altE’s Take on Tesla’s PowerWall Battery Announcement

Amy Beaudet May 12, 2015

We’ve all heard the excitement over Elon Musk’s Tesla Energy battery announcement. We thought we’d take a moment to try to help sort through the bits of information released. Additional pieces of information are slowly coming through, so we are all learning more details each day. Then we’ll compare, cost-wise, Tesla’s batteries with ones that already exist on the market.

What We Know About the PowerWall Batteries

Tesla Motor’s new subsidiary, Tesla Energy, announced two residential models of their lithium-ion battery, the PowerWall, that they will eventually be making in their $5B gigafactory they are building in Nevada. The PowerWall will be available in a 10kWh, occasional use, back-up version for $3,500 to distributors and a 7kWh daily use version for $3,000. Up to 9 of these batteries can be stacked to increase capacity. Just for a reference point, the “average” US household uses about 30kWh a day.

SolarCity has announced that the install options for the 10kWh backup system will have an option of a nine year lease for $5,000, or one can buy the system for $7,140. The price includes the inverter, installation and maintenance. Installations are scheduled to begin in October 2015.  They do not intend to initially provide the 7kWh daily use version, as they don’t believe it is economically viable in the US market. Until the gigafactory is up and running, supplies will be limited.

How Do Tesla Batteries Compare to Other Options Already on the Market?

The battery bank is listed at 350 – 450VDC, so they will not work with a typical battery-based inverter, which are usually 12V, 24V, or 48V. Tesla announced a partnership with SolarEdge and Fronius, who currently make grid-tied inverters. Both companies are stating that a single inverter will manage both the PV array and the battery bank for new installations. For existing grid-tied systems with another brand of inverter, an additional inverter will need to be added if the customer doesn’t want to replace their existing one. SolarEdge has announced that the change will be simply a firmware update to their existing inverter to turn it into a StorEdge, it will treat the battery like an optimized PV string, so no additional hardware is needed. Fronius’ Symo Hybrid will use one of their two MPPT inputs to connect to the battery bank, the other to the solar array.

Tesla also says that the battery “pairs with a growing list of inverters,” so presumably, they will be working with other manufacturers soon. However, Fronius announced they will initially be launching their inverter in Europe, and SolarEdge has said theirs will be available by the end of 2015.

So what is the advantage of adding batteries to a solar grid-tied system?

While Musk’s ultimate goal is to eliminate the need for fossil fuels, his shorter range plans are to help reduce homeowner’s power bills by peak shaving. Instead of selling excess power generated during the day back to the grid, the 7kWh system can store the excess power, and use that power later in the afternoon and early evening when power demands, and in some locations, power prices increase. Most residential solar systems generate their power when the homeowner is away at work, whereas most home power use is in the morning and evening when the solar isn’t producing much, if any power..

By making the power generated during the day available at night, the net-metering by the power company is no longer needed and you don’t need them to store it for you anymore. In regions where there is no time of use or peak pricing, where the power company charges you more for power bought during certain hours, this doesn’t provide much advantage. If you sell a kWh for $0.15 and buy it back for $0.15, there’s not much justification for your own battery. But if you are in an area that does not have solar friendly net-metering agreements, this can provide a use. If you are selling for $.04 and buying at $0.15, suddenly there is an economic advantage to using your own power later instead of selling it. However, that is currently a very small percentage of the US market.

Another grander benefit of storing your excess power instead of selling it is that it helps even out the load on the grid. Instead of the power company having to start up additional power generators to handle the late afternoon load of people getting home and turning on their air conditioners, the distributed network of solar installations will be able to handle that increase, allowing the grid to be more stable. If enough households have their own solar generated power available when they need it rather than when the sun is peaking, a lot of the grid’s peak power generation issues get resolved.

Comparing Tesla PowerWall batteries with products already on the market

What makes the Tesla Energy PowerWall so special? That’s the big question of the day. They certainly have the Elon Musk name and Marketing power behind it, which certainly gives it a leg up to begin with.  Tesla is now claiming 5,000 cycles at 80% Depth of Discharge (DoD) for the life of the battery.The new, non-toxic, salt water batteries by Aquion claims 4,000 cycles at 80% DoD. With this information, and the information of more traditional types of batteries, we can now compare the cost over the life of the system. By taking the kWh capacity and multiplying it by the DoD and cycle life expectancy, we can get a fairly good look at the cost per total kWh the battery can store through its life expectancy (until the battery is has degraded to 80% of its rated capacity).

We can see that the price per kWh for the life of the battery is close to the new Aquion batteries, but about 40% less than the cost of a flooded lead acid battery, and 65% less than the lifetime cost of an AGM.

However, the Tesla batteries are limited to 63kWh, the lead acid batteries in a 3 string 48V system would be limited to 53kWh, but different battery models in the same range can be used for larger systems. However, the Aquion batteries can be stacked in a limitless number, allowing for MWh systems.

More reports are coming out about the availability, or lack thereof, for the 7kWh battery.  SolarCity’s focus on the back-up only 10kWh version in the USA, and its initial offering of the 7kWh battery in Europe and Australia shows that Tesla does not intend to have the daily use battery available in the USA any time soon.

While the news is very exciting, and it is great to have people talking about the subject of affordable battery storage, the ability to store your solar power in batteries has been around for many years, with time proven technology, and is readily available now.