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Charging Tesla Powerwall – How Many Solar Panels Are Required?

In Education 10 Comments

First, let me be clear – unless you’ve already bought one, you are not getting a Tesla Powerwall in the next 18 months. They are simply not available. There are plenty of other battery options out there, but this article focuses on the Powerwall because of the media buzz that has been generated recently. With that said…

In short, there is no answer to “how many solar panels you need to charge a Tesla Powerwall” without knowing the time-frame over which you want to charge it. In fact, you could theoretically charge up a Powerwall battery with a tiny 10 watt solar panel, but it would take almost 6 months!

Let’s get down to some battery basics:

Batteries do not produce energy – they store energy. The rating of a battery is its capacity. The larger Tesla Powerwall battery can store 10 kilowatt-hours (kWh) of energy. That energy can be used in your home, but it requires an inverter, which converts DC electricity to AC electricity. Batteries capacity ratings are dependent on the amount of time over which the energy is used. The assumption in this case is that you will use 10 kWH over a 24 hour period, which means you can use 0.416 kilowatts (kW), or a steady 416 watts for 24 hours before your battery is fully depleted. If you attempt to use electricity faster, the capacity will be less. The maximum rate at which you can use electricity from the Powerwall battery is 2 kW. At that rate, the battery will last under 5 hours.

Solar panels generate electricity that can be stored in the Tesla Powerwall battery. A charge controller is required to regulate the charging rate. You can charge a battery at a variety of rates, and while Tesla has not published the maximum charging rate, we can assume it is the same 2kW as the output rating. That would equate to about 8 solar panels at 280W each (adjusting for system losses). In Florida, 8 panels rated at 280W each would produce about 11 kWh of energy per average day, but since there are losses involved in battery charging, realistically you would probably be able to store about 7 – 8 kWh of that energy generated. If your battery is 70-80% depleted, you could realistically charge it up fully on an average day with 8 solar panels.

Now that is an “average” day. Since solar energy is weather and time-of-year dependent, some days will be better, and some will be worse. If you install a large number of panels, you may be able to charge up your 7 kWh Powerwall in under 4 hours. If you install a small number of panels you could charge up your Powerwall over a period of multiple days. If you’re looking for panels that will charge a fully depleted Powerwall battery on an “average” day, about 10 panels rated at 280W would be a good estimation.

To summarize:

  • Batteries don’t produce energy – they store energy. Batteries don’t save money – they cost money.
  • You need a charge controller and an inverter (and other distribution equipment) to be able to use a Tesla Powerwall in your home. Don’t be misled by the seemingly low price of the battery itself.
  • The average home requires more than one battery to sustain energy needs during an outage. In fact, the average home in Florida would need about 4 batteries to maintain full power for a single “average” day of use, and there would still be limitations.
  • You can charge a Powerwall with any number of solar panels. The number of panels will determine how fast it charges.
  • Solar panels have variable power output, so the charging rate is not guaranteed.

In reality, your Tesla Powerwall battery would most likely be part of a grid-interactive solar energy system, and the battery would charge from the utility grid and solar panels simultaneously. The relationship between storage capacity and solar energy production is complex, variable, and imprecise. The first step is understanding how energy is used, stored, and delivered. A battery is not a magical perpetual motion machine that solves all of our energy woes. In fact, it is more of a necessary evil to cushion variability in solar energy production, utility power time-of use cost, and utility availability (outages). Determining how many solar panels you need is more a function of how much energy you use (power use over time), not how much energy you can store.

Comments

    1. Author

      You don’t really charge a Tesla Model S directly with solar panels. What I think you are asking is how many solar panels do you need to offset the utility electricity necessary to charge your car.

      That’s not easy to answer. I would need to know your battery capacity, how many times per year you recharge your car and what percentage of depletion you reach each time you charge it.

      Let’s say for argument’s sake that you have the 85kWh battery. Let’s assume you drive the car 250 days each year, and each day you deplete the battery to the 75% level (25% depleted), and you recharge it each day after you drive it.

      The amount of energy you would need over the year is approximately:

      250 x 85kWh x 0.25 x 1.1 (efficiency factor) = 5,844kWh (kilowatt-hours)

      To generate that much electricity with solar panels in Southwest Florida you would need a system of approximately 4.1kW (kilowatts).

      The number of panels depends on the rating of each panel. Let’s assume you use a 260W panel that is popular today. You would need:

      4,100W / 260W = 16 solar panels

      Note that these are very, very rough numbers. It depends on a lot of factors like the charger efficiency, the solar panel orientation, maintenance charging when idle, temperature, battery age, solar panel age, and a laundry list of other factors. This answer is intended to be a rough guide for you. If you live in Southwest Florida and want a more personalized answer and education and advice on solar panels, please contact us!

  1. Hi Jason,

    Thank you for a very informative article. I just wonder, currently the Tesla batteries costs 350 $/kWh. If for example, in year 2025, when batteries are already cheap and costs around 100 $/kWh or possibly much lower, do you think it will be viable to go completely off-grid using solar pv and battery storage systems?

    Thank you.

