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The Tesla Powerwall is Useless

May 15, 2015

Musk’s favorite phrase when trying to disparage something is to say, “It sucks.”

Should this crude term be applied to the Powerwall?

The Tesla Powerwall battery is to be used in conjunction with PV rooftop solar installations, and not for commercial or utility applications.

The 10 kWh Powerwall costs $3,500. Reportedly, it costs over $7,000 installed.

Can the Powerwall provide the following fundamental benefits?

  • Backup power for grid failures
  • Monetary savings by not using electricity from the grid
  • Demand response for shaving peak load
Tesla Powerwall

Tesla Powerwall

To be useful and worth the cost of installation, the Powerwall must be able to do at least one of these functions well.

Let’s examine each function to see whether the Powerwall provides the anticipated benefits.

Grid Backup

Essential household loads include:

  • Refrigerator-Freezer
  • Heating system blowers
  • Air-conditioning
  • Hot water heater (if electric)
  • Microwave oven

These are needed for any household to function at a basic level: The ability to cook meals, to keep food safely in a refrigerator/freezer, to maintain home temperatures that are safe, and to provide hot water and lighting.

These loads will vary by time of year and by whether the activity is provided by electricity.

During the summer the Powerwall can provide backup power for 2 to 3 hours. The air-conditioning load is large, so in the winter the Powerwall might be able to provide backup power for 5 – 6 hours.

Adding television usage would slightly decrease the length of time the Powerwall can provide backup.

While many utility interruptions are fairly short, less than an hour or two, the major outages are caused by ice storms, wind storms and hurricanes.

These outages last for several days, so a Powerwall unit cannot supply backup power for these outages.

If backup is to be provided for lengthy outages, the Generac, or equivalent, costing $4,000 and using natural gas, is a much better investment. It also provides backup for the short nuisance outages.

Objectively, the Powerwall is unsuitable for providing backup power.

Saving Money

Can the Powerwall avoid using electricity from the grid at night, after the sun goes down? If so, this might make a $3,500 investment in a Powerwall unit worthwhile.

Fully recharged, the Powerwall might be able to save $1.10 per day by providing power after dark, with a utility rate of 11 cents per kWh. This amounts to approximately $400 per year in savings, so it would take approximately 9 years to recover the Powerwall $3,500 investment. Or 18 years if the total cost of installation is included.

With a 30% rebate for batteries used in PV rooftop systems, the payback would be closer to 6 years, or 12 years if the $7,000 installed cost is used.

But why degrade the Powerwall battery to save on electricity usage when the owner of a PV rooftop system with net metering can sell the excess electricity from his PV rooftop system to the utility for the retail rate of 11 cents per kWh?

Or, if the utility only pays 5 cents per kWh, the real money saved by using the Powerwall battery to save on using electricity from the grid is actually 6 cents per kWh, not 11 cents. This increases the payback period to 14 years without including installation costs.

There is also some question as to how often the Powerwall battery can be fully discharged and recharged.

A payback period of 9 to 14 years, or 6 years with the 30% rebate, (not including installation costs) is terrible for a product that may only last 10 years.

Clearly, the Powerwall battery is not suited for storing electricity to avoid buying electricity from the grid.

Demand Response

Theoretically, utilities could group a large number of Powerwall batteries from homeowner PV rooftop installations, and install the necessary controls to use electricity from the batteries during peak periods.

This is obviously impractical since the homeowner would stop using solar power and start using power from the grid, offsetting any reduction of load that might be achieved by the utility drawing power from the Powerwall battery. The Powerwall battery can’t simultaneously serve two masters.

This differs from commercial and industrial customers in states where time of day pricing is used. In these situations the price for electricity can be very high during peak periods and commercial and industrial customers can avoid those high prices by using electricity from batteries they own.

Powerwall batteries cannot routinely provide demand response.

In summary:

The Powerwall battery is inadequate for providing backup power, doesn’t save much money and can’t contribute significantly to demand response.

What term does Musk use to describe an inadequate product?


