Tesla Motors was the first carmaker to branch out from selling electric cars to offering standalone battery packs for energy storage, but others have followed the company's lead.
Mercedes-Benz and Nissan have stated their intentions to enter the energy-storage market, and now BMW is jumping on the bandwagon as well.
At the Electric Vehicle Symposium & Exhibition 29—known as EVS29—held this week in Montreal, Canada, BMW unveiled an energy-storage system that uses battery packs from i3 electric cars.
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Developed in concert with German firm Beck Automation, BMW's system is designed to use either new battery packs or "second-life" packs that have degraded too much for continued use in electric cars.
Battery packs that can no longer function in cars still have enough usable capacity for energy storage.
BMW has tested the concept over the past five years with various research projects, including a 2013 "micro-grid" project with the University of San Diego, and a 2014 collaboration in Germany with utility Vattenfall to use electric-car battery packs as buffer to help stabilize electricity grids.
2017 BMW i3
General Motors, Nissan, and other makers have demonstrated "second-life" batteries in various similar projects, while Toyota provided used Camry Hybrid battery packs to store energy from solar panels at Yellowstone National Park.
BMW says it will offer the energy-storage batteries in 22-kilowatt-hour and 33-kWh sizes.
This corresponds to the battery-pack sizes of the current 2016 i3 and the larger-capacity version that will arrive for the 2017 model year.
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Assuming daily energy usage of 15 to 30 kWh for a home—the U.S. national average is slightly over 30 kwh—BMW claims a pack can provide power for "a variety of appliances and entertainment devices" for up to 24 hours.
As with other energy-storage systems, however, the main benefit will likely be increased use of renewable energy.
Having a battery pack to store electricity from solar panels for later use allows homeowners to rely more on solar power, and buy less electricity from the grid.
2017 BMW i3
It also lowers the well-to-wheels carbon footprint of electric cars charged at home, because a greater portion of the power used comes from a zero-emission source.
BMW's use of second-life batteries could also provide an added benefit for i3 owners.
When battery packs need to be replaced, owners could get more use out of them by re-purposing them for home energy storage, notes i3 advocate Tom Moloughney in his BMW i3 blog.
MORE: Mercedes Follows Tesla, Will Offer Home Energy Storage Batteries Too (Jun 2015)
In its announcement, BMW did not specifically say that owners would be able to use their own decommissioned battery packs in this way, though.
Nor did it discuss pricing or an on-sale date.
We'll report back when we have more details.