There’s the battery in your watch. There’s the battery in your mobile phone. And then there’s the battery at Green Mountain Power’s Stafford Hills solar farm in Rutland, Vt.

The lithium-ion gargantuan is housed in two trailer-truck-size green metal containers. It sits atop a 10-acre former landfill and captures electricity from 7,722 nearby solar panels—enough to power 2,000 homes on a sunny day. What’s revolutionary about this system isn’t the solar farm; it’s the size and purpose of the battery, which offers 3.4 megawatt-hours of storage, enough to supply backup power to about 170 homes for a day, if needed.

The rap on solar and wind is intermittence—they don’t produce power when the sun isn’t shining or the wind isn’t blowing, making them unreliable as the primary source for power grids. But if vast amounts of renewable energy—say, enough to power entire cities—could be captured and stored in giant batteries and deployed when needed, that downside would fade away.

‘Missing piece’

This has been the “missing piece” in the renewable-energy revolution, says Venkat Srinivasan, director of the Energy Department’s Argonne Collaborative Center for Energy Storage Science. But it’s starting to come into view, thanks to the scale and progress of current research and real-world applications such as Green Mountain Power’s.

Green Mountain’s project puts it in the vanguard of power companies that are showing that utility-scale battery storage can be technologically and economically viable—depending on the scale and how it is used.

Like all utilities, Green Mountain faces issues meeting “peak demand,” the high-use period, typically in the early evening, when people return from work and school and crank up air conditioners and energy-hungry appliances.

Many U.S. utilities fire up natural-gas-powered generators to help their baseload plants meet peaks, often imposing hefty surcharges on customers to offset the extra generating costs. Green Mountain instead stores energy from its solar-and-battery combination to meet peaks—shaving as much as $200,000 an hour off demand charges for its 265,000 residential customers, according to the utility. Green Mountain, which says it invested $12.5 million in the project and received a $285,000 government grant, benefits in that it doesn’t have to build additional plants to meet peak demand and gets a public-relations boost among its green-leaning customer base.

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SPARKS, Nev.— Tesla Motors Inc. is scrambling to finish building its massive $5 billion battery factory here years ahead of schedule to meet demand for its coming cheaper sedan and provide power for new types of vehicles Chief Executive Elon Musk says are under development.

Tesla has doubled the number of people constructing the “gigafactory,” which sits on more than 3,000 acres near Reno. Now, 1,000 workers build seven days a week on two shifts in an effort to start churning out lithium-ion cells by late 2016.

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Forget about Tesla Motors Inc.’s batteries: The hottest way to store energy in the electricity business today is a century-old technology that involves moving water to stash power.

Known as pumped storage, the giant facilities push large volumes of water uphill when there is surplus electricity, and then let gravity move it back down through turbines to generate hydroelectric power when extra kilowatts are needed.

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