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29 Comments leave one →
  1. Craig Mott permalink
    May 15, 2015 9:40 am

    Hi Donn,

    Insightful comments on the powerwall for homeowners. I agree that the powerwalls are too expensive for home use. The natural-gas powered Generac is much more affordable and would provide backup power for long outages due to storm-generated power outages.

    Will the power companies in California buy Tesla’s utility backups or will they use hydro or other means of energy storage?

    Do you think Elon Musk will make any changes in pricing to get a better pay-back for homeowners?

    Do you think that Elon Musk needs powerwall sales in order to support Solar City sales of homeowner solar arrays?


    Sent from my iPhone


    • May 15, 2015 10:01 am

      I plan to discuss the Powerblock unit that’s designed for commercial and utility use and the gigafactory’s total capacity next week.
      I doubt whether Musk will make any pricing changes in the near future. His acolytes will buy enough Powerwall units to exhaust near term supply. He can barely meet demand with his current facilities.
      I don’t think the Powerwall matters very much to SolarCity, except for publicity purposes. From what I have read they seem to be downplaying the Powerwall.
      I’m finding it virtually impossible to find any reliable estimate for storage requirements in California, let alone for the entire country. This, as you will see in my article on the Powerblock, makes it very difficult to reach any concrete conclusions about the gigafactory at this time.
      With respect to hydro, California doesn’t include it as a renewable source. I get the impression that CAISO counts it as part of its baseload source and isn’t counting on it as an adjunct to storage. Germany, on the other hand, is going to connect to Norway’s hydropower using an underwater DC transmission line to help level the load curve.
      I think California is still struggling to figure out how all this is going to work, so they may eventually figure how hydro might fit into the storage equation.

  2. sandek1 permalink
    May 15, 2015 10:55 am


    And the answer to your question is “It sucks.”

    Interesting discussion.


  3. John Ch. permalink
    May 15, 2015 12:56 pm

    My worry is that Elon Musk’s plan is to have a great product in play for when the grid reliability plummets, and the price of power skyrockets, which seems to be an excellent way to skim more wealth from the masses, in this case ratepayers more so than taxpayers. Don’t do calculations on current delivered KWh rates.

    When Energy companies separated generation from distribution and both started telling us to buy less energy, the writing was on the wall: A comprehensive and all encompassing rent seeking behavior would supplant production for competitive price, and the deep glean from the masses would begin.

    If a widget company tells you to buy fewer widgets, we have to know something else is in play. There are now far too many elements of our plutocracy required to have unreliable electrical provision and at a high price per KWh.

    • May 15, 2015 1:52 pm

      There is no question the utility business and grid are under siege. How that will ultimately play out is hard to establish.
      No question that electricity prices will eventually sky rocket if this scenario continues unabated.

    • kakatoa permalink
      May 16, 2015 12:57 am

      John says “Don’t do calculations on current delivered KWh rates.” I concur!

      The advisory panel for the CA RPS study said this over a year ago-
      “Higher RPS requirements at the 50-percent level would likely additionally increase electricity rates in 2030 by a wide range, compared to the expected rates based roughly on current policies and plans: the estimated increases were from 9 percent to 23 percent, depending upon the scenario under base case assumptions. The range was 3 percent to 36 percent under different sensitivity analyses, depending upon scenarios that changed combinations of variables. These estimated rate increases in 2030 were above and beyond the already-higher rates assumed to occur by then in the base case (which are estimated to be 47-percent higher than today’s rates).” Reference-

      Click to access Advisory_Panel_Report_on_the_CA_RPS_Study_FINAL_1-2014.pdf

      The AVG price of a kWh is around $.20 for CA’s three private service providers today. My little PV system has been working fine (20% capacity factor) for nine years. Given the overcapacity issues noted in the report-

      I will be evaluating some alternatives over the next few years. Trying to figure out what my cost allocations will be for my future usage from PG&E is not something I am looking forward to doing. At the moment the marginal price for a kWh is around $.27 from PG&E for me, and most PG&E residential customers, for half of the year (last month I generated more power than I used from the grid) , so I guess I should consider using an increase of 50% in this rate given the information above as my low estimate and 70% more on the high end when evaluating energy efficiency, additional PV with battery and generator options, etc.

