On Wednesday night, Tesla owners in Northern California received an unusual message. “A utility company in your area announced they may turn off power,” it read. “We recommend charging your Tesla to 100% today to ensure your drive remains uninterrupted.”

The utility there is Pacific Gas & Electric, aka PG&E, which did indeed cut power to some 500,000 customers in the region in a bid to limit the risk of wildfires amid high seasonal winds. (A fiery fate, FYI, is not so easily avoided.) As of Wednesday afternoon, more shutdowns were on the way. It’s possible the juice won’t start flowing again for up to five days.

The notice from Tesla was certainly a handy reminder to always be charging, but it seems to have inspired some Tesla owners to tweet a follow-up question to Elon Musk: Can’t they use their cars’ batteries—which can hold enough juice to power a house for days and are now the object of a Nobel prize—to keep their lights (and TVs and refrigerators and so forth) on? The answer, we’re sorry to report, is no. Not yet, anyway.

This idea of a vehicle-to-grid power transfer system has been knocking around for decades. It has picked up currency (so to speak) in recent years as EVs have surged in popularity, and regulators and utilities have moved to modernize the way electricity is produced and distributed.

The benefits go far beyond letting Tesla owners keep the Netflix marathon running while their blacked-out neighbors play cards, because these batteries’ ability to store power could prove key in the shift to renewable energy sources. Today’s grid works on a just-in-time basis, producing however much electricity the people want at any given moment, and delivering it immediately. Because humans can’t dictate when the sun shines or the wind blows, we need the ability to stash away the power these sources provide. Which is why, along with a pledge to make all its electricity from renewable sources by 2045, California has ambitious energy storage targets.

Tools for holding onto that energy include Tesla’s home battery system (which works independent of its cars), moving compressed air between caves, and filling a train with rocks. But given that global annual sales of passenger electric vehicles are forecast to hit 10 million in 2025 and 56 million in 2040, some see an opportunity to use Tesla, Leaf, Bolt, and other batteries as a massive, distributed energy storage network.

Indeed, a 2017 study by researchers at Lawrence Berkeley National Laboratory found that if California hits its goal of getting 1.5 million EVs on its roads by 2025, and “some” of them had the ability to transfer energy into the grid, their batteries would easily exceed the state’s energy storage needs. “Substantial capital investment, as much as several billion dollars, can be avoided if EVs are used in lieu of stationary storage,” they wrote.

These vehicles, though, are not like penguins, designed to take in energy only to spew it back out and into the mouth of whatever chick wanders by. Getting power out of an EV battery requires work. For one thing, any house hoping to use power from the car in its driveway must be able to disconnect from the grid, which requires specialized hardware. Otherwise, the power won’t stay where you want it. “It could end up going backwards all the way into the grid,” says Sam Saxena, one of the Livermore researchers who authored the paper on EV battery storage. (The lab, by the way, is closed in anticipation of having its power cut.) And that’s bad news for anyone working on a power line they think is disconnected. Also, to convert the battery’s DC power into the AC goodness running through the grid, the car or its charger needs an inverter. You need the software to tell a car when to discharge its power, and when to hoard it. And you have to address concerns of degradation: Extra demands on batteries can limit their useful lifespan.