The systems will get cheaper—not a lot—but cheaper.
Duvall said a system like Tesla's will probably get as much as 50 percent cheaper in the next 10 years. Part of that is the difference new technical innovations will make, but more of the price reduction will come simply from economies of scale as more people adopt at-home storage, using it in place of power from the utility grid or, more often, working in tandem with the grid. Another key factor will be how rapidly contractors learn how to install systems more efficiently, which has already helped drive down prices of residential solar-power systems.
"It's a lot more expensive now because of how small the market is," Lux's Frankel said. That's especially true of the electronics, which are less advanced commercially than the batteries themselves, he added.
Finally, emerging business models will help consumers manage the cost more easily.
For years solar power stagnated, because it cost too much upfront to install, until SolarCity and others came along with plans to finance and install the panels, letting consumers recoup the investment by electricity savings each month that typically exceed the payments. That model has helped SolarCity, where Tesla CEO Elon Musk is chairman and his cousin chief executive Lyndon Rive, become a huge hit, with shares rising sevenfold since a late-2012 initial public offering.
SolarCity, or someone like it, will soon offer plans to finance the battery-storage systems, Frank and Duvall said. After 195,000 installations in 2014, nearly 645,000 U.S. homes and businesses now have solar arrays, according to the Solar Energy Industries Association.
Utilities may also offer some assistance, because having more power available from battery owners may let them avoid the capital cost to build as many new plants, usually burning natural gas, to generate peak-load power. One of the earliest plans is a Sacramento Municipal Utility District pilot study on how to integrate at-home solar and storage systems into a smart grid.
"There's a lot of room for incentives, and there are certain to be a lot of options,'' Duvall said.
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"Powerwall looks like it will do OK in high-cost places with high-cost photovoltaic solar systems—Hawaii especially, and California," said Richard Sedano, of the Regulatory Assistance Project, a nonprofit focused on the power sector. He said reforms in New York will help, too, but broad success is uncertain and will depend largely on how pricing evolves.
Rábago said the Powerwall system isn't, strictly speaking, necessary to keep the electric grid from breaking down. But, the energy consultant added, "I think there is an opportunity for it to be successful." For one, he said it hits the antisolar argument from some utilities—that you can only use solar when the sun shines—on the head. The Tesla bet is that there is a reasonable, affordable way to solve the utility problem with at-home energy storage.
Rábago also said that early adopters wouldn't necessarily be motivated, or discouraged, by cost savings. He recalled an interview he was present at a decade ago in Colorado with a guy who had a 9-kWh solar system and was asked if it was cost-effective. "He said, 'You're sitting on a $9,000 couch. Do I look like a price-sensitive consumer?'"
"People who will want to do it first are those who desire energy self-sufficiency and autonomy. They will think it's worth it," Rábago said. "And the more of them we get, the cheaper the next version will be for the rest of us."
—By Tim Mullaney, special to CNBC.com