      • May 16, 2015 8:46 am

        Thanks for your comments.
        I agree that today’s rates shouldn’t be used to estimate future rates when the percentage of renewables is higher at 40 or 50%.
        This is difficult for the non-technical person to understand.
        The advisory panel report clearly establishes that rates will be higher than expected if today’s rates are used to estimate future rates, but most people won’t read the report.
        It’s too bad that RPS advocates can paint a rosy picture, when it’s actually a false picture.
        Thanks also for the reference. I will use it in an upcoming article.

  4. May 15, 2015 3:02 pm

    I sense one motivation for the home battery is to provide a source of power for people who own both electric cars and solar panels. The sun is down when they get home from work. They park their car in the garage and plug it in to recharge. Where does the power come from during intervals when solar panels are effectively useless? Either the grid or from battery storage from daytime solar or the grid. I suspect off-peak grid power is cheaper.

    • May 15, 2015 3:57 pm

      Thanks for the comment.
      Even if fully charged, the Powerwall cannot provide more than a small, partial recharge of a car battery that has been fully discharged. Depending on the size of the car battery, a Powerwall would only provide a 12 to 15% charge so it’s not suited for this purpose.
      It was for this reason I didn’t include it as a possible function for the Powerwall.

      • May 19, 2015 5:16 pm

        I assumed this was one reason why Tesla is marketing them as daisy-chained devices so people who need more buy more. Though, it does seem foolish from a capital perspective vs. just paying $0.09/kWh off peak.

  5. Bryan Leyland permalink
    May 15, 2015 11:52 pm

    Dear Donn,

    I have a suspicion that the powerwall inverter will only produce 2 kW.

    I suspect that it will not control frequency and voltage in an isolated system – but I have not checked.

    This from Wikipedia. Which one do they use? Grid tie i suspect. but it might be a battery backup inverter

    Solar inverters may be classified into three broad types:[citation needed ] Stand-alone inverters , used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays. Many stand-alone inverters also incorporate integral battery chargers to replenish the battery from an AC source, when available. Normally these do not interface in any way with the utility grid, and as such, are not required to have anti-islanding protection . Grid-tie inverters , which match phase with a utility-supplied sine wave . Grid-tie inverters are designed to shut down automatically upon loss of utility supply, for safety reasons. They do not provide backup power during utility outages. Battery backup inverters, are special inverters which are designed to draw energy from a battery, manage the battery charge via an onboard charger, and export excess energy to the utility grid. These inverters are capable of supplying AC energy to selected loads during a utility outage, and are required to have anti-islanding protection. The charge/discharge efficiency is about 80% so you put in 1.25 kWh to get one out.

    If the battery will last 1000 cycles, it costs $.35 just to store one kilowatt-hour.

    Kind regards,

    Bryan Leyland

    Phone +64 9 940 7047 Mobile +64 21 978 996


    • May 16, 2015 8:55 am

      Thanks for your comment. It’s my understanding the inverter doesn’t come supplied in the Powerwall but is added by the company installing the PV rooftop system. If SolarCity installs the Powerwall they must be conforming with the requirements for grid compatibility. This may be why the installed cost is reported to be $7,000.
      I have’t seen where the degradation of the Powerwall is mentioned, but the Powerblock is rated for 5,000 cycles. The Powerblock will need to be replaced after 15 years. I’m not sure about the Powerwall.

    • May 17, 2015 11:42 am

      Hi Bryan,

      I was reading this post by Navigant
      that said this-
      “Enhanced Intelligence

      Basically, Tesla will provide the storage hardware to a facility and EnerNOC will provide the software smarts to tell the unit when to charge or discharge in an optimal manner. The software does this by taking in signals from inside the building and from external markets and figuring out how to gain the most financial benefit on an ongoing basis. Remley said that the Tesla unit has some smarts as a standalone unit, but the gains from EnerNOC’s software should outweigh the costs,”….