    1. Author

      Paul,

      Thanks for the question. It’s a bit more complicated than just monitoring the future $/kWh for battery capacity to determine when it becomes viable. You have to look at it on the system level including all of the components that go into a system. The battery is just one component, currently representing perhaps 1/4 to 1/3 of the total cost of going completely off grid. Battery prices dropping by 1/3 would only reduce the system cost by 1/9th to 1/12th the current prices.

      In fact, we have flooded lead acid batteries in the $100-$125/kWh range today. So if that’s the magic number, it would be viable today. But viability is a matter of opinion, budget, and risk tolerance, not just raw math. Where are utility electric rates headed? What ROI (if any) would satisfy you? Do you personally consider important the environmental costs and benefits of battery based off-grid technology?

      Finally, you can’t just look at $/kWh of rated battery capacity. You need to look at discharge cycles. How deeply will the batteries be discharged and how many times? Lithium-ion and Lithium Iron Phosphate (LiFePO4) batteries are generally rated for more discharge cycles than lead acid counterparts with similar rated capacity. You need to look at the lifetime kWh supply capability.

      There is no simple answer to when off-grid viability will come, and it probably won’t look like what we imagine. The whole paradigm of energy generation and distribution will probably shift, with a new generation of smart grid that benefits producers, storers, and distributors of power and energy differently. In my mind we will see individual homes and businesses being producers and/or storers, with utility companies becoming energy distributors and traders. Everyone can participate in their own way to their own economic benefit.

  2. Hello,
    Interested in powerwall for backup electricity. For an extended outage my plan is to recharge it using a small generator. My question is does the powerwall have the ability to limit incoming current? If not, connecting the generator would immediately overload it and trip the generator’s breaker.

    I have not been able to find anything about charging the powerwall from any source other than the grid or solar?

    1. Author

      Yes, the inverter will not be able to tell the difference between the utility company and a generator. In either case it will only draw the current required for the integrated battery charger.

  3. Hi Jason

    I hope I’m not too late to the party (or to early?) for you to still follow this article of yours, but I have some questions.

    I have pre ordered the powerwall 2.0 AC version and awaiting conformation of delivery summer 2017. Are you familier with this model? There’s not much information out yet on how this PW2.0 actually works.

    I’m in contact with tesla, but they do not have too much information to give out unfortunately.

    If I use 2x 300W solar panels at 40-60vdc, what extra component would I need to use this to charge the PW at 230VAC? You say the internal charger of PW would draw the current it needs, wouldn’t this kill a directly connected 600W dc/ac converter?

    I still really don’t know how to use my solar panels to charge PW.

    In addition and continuation of what George is asking, how can one use a generator to charge? Is a 5kW generator needed?

    Can maximum charging current be set using the app?

    Any info on this would be greatly appreciated.

    1. Author

      Tore,

      In true Tesla fashion, the hype comes with few details, specifications, and manuals. They make these announcements when the products are essentially vaporware – they don’t exist. The details are up in the air. While they might be great products someday, they are expensive relative to existing technology. Early adopters will probably be happy, albeit a little lighter in the wallet than others that go with traditional battery storage.

      The idea is revolutionary. The idea is to combine a battery, charge controller, and inverter in one box. Well, it’s not so revolutionary, since it’s been done before – It’s just a new battery chemistry in a much lighter, smaller, and more consumer friendly package.

      I don’t even know where to start with your questions. Two panels at 300W is really a drop in the ocean. That’s the first thing you should realize. You will not be able to recharge the Powerwall in a single day.

      The Powerwall 2.0 will come in an AC coupled and DC coupled model apparently. I favor AC coupled systems, in which case you would use a microinverter on each solar module to connect on the AC side of the Powerwall. But it’s not as simple as plugging in a couple of wires. Your design needs to be done by a competent contractor or engineer. And it would probably prove cost prohibitive to do this for such a small PV system. If you go with a DC coupled system, you will need a compatible charge controller. Your guess is as good as mine at this time about compatible models. And it will be expensive to go this route for a small PV system, too.

      The generator question has the same answer. You can’t AC couple a generator in most cases. You would probably need a compatible external battery charger unless the Powerwall 2.0 has a separate input for generators to use the internal batter charger.

      In all likelihood you will not have access to batter charging parameters. The integrated smart charger will do that for you.

      I wish I had more answers for you, but Tesla likes to keep solar pros in the dark. Maybe they don’t have the answers yet. Maybe they want to control the whole supply chain. It’s really not a good relationship model with smaller solar installers in my opinion.

  4. Hi Jason, I was wondering if I could get your opinion on the power wall if it is worth getting if you are going solar, thanks.

    1. Author

      There is no economic argument for it in Florida. I’m not sure what all the hype is about. Maybe I can turn this around and ask what people think the Tesla Powerwall will do for them? If you are thinking about this product for backing up power in your home, it is a terrible economic investment. Just buy a generator for the brief and infrequent times that you need backup power. It is FAR less expensive. You might also be thinking the Powerwall will somehow save money. It doesn’t. In fact, it will just cost you money. Here is a link to another article on the subject:

      https://floridasolardesigngroup.com/solar-battery-backup-does-not-save-money/

      If money is no object and you are an early adopter type, we can talk… but I would recommend a much less costly solution with what’s available today, and the Powerwall is really not (here).

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