The Juice

How Do You Solve Solar Power’s Nighttime Problem?

Batteries. Really big batteries.

Not a night owl.

Photo illustration by Lisa Larson-Walker. Photos by Thinkstock.

How do you get solar power when the sun goes down?

This isn’t a trick question.

One answer: You generate power from solar panels during the day. And instead of funneling that electricity into the grid, you simply charge up lots of batteries, which function as a temporary power plant for several hours much later in the day—and even after nightfall.

That’s the business plan behind the Powerwall, Tesla’s home-based battery storage system. And now a small utility on Hawaii’s lush, sparsely populated, and fabulous island of Kauai, has inked a deal with the solar-panel arm of the Musk-Industrial Complex to build a truly massive powerwall—big enough to supply up to one-fifth of the island’s electricity needs for several hours. Earlier this month, Musk-chaired SolarCity and the Kauai Island Utility Cooperative  announced they would build the “first utility-scale solar array and battery storage system designed to supply power to the grid in the evening, when demand is highest.”

Here are the specs: SolarCity will build a large solar array capable of producing 17 megawatts at its peak on 50 acres of a former sugar plantation near KIUC’s Kapaia Power Station, near Lihue. A small slice of the property, about two thirds of an acre, equal to half a football field, will be set aside for packs of lithium ion batteries. Lots of them. During the day, the glorious Hawaii sunshine will produce electricity that will be used almost solely to charge up the batteries. Then, in the later afternoon or early evening, when solar panels elsewhere on the Garden Island are growing less productive, the batteries can flood a large amount of power into the grid for several hours. Together the batteries have a collective capacity of 52 megawatt-hours, meaning they can dispatch 13 megawatts an hour for four hours, or one megawatt an hour for 52 hours, or anywhere in between.

That isn’t much power. But given Kauai’s relatively small peak load—about 72 megawatts—the batteries can provide a meaningful amount of juice. As important, they’re displacing the usual source of electricity—burning diesel oil—and at an equivalent or lower price.

Solar has boomed in Hawaii, whose electricity costs are generally the highest in the U.S. thanks to the state’s reliance on burning diesel. But it has created a conundrum. This daily chart of Oahu’s electricity use shows the challenge of managing shifting demand when you have a lot of solar in the system. Overall electricity demand ramps up rapidly early in the morning, and the total load (including from solar panels on rooftops) climbs steadily through the day, and then shifts up again at 6 p.m. It peaks at 8 p.m., and then falls by about a third into the evening. The problem on Oahu and the other Hawaiian islands: The system needs more electricity at precisely the moment that sun power starts to fade.

“We have this issue with a lot of solar on during the day, and we want to figure out how to move that through the night,” says Jim Kelly, member services and communications manager at KIUC, which is a not-for-profit utility that is owned by its 23,000 members. Kauai is a geographical and electrical island (there are no transmission lines connecting it to other landmasses). As a result, local diesel-fueled generators have been the sources that get the island through the night—and through the day. But Kauai is shifting quickly away from fossil fuels. From virtually nothing several years ago, renewable energy—in the form of solar arrays, hydroelectric plants, and biomass plants—should account for close to 40 percent of total electricity production on this island by the end of 2015. KIUC has a plan to get to 50 percent renewables by 2023, which it should easily meet. (The entire state is going to 100 percent by 2045.)

Last year, KIUC put out a request for proposals for energy storage. When SolarCity, which built a 12-megawatt array for KIUC that went into operation last year, responded, the utility was skeptical. Kauai already had plenty of solar. But SolarCity essentially asked KIUC to ignore the panels and focus on the batteries, and to regard them as a swing power plant that could produce lots of electricity at the flick of a switch. (The term of art in the industry is “dispatchable,” meaning it can be supplied on demand.) With the batteries, says Peter Rive, chief technology officer at Solar City, “you have the ability to discharge this enormous amount of power during your peak periods.” The batteries can be fired up and turned down more rapidly than diesel generators, he adds. And SolarCity was willing to fix a price for 20 years.

Aside from saving emissions, the deal is likely to save KIUC money. Kelly says KIUC will be buying the electricity from SolarCity at 14.5 cents per kilowatt-hour. That’s less than the 22 cents per kilowatt-hour KIUC was paying for oil-based generation last year, and roughly the same as the 14 cents per kilowatt-hour it is paying for oil-based generation at this year’s lower prices. And it comes with none of the negative externalities of diesel—no emissions, no environmental impact, and no wild swings in the cost of fuel. “We’re completely whipsawed by the vagaries of the world oil market,” says Kelly, who notes the utility has to jack up prices when oil becomes more expensive. “What we’re looking for is a hedge against that next oil spike.”

The solar-battery facility, which should be completed by the end of next year if permits are forthcoming, makes economic and engineering sense in Kauai due to the unique circumstances on the island. The baseline fuel is diesel, not cheaper natural gas, coal, or nuclear. Hawaii has fabulous sun resources. And with a small load, Kauai can make up a significant amount of capacity with a single large field.

But as time goes on, it may also make sense in other regions—especially if battery and solar prices continue to decline. Batteries paired with solar fields may become an industry standard. As Rive puts it: “Over time, the next decade, it’ll always be deployed with a battery.”