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Now, let's come back home for our next story. Here in the U.S., most of us tap into the nation's electricity grid every single day. This web of power stations, transformers and transmission lines spans the continent, distributing electricity, a central part of the economy, and possibly even powering the radio you're listening to now. But the grid is outdated. The problem is that the kind of electricity that's now flowing through the grid is changing. As NPR's Christopher Joyce reports, energy experts say the grid is not ready for it.

CHRISTOPHER JOYCE, BYLINE: The National Academy of Engineering in Washington, D.C. once asked its members to pick the greatest engineering achievement ever. Their choice, the electrification of the country through what's known as the grid. Ernest Moniz, director of the Energy Institute at MIT, says they were right on the money.

ERNEST MONIZ: That reflects what an amazing machine this is, spread out geographically, always having to balance demand and supply because electricity is not stored.

JOYCE: Electricity has to keep flowing all the time. Grid operators constantly match what power plants are producing with what people and their TVs, microwaves and air conditioners need. It's the world's biggest balancing act. And that's doable largely because big power plants run almost constantly and produce a predictable amount of electricity.

So what happens when you add in unpredictable sources of electricity, like wind or solar power?

MONIZ: The operator does not have control of when to turn it on and off. It's a new challenge that we just have to meet. And we're not doing it at anything like the pace that I think we need.

JOYCE: That's the conclusion of a study that Moniz's group at MIT is issuing today. It's all about how the grid must change to handle the fickle flow of electrons from renewable energy.

Steve Berberich runs the California Independent System Operator. That's the California grid. They're trying to sort out how to handle this on-again/off-again source of electricity

STEVE BERBERICH: We have to have a backup. There are times when Mother Nature decides to bring in clouds and turn off the wind, but I think everybody in that case still wants to have power.

JOYCE: In California, most of that backup power comes from plants that burn natural gas; they can switch on and off in a matter of minutes. But Berberich says natural gas plants face some obstacles. Gas plants have to compete against the very renewable energy sources they're supposed to back up.

BERBERICH: They're not getting as much revenue as they once did because they're not selling as much power, because it's being displaced by wind and solar energy, which is exactly what we want. But we have to find a way to maintain those things.

JOYCE: Gas plants have to make money to survive. Keeping them idle until a rainy or cloudy day to back up renewables won't pay their bills. Coal and nuclear plants - thermal plants, as MIT's Moniz calls them - are not a good option for backup. It's costly to start and stop them on short notice.

MONIZ: Another set of costs is the additional operating costs and maintenance costs, wear and tear, on some of these thermal plants that we may be asking to go up and down a lot more than they were planned for.

JOYCE: As the cost of solar and wind energy drops though, the grid is going to use more of it. Many states demand it, so the grid must adapt.

Michael Goggin, at the American Wind Energy Association, is trying to figure out how to do that. They're the biggest industry group for wind energy.

MICHAEL GOGGIN: We're adding new energy sources and obviously the old rules don't necessarily always work.

JOYCE: Goggin says, however, solving this problem is not as hard as MIT makes it out to be.

GOGGIN: I think there's a lot of misconceptions about backup power. The reality is that all power plants are backed up by all other power plants.

JOYCE: And he says grid operators could accommodate the vagaries of wind and solar if they moved power around the grid minute by minute, instead of hour by hour as most do now.

One thing the experts agree on - since wind and solar energy are all about the weather - grid operators will need to hire a lot more weather forecasters.

Christopher Joyce, NPR News.

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