Solar + Batteries: Is Tesla’s PowerWall the iPhone of Battery Storage?
Solar Technology | June 10, 2015 |Posted by Fred Greenhalgh
Share this post
Part 1 of our series on Battery Storage, also see Part 2 (Solar + Batteries: The Right Thing to Power my Home?), and Part 3 (Solar + Batteries: Technologies on the Bleeding Edge)
Elon Musk, in an event that may be for battery technology what the announcement of the iPhone was to cell phones, announced the Tesla PowerWall in a speech that spoke of a coming dawn of grid-tied photovoltaic arrays with battery storage. We originally published this three-part series in June of 2015 and now, two and a half years later we have seen the rise of the Tesla PowerWall and PowerWall 2 and the fall of prices battery systems across the board. What can we say about the concept of Solar + Batteries now that we could only guess at then? What can we guess at now that we are a little further down the road?
Musk’s vision is an era where power utilities become optional and the average suburban home can realistically be grid independent. Don’t like the power company? Fine, cut the cord. At the end of 2017, this is a goal closer to being a reality, but we don’t think that the fossil fuel utilities are going extinct yet. Innovative companies like Green Mountain Power are instead embracing the resilience that renewable energy and products like the PowerWall offer to evolve into a smarter entities set to usher us into the future.
The excitement and buzz surrounding Tesla’s announcement in 2015 makes now a good time to check in on the development of storage markets and technology and to help separate fact from fiction and marketing hype. To start, let’s take a quick trip through battery history so that we can discuss the PowerWall announcement in context and provide insight into the future of grid-tied solar + battery storage.
Batteries and Solar Energy Systems
Though it seems like ancient history in the context of the fast moving solar energy industry, it wasn’t that long ago that pretty much all solar electricity systems included batteries (and in many parts of the world, they still do).
Before the popularization of the grid-tied inverter in the late 1990s, all solar energy systems were grid independent, which meant that if you wanted to have power after the sun had set, you had to have a battery of some kind to store the energy, usually a big bank of ‘golf-cart-type’ lead-acid batteries.
Fast forward twenty years and, at least in the US, the vast majority of solar energy systems no longer use batteries at all, instead they take advantage of the utility grid and net metering to export excess electricity when the solar production exceeds the load, and to import electricity back from the grid when the loads exceed production.
Grid-tied and net-metered solar energy systems work great and—notwithstanding the propaganda of the utility monopolists—are a real win-win. The solar customer benefits by having the advantage of using the grid as storage (without the cost and complexity of a battery based system) and the rest of the grid and other ratepayers benefit too because solar customers typically export excess electricity to the grid at periods of high wholesale prices and peak demand, and import power at periods of low wholesale prices and low grid demand.
The benefit that net-metered solar systems provide to the grid is well documented in a variety of Cost/Benefit analysis of net metering including Maine’s own Value of Solar Study.
Grid-tied solar energy’s rapid growth has been fueled by staggering drops in equipment cost, roughly 75% over the past 10 years. And though batteries have not experienced the same meteoric progress in performance and cost, many industry observers think it is only a matter of time. Investment in battery R&D is growing worldwide, thanks largely to the surging popularity of hybrid and plug-in electric vehicles; the cost of vehicle batteries have dropped by around 80% over the past 8 years. What that means is that the distributed solar industry is well poised to benefit from those investments and the economies of scale created by that market.
Tesla’s PowerWall announcement illustrated that perfectly as Musk’s expectation was and is that the home energy storage market can ride the coattails of his Gigafactory, designed primarily to produce batteries for next-generation Tesla vehicles. As of the end of 2017, other vehicle manufacturers, like Renault, have launched similar initiatives, offering home battery storage options from used vehicle batteries.
How Batteries Work
Whether it is the Tesla PowerWall, the Pika Energy Island, a cell phone, a cordless drill power-pack or a golf cart battery, the basic concept is the same: a charged battery stores electrical energy in chemical form for later use.
In most renewable energy applications, a special class of battery, deep cycle batteries, are required, as the constant discharge and re-charge of the battery requires more durable construction (in contrast to your non-EV car’s battery, which is used for a short burst of energy before it is quickly re-charged by your alternator).
In addition to being able to survive the wear and tear of frequent use and deep cycles, a renewable energy battery will also have a number of other important performance characteristics that determine how well it works in a particular application. Among those are:
- Energy Capacity: Maximum electric usable energy (kWh) stored in a battery.
- Maximum Discharge and Charge rates: The peak Power (usually given as maximum current) the battery can either provide or accept without damage.
