Tesla has already built a massive battery in Australia, and now plans to fit more in 50,000 homes to create the world’s largest virtual power plant

Are you ready to cut the cord to your big utility and turn your home into a power plant?

Elon Musk certainly hopes so. The Tesla Motors chief executive on Thursday night unveiled the Powerwall, a home version of the lithium-ion battery pack that powers the company’s luxury electric cars. The $3,500 Powerwall is designed to store electricity generated by a home’s solar panels for use during a blackout—or when power prices spike and homeowners want to tap free and clean energy generated by the sun.

“The issue with existing batteries is that they suck,” Musk said at a press event Thursday that was powered by Tesla batteries storing electricity produced by the company’s rooftop solar panels. “They’re really horrible. They’re expensive, they’re unreliable.”

With the sculpted white lines of an Apple iPhone, the Powerwall looks more like something you’d hang in your living room rather than in a dusty corner of your garage. The device, which will be available in three to four months, can store 10 kilowatt-hours (kWh) of electricity—enough to keep the lights on and the refrigerator cold during a power outage or during the early evening when power rates tend to rise.

“The potential market size is huge —any solar customer (there are 700,000 to date and more every day) could theoretically add energy storage for backup or to load-shift,” Shayle Kann, a senior vice president at GTM Research, said in an email. “The economics are the bigger barrier, however.”

That’s because cutting the cord to your utility is pricey if you want to install enough energy storage to operate your home. (For instance, a clothes dryer consumes about 3.3 kWh each time you turn it on.) For those customers who really want to declare their energy independence, Tesla will install multiple Powerwalls to store as much as 90 kWh. The price tag, though, would be $31,500.

“It makes no sense to cut the cord,” said Kann. “It’s more expensive (especially because you need to oversize the battery) and it’s far less reliable. For most customers, it will make much more sense to stay grid-connected and use their battery systems to load-shift and provide backup power in the rare event of a blackout.”

But buying massive amounts of battery storage could make sense for companies that pay exorbitantly high electricity rates when demand jumps. And so Tesla is also selling a supersize version of the Powerwall for businesses, called the Powerpac.

Musk, of course, has bigger ambitions, envisioning solar panels and Tesla batteries supplying carbon-free electricity to remote communities around the world.

Or the entire world.

“We could power a small city with a 1 gigawatt-hour pack,” said Musk, who also serves as chairman of solar panel installer SolarCity. “With 160 million Powerpacs you can transition the United States [to clean energy]. With 900 million you can transition the world.”