Green Mountain Power said the idea started after a power outage knocked out over 15,000 homes. "Three customers who had Powerwalls never lost power, so it carried their home through," GMP CEO Mary Powell told WCAX-TV. "And unlike a generator, they didn't have to worry about hooking it up, they didn't have to worry about whether it was fueled."

Once in operation, the Powerwalls will stay charged in your home. During times of peak electricity usage when its normal power sources (hydro, nuclear, wind, etc.) max out, GMP will draw from its Powerpacks and the consumer-installed Powerwalls. At night, when demand is low, the batteries are recharged.

Tesla Powerpacks installed at a Hawai'an utility.

Tesla says GMP is the first utility to do such a large-scale "grid-smoothing" installation. "There hasn't been any really successful large-scale trial, so that's why this is so exciting," said Tesla CTO J.B. Straubel. "It's been in development at Tesla for quite some time, but this is our first real deployment."

GMP is offering 7kW Powerwalls for $15 a month or a flat fee of $1,500. That's quite a bargain compared to the regular $3,000 price, but again, it's only available for 2,000 homes. That's presumably enough, however, to provide peak power backup in conjunction with the company's industrial Powerpacks.

GMP thinks the Tesla batteries are not only less polluting than regular generators, but more economical too. "[Backup generators] are some of the dirtiest and ... costliest forms of generation," says Powell. "So when we can produce 10 megawatts of energy, that is an alternative to that peaking generation, that has tremendous economic value."