- Depth of Discharge: What % of the battery’s capacity may be used before it needs to be recharged.
- Cycle Life: How many recharge cycles a battery can undergo before it reaches end of life.
- Calendar Life: How long, in time, a battery can be expected to last before it reaches end of life.
- Energy Density: How much energy is stored per unit of volume.
- Specific Energy: How much energy stored per unit of mass. Energy density and specific energy are related but distinct. For example, a battery with the same energy capacity may be large and heavy (low energy density and low specific energy), large and light (low energy density but high specific energy), small and heavy (high energy density and low specific energy), or small and light (high energy density and high specific energy).
- Temperature Limitations: The acceptable operating temperature range of a battery. Some battery chemistries may not operate below freezing temperatures, or at very high temperatures, for example. Others generate significant heat either while charging or discharging and thus may need an active thermal management system to keep them from overheating.
- Self Discharge Rates: The rate at which a battery loses charge while not producing energy.
There is no one single battery which optimizes all these characteristics, and in fact even within a particular battery chemistry (Lithium Ion or Lead Acid, for example), battery designers constantly have to make tradeoffs between different characteristics; for example sacrificing maximum discharge rate to increase overall capacity, or trading off cycle life against calendar life. Or trading any of the above against cost. Clearly, there is no such thing as ‘the best battery’, there is only ‘the best battery for a particular application.’
So Why is the PowerWall a Big Deal?
The PowerWall is a lithium ion (Li-Ion) battery, the same high power, high specific energy and energy density rechargeable battery technology that runs your cell phone or laptop computer. Up until now, Li-Ion batteries were disproportionately expensive relative to their lead-acid counterparts, necessarily limiting the applications where they made sense, such as in portable consumer electronics or in electric vehicles where power and energy density are extremely important.
Perhaps the most eye-grabbing headline of Musk’s product announcement was the PowerWall’s price: $3,500 for a 10kWh battery. Though a lot remained unknown about the battery then, the stated price was still exceptional, making it competitive with maintenance-free heavy duty lead acid batteries. Further, the PowerWall and the subsequent PowerWall 2 have several properties (less weight, higher power, and possibly higher cycle life) that are superior.
It’s that superiority to traditional battery storage options that has contributed to making the PowerWall such a big deal. Musk’s product spurred interest in what would become an emerging industry. This is what happened in the PV market; PV cells today are essentially the same core technology in use since the 1970s, but benefiting from vastly improved manufacturing processes. Tesla is aware that even at their impressive announced price per kilowatt-hour for their battery, the pure economic case for adding energy storage to a GTPV array is not yet compelling enough to make it a no-brainer for the average residential solar customer. And so, they and other manufacturers have worked tirelessly to reduce that price.
One thing we can say with certainty: the iPhone changed the mobile phone industry forever and changed the trajectory of hundreds of competitors. Musk’s vision (and his extraordinary ability to use PR) has truly recharged the home battery storage marketplace.
Like the iPhone was at its time of release, the Tesla PowerWall was a milestone and only the beginning of a new era of battery storage. And like the iPhone, it’s introduction has pushed others (Pika, Sonnenbatterie, etc.) to innovate and improve their products in that space as well. Residential backup batteries are not a new technology, but the PowerWalls’ and their competitor’s better packaging and user experience are an update of an existing technology; as has been proven before, a product that has pizzazz, consumer appeal, and strong engineering to back it up, can radically transform the world.
Next Time: Applications of Energy Storage, Past, Present, and Future
Our follow-up installment dives into the applications of battery storage in more detail than what we’ve done today. We expect our customers to be most interested in residential battery backup power (to replace/supplement a generator) but other applications include grid ‘arbitrage,’ commercial demand charge management, and providing ancillary services to the grid (such as regulating voltage and frequency).
We’ll dive into each of these areas and explain where battery technology is and can be, for each. Our final installment will be a more comprehensive overview of the various battery chemistries on the market, and in development, and explain which applications each battery may be best for.
As a leading solar energy provider, ReVision Energy has experience with a wide variety of systems and battery storage options to supplement grid-tied arrays, or in some cases even for totally stand-alone systems.
Please contact us if you’d like to discuss solar + battery storage options in more detail and we’ll be glad to help you separate fact from hype and to design a robust and reliable energy storage solution that suits your needs.
If you are not yet in the market for a battery energy storage system, you can still take comfort in knowing that nearly all solar electric systems we install for customers are backward-compatible with battery storage so you can install a grid-tied inverter and solar array today and add batteries years in the future with no major system modifications.