      The enhanced intelligence reference reminded me of the movie Apollo 13 and how many engineers it too to figure out how to manage the loads to get the crew back to earth. Lots and lots of details had to worked out technically speaking. How to leverage the capability of the Tesla batteries is going to take a lot of code to digitize all the decision trees.

      • May 17, 2015 12:14 pm

        Thanks. I saw the Navigant statements and agree with you.

  6. David Koch permalink
    June 21, 2015 8:34 pm

    It is not useless, it is not for everyone. As a power source for RV, power source for golf cart, etc it may be wonderful improvement… matter if you can buy it alone rather than installed.

    It allows the *right* person to go off grid, the right person has enough solar panels during day to power his house, he runs most of his loads (eg washing clothes) when sun is shining. The big savings with off grid is no longer need utility hookup.

    As far as grid tie utility savings, places like Hawaii and germany the numbers make sense.

    For me sounds *very* interesting… the big limitation is 300 volts rather than something more standard like 48V, or 24V.

    • David Koch permalink
      June 21, 2015 8:41 pm

      “no longer need utility hookup” – you may save $15/month for not being on grid at all, tends to be a fixed charge every month. If you are in more remote rural area may be $3000 to $10000 to even get hooked up to grid.

      Off grid is different sort of mentality. Example refrigeration… freezer can be “outside’ in winter, use cold weather to reduce need as less sun. fridge can be a very well insulated icebox where you switch ice between it and freezer every few days. Heat can be wood stove, house can be insulated to several times the normal standard, and ways with thermal mass to keep house above freezing for many days if you are not home and you always have the cheap diesel generator in emergency.

      • June 22, 2015 9:31 am

        If you read my other articles you will see that it’s not possible to go off grid in the US anywhere except in the southern US. Even there it’s not really practical.
        Of course, people could go to extremes and live without electricity. Or connect to a gas generator etc. Powerful batteries are not practical.

  7. August 14, 2015 1:33 pm

    I feel the actual use of the powerwall is completely being overlooked. The powerwall is not a means by which to store electricity during it’s cheap period, nor is it specifically for households with solar energy (although it can be used as such).
    The powerwall is a way of solving the issue that solar and wind energy bring to electricity production. What happens when the sun is down and the wind isn’t blowing? Where will the electricity come from?
    So now the question is, how do we store the electricity for later usage? One large battery for the entire city? Not very feasible. Personal air compression storage units that are currently being developed where spare electricity compresses gas into chambers and when electricity is required it releases the gas to run a motor to generate electricity? Well that’ll need a lot of space.
    The powerwall is a battery for personal home usage that stores naturally produced electricity when created in surplus to be later used by the consumer. The goal is to have each household in a community (a whole community which is powered by solar or wind) to store the electricity when not in use for times when there is no sun or wind. This is a more practical solution for the problem of energy storage amongst an entire city. It also has benefits by which you can store energy when it’s cheap, store your own generated electricity, and even not be concerned with area restricted power outages as you’ll have your own personal power source as backup.

    • August 14, 2015 5:21 pm

      Thanks for the comment. Actually this use is addressed in the article, and in the next article on the subject. Unfortunately the Tesla Powerwall is an expensive fix that doesn’t really work.

  8. dee1001 permalink
    September 23, 2015 1:18 pm

    What about in the situation, when no PV is available, and on-peak rates at $0.16/kWh and off-peak are $0.08/kWh, Will the Powerwall help not to use on-peak power? Also, what if you have a house with a 200A panel, can this theoretically provide 200A?

    • September 23, 2015 2:02 pm

      Yes, but at a high cost. It’s rated 8.6 amps continuous, so can’t provide 200A
      200 is a lot of current.

      • dee1001 permalink
        September 23, 2015 3:13 pm

        So I assume, if one installed a powerwall, you would have to splice it into a sub-panel with specific sub-circuits to avoid exceeding 8.6 amps. Would this be a correct assumption? In other words, it will only peak-shave for a these specific circuits, and not help, say when, the big heat pump comes online.

