Clean Energy SF: San Francisco, fog then sun, est. 2026

Vol. 1 - No. 14 - Thursday, September 24, 2026 - Free to readers

Hydro ยท Analysis

Helms Pumped Storage: the grid's Sierra battery, now charged by midday solar

A plant 200 miles east of the fog stores power by moving water between two mountain lakes. In the solar era, its daily job has quietly reversed.

Filed October 25, 2026 By the Clean Energy SF editors Reading time: 6 minutes

I. How a mountain reservoir works as a battery

The mechanics are older than the smartphone and simpler than most people expect. Helms links two reservoirs on the North Fork Kings River in Fresno County: Courtright Lake up top and Wishon Reservoir below, separated by roughly 1,600 feet of elevation, per PG&E's project description. When power is cheap, the plant runs its pumps and pushes water up to Courtright. When power is dear, it opens the gates and that water falls back down through the same machines, now spinning as generators, at a capacity PG&E puts at roughly 1,200 megawatts across three units.

One honest caveat belongs here. Pumped storage is not a power source; it is a timing device, and the round trip loses energy, on the order of a fifth or so, as heat and friction. It returns less than it takes in. What that loss buys is control: the ability to hold a few hours of energy and release it at the fog-dark moment when the city actually wants it.

II. The job that solar flipped

When Helms came online in the mid-1980s, per PG&E's history, the cheapest hours were overnight, when demand fell and large plants kept running regardless. The plant pumped uphill in the small hours and generated through the afternoon peak, and that logic held for roughly thirty years.

Solar rewrote the clock. California now produces so much midday solar that CAISO, the nonprofit that runs the state's grid, regularly sees its net demand (total demand minus wind and solar) sag to a midday trough and then ramp steeply as the sun sets. That evening ramp is the hard part, and it is exactly when the fog tends to roll in over the city and the lights come on. Helms now charges in the sunny trough and discharges into the ramp, which is the clearest way to watch the energy transition change an old machine's daily routine.

III. Who runs it, and who it serves

Three names matter here. PG&E owns and operates Helms. FERC, the federal energy regulator, licenses it as a hydroelectric project, which means its operations and any relicensing run through a public federal docket. CAISO decides, hour by hour and through its markets, when the plant actually pumps or generates, based on what the whole grid needs that day.

For a CleanPowerSF customer, the link is real but indirect. CleanPowerSF, the city's community choice program run through the SFPUC, buys power on residents' behalf, but that power still travels the CAISO grid and arrives over PG&E's wires. No one can trace a given evening's kilowatt-hour back to Courtright Lake. What can be said honestly is that a steadier, cheaper evening ramp, which Helms helps provide, shows up eventually in the grid everyone in the city shares.

IV. Worth watching this month

1. CAISO's public Today's Outlook page shows the net-demand curve every evening, and watching the ramp steepen after sunset is routine, but it is the clearest free window into why plants like Helms exist.

2. Any relicensing or operating filings for the Helms project move through FERC's public docket system, which is worth a periodic check, though these proceedings move slowly and rarely make news.

3. CAISO's quarterly reporting on new battery storage is worth following, because grid-scale batteries are the resource most likely to reshape the evening ramp Helms has long handled.

4. CleanPowerSF posts rate and enrollment notices on its site, and those, not the Sierra reservoirs, are what will actually show up on a San Francisco electric bill.

5. Sierra snowpack and the levels at Courtright and Wishon are worth a glance heading into winter, since a pumped-storage plant still depends on having enough water in the system to move.

Frequently asked questions

Does Helms actually make electricity, or just store it?

Both, in a sense. It generates real power by letting water fall through turbines, but over a full cycle it uses more energy than it returns, so it is best understood as storage, not a new source.

Is my CleanPowerSF power coming from Helms?

Not in any traceable way. You draw from the shared CAISO grid, and Helms is one of many resources helping keep that grid balanced, especially in the evening.

Why pump water uphill if the round trip wastes energy?

Because timing is worth more than the loss. Cheap, abundant midday solar held for a few hours is more valuable when released into the expensive evening ramp, even after the energy lost in the cycle.

Could batteries just replace a plant like this?

Increasingly, yes, for short bursts. Helms can hold several hours of energy at large scale, which the battery fleet is only starting to match, so for now they tend to work alongside each other.

References