Hydro · Analysis

Castaic: LA's biggest battery is a 50-year-old lake

LADWP already owns a battery bigger than most new lithium projects in the basin. It is a 50-year-old lake, and the solar grid is quietly changing its job.

BY THE RENEWABLE ENERGY LA EDITORS · FILED OCTOBER 13, 2026 · 6 MIN READ ·

How a lake becomes a battery

Pumped storage is a trick older than the word grid itself. When power is cheap and plentiful, the plant runs its machines backward, pumping water up through a tunnel in the hills from Castaic Lake into Pyramid Lake above. When the grid needs power, operators let that water fall back down through the same machines, which spin and generate on the way. The same equipment pumps and generates, depending only on which way the water is moving.

Some energy is lost in the round trip, roughly a sixth to a fifth of it, because pumping uphill always costs more than you get back coming down. What that loss buys is time. A pumped storage plant can pour out power for hours at a stretch, far longer than a typical rooftop or utility lithium battery, which is why engineers call it long-duration storage (the kind that can cover a whole evening, not just a short spike).

The math of a slow, giant battery

Put the numbers side by side and the scale stands out. Castaic's roughly 1,200 megawatts, at last public report, is more than many of the new lithium projects in the basin can manage even added together, and it can run for hours rather than the four or so hours most lithium installs are built for. That combination, large and slow to drain, is close to what a solar-heavy grid is short on after sundown.

Water is the honest catch. How much the plant can deliver depends on how full Pyramid Lake is, and that depends on State Water Project deliveries, which rise and fall with drought, snowpack, and the needs of cities and farms downstream. In a dry year the lake is a drinking and irrigation resource first and a battery second. The power is real, but it is not unconditional.

Who owns the water, who owns the wires

Castaic sits at a seam between two public agencies. The California Department of Water Resources operates Pyramid and Castaic lakes as part of the State Water Project, the network of dams and canals that moves water across the state. LADWP owns and runs the power plant between them. So the water levels that decide how much electricity is available are not, strictly, an energy decision at all.

Wires add another wrinkle. Most of California's grid is run by the California Independent System Operator, but LADWP is not a member. It runs its own balancing authority, keeping its own supply and demand in step, which means Castaic is scheduled by LA's own operators rather than the statewide market. Anyone reading about California's evening ramp should remember that LA manages a large piece of it on its own.

Built for a coal era, asked to chase the sun

When Castaic came online, the cheap power worth storing came at night, much of it from coal and large thermal plants that ran steadily around the clock. The plant pumped uphill in the small hours and generated during the daytime peak. The logic was sound for that grid, and for decades it held.

Then the grid flipped. The cheapest, most abundant power now often arrives at midday, when solar across the basin and the desert runs hard and sometimes makes more than the region can use. The steep climb comes in the evening as that solar fades, a pattern planners call the duck curve for its shape on a chart. A plant built to store night power and sell day power now does close to the reverse, soaking up surplus midday sun and releasing it into the evening ramp. The machines do not care which way the economics point, which is what makes a 50-year-old lake useful to a grid LADWP aims to run carbon-free by about 2035.

Worth watching this month

1. LADWP Board of Commissioners agendas, posted on the utility's site, for any maintenance or upgrade item touching Castaic, most of which will be routine rather than headline news.

2. Department of Water Resources State Water Project allocation updates, which signal how full Pyramid Lake is likely to run and how much the plant can lean on it this season.

3. CAISO and LADWP conditions during the next Santa Ana heat event, when the evening ramp bites hardest and long-duration storage earns its keep.

4. Any LADWP long-term resource planning document tied to its roughly 2035 carbon-free target, which should show where pumped storage fits among new lithium and transmission.

5. Notices of any federal relicensing or major overhaul at the plant, a genuinely consequential item given its age, though such processes move slowly and rarely on a public clock.

Where this dispatch reads

Los Angeles Department of Water and Power, www.ladwp.com, Operator of the Castaic plant and LA's municipal power supply; where board agendas and power system information are posted.
California Department of Water Resources, State Water Project, water.ca.gov/Programs/State-Water-Project, State operator of Pyramid and Castaic lakes; reservoir conditions and water allocation updates.
California Independent System Operator (CAISO), www.caiso.com, Grid operator for most of California; source for the evening ramp and the duck curve pattern.
NREL, LA100 study, www.nrel.gov/analysis/la100.html, Federal laboratory study of Los Angeles's path to 100 percent clean energy by about 2035.
California Energy Commission, www.energy.ca.gov, State agency with energy data, including pumped storage and statewide supply and demand.

Questions readers have asked

Is a pumped storage plant really a battery?

In function, yes. It stores energy by lifting water to a higher lake and releases it by letting that water fall through turbines. The storage is gravity and height instead of chemistry.

Does running it use up our water?

Mostly no. The water cycles between the upper and lower reservoirs rather than being consumed, though some is lost to evaporation and seepage. The bigger limit is how full the reservoirs are to begin with.

Is Castaic on the same grid as the rest of California?

Not exactly. LADWP runs its own balancing authority, separate from the California Independent System Operator that manages most of the state. Castaic is scheduled by LA's own operators.

Could LA just build lithium batteries instead?

It is building them too. Lithium is fast and easy to site in small pieces, while pumped storage offers long duration at large scale. The two cover different jobs, so the basin is likely to use both.