  9. September 23, 2015 4:58 pm

    dee101: You need an electrician. I can’t wire the panel from the inverter for you. A 200 amp panel merely reflects the maximum amount of current that the panel can handle.

  10. dee1001 permalink
    September 24, 2015 9:39 am

    Hi Donn, I understand that. In my opinion, Tesla’s advertisements of providing whole house backup are suspicious, when the Powerwall only provides 8.9A of current. A backup generator provides 20-30A (limited by a sub-panel), Powerwalls will never fly if they cannot provide amperage for the entire house demand.

    • September 24, 2015 9:41 am

      Thanks for your comments. I agree that the Powerwall doesn’t live up to the hype.

    • Mac Micheals permalink
      September 29, 2015 5:04 am

      You are all really mixing up your units here. The powerwall battery provides 5.8A continuous and 8.6A peak at 350v – 450v. These all and voltage rating are for the battery pre-inverter. The inverter will drop the voltage to standard home110v – 120v which means an increase in current (amps). Volts x amps = watts (power) which the powerwall is rated at 2000w continuous and 3300w peak. At those ratings it can supply approximately 16.7A – 18.2A continuous and 27.5A – 30A peak. The avg US home use 27kwh per day. Averages can often be much higher then a mean though and powerwall consumers I would assume are more energy effecient. Some homes use 5kwh or less per day. So yes it could power some homes for 2 days or so and others for 1/3 of a day say at peak hours like it was designed for. Nobody ever uses close to 200A on your main breaker. that matches the rating of the huge wire coming into your house and stops it from melting in event an electrical short or some crazy surge. If you ran at 200A for a day it would be 1152 kwh enough to power 43 avg homes for the day.

      Also to the person who mentioned the limitation of 300V vs the standard 48V (powerwall 350V – 450V) you are confusing grid tied solar with off grid. Solar panels are wired in series quickly jumping to hundreds of volts a typical grid tie invert input voltage is 300V – 600V with up to 1000V not uncommon they actually won’t operate under 300V. Off grid a charge controller (DC-DC converter) will take the high solar panel voltage and drop it to the standard 48V to charge the battery bank which is then connected to a off grid specific 24V, 48V inverter.

      So is the Powerwall useless? Comparing it to the alternative? You could do the same thing with some golf cart batteries. A golf cart battery will provide about 1kwh at a cost of $150. You would need 10 cart batteries totaling $1500 to equal the 10kwh Powerwall for $3500. The Powerwall has a 10 year warrenty, weighs 220lbs, can discharge lower (more useable power), maintenance free, indoor mountable. The cart batteries last half as long, weigh 600-700lbs, need to be outside but sheltered, and balance of charge needs to be monitured. For either system you still need an inverter that costs a couple thousand. So really batteries for this purpose suck. Buy a generator makes way more sense for power backup and neither solution makes sense for use during on-peak unless you just want to do your part of powering peak demand at your own cost.

      The usefulness of the powerwall comes in for off grid situations and other various applications where batteries are the only choice. Li-on batteries are battery in every way compared to lead acid, the only problem was price, but it seems the powerwall is comparable if not more econmoucal now on comparison. This is how it works though, someone really pushes a new technology that is kinda useless in the moment for the price. There is always going to be the early adopters people who want the newest, want to show off or make a statement or people that just buy into the hype. Production goes up prices go down and then the now kinda useless kinda sucks powerwall was the start of the new standard.

      • September 29, 2015 8:33 am

        Thanks. Great comments. Li-ion batteries are currently (sorry for the pun) the best technology for the application. Whether the price can come down sufficiently within the foreseeable future to make the Powerwall useful is questionable. When the entire PV rooftop system is considered, the Powerwall merely makes the system less economically viable. The only state in the union that can obtain a reasonably good return on a PV rooftop system is Hawaii. In this context the Powerwall has little value elsewhere.


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