenario would be paying the PV &/or battery owner. The size of the subsidy would have to be carefully analyzed and adjusted over time to make it rational for all stakeholders, but even just financing it is a big shift away from utility Rev.1 business as usual. Utilities have access to cheaper capital than homeowners and small businesses, and should be able to make it a very good deal- maybe even zero interest, given the true value of those assets to the grid (and by extension, all ratepayers.)

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