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

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

Markets ยท Analysis

Where your noon solar goes when the fog burns off

Around midday, San Francisco and the rest of California make more solar power than they can use. A real-time market called the Western Energy Imbalance Market ships the surplus west in five-minute trades, and CAISO's public reports show roughly what it saves.

Filed October 26, 2026 By the Clean Energy SF editors Reading time: 6 minutes

I. How the noon surplus leaves town

The Western Energy Imbalance Market (a real-time trading platform run by the California Independent System Operator, or CAISO, the nonprofit that manages most of the state's grid) lets utilities across the West buy and sell power in five-minute intervals. When Bay Area rooftops and Central Valley solar farms overproduce around midday, that surplus can be sold to a utility in Oregon, Arizona, or Wyoming that needs it right then, instead of being curtailed (dialed down and left unused).

Trades run both ways. After the sun drops and California's evening demand climbs (the steep ramp grid planners call the duck curve, for the shape it draws on a chart), the same market lets California buy cheaper power back from wherever it is available. The market started small around 2014 with a single out-of-state utility and now covers a large share of the western grid at last public report.

II. What the trades are actually worth

CAISO publishes a quarterly benefit report for the market, and it is public. At last public report the cumulative gross benefits since 2014 were on the order of several billion dollars across all participating utilities, with hundreds of millions added in a strong recent quarter. Treat those figures as estimates: they come from CAISO's own model of what the grid would have cost without the market, not from a line on anyone's bill.

For a CleanPowerSF customer, the connection to your own statement is real but indirect. The savings accrue to utilities and balancing areas, not to individual ratepayers as a labeled credit, and CleanPowerSF buys power through its own contracts and the CAISO markets rather than pocketing a market check. So the honest framing is that this market lowers the cost and waste of running the western grid as a whole, and some of that flows back to California buyers over time. How much reaches a single bill is genuinely hard to trace.

III. Who is at the table

Several players sit between a San Francisco roof and that five-minute trade. SFPUC runs CleanPowerSF, the city's community choice program (where San Francisco buys the power and PG&E still delivers it over the poles and wires). SFPUC also runs Hetch Hetchy Power, the municipal hydroelectric supply from the reservoir in Yosemite that has served city buildings and the Muni system for generations.

CAISO sits above all of them, matching supply and demand second by second and running the imbalance market. PG&E owns the local distribution grid that carries power down your block. And dozens of western utilities, from Arizona to British Columbia, are now market participants at last public report. When panels on a Bernal Heights Victorian overproduce, the path out of town passes through CleanPowerSF's portfolio, into the CAISO system, and potentially onto a utility a thousand miles away, all inside a few minutes.

IV. Worth watching this month

1. CAISO is expected to publish its next quarterly Western EIM benefit report, the public scorecard for how much the market saved, on the market's own site.

2. CAISO's larger day-ahead market expansion, known as EDAM (the Extended Day-Ahead Market), is scheduled to bring participating utilities on in phases over the next few years, and the first start dates are worth checking against CAISO's posted timeline.

3. The California Public Utilities Commission continues to oversee the state's net billing tariff (the successor to net metering that sets what rooftop owners earn for the power they export), and any proposed changes show up on the CPUC docket.

4. CleanPowerSF usually updates its rates and generation mix on its program page around the start of the fiscal year, so a rate notice there is routine rather than alarming.

5. Watch CAISO's Today's Outlook page on any sunny, low-demand spring day, when real-time prices can briefly turn negative, the clearest public sign of midday surplus at work.

Frequently asked questions

Does this market put money directly on my CleanPowerSF bill?

Not as a labeled credit. The savings accrue to utilities and the grid as a whole, and reach ratepayers only indirectly through lower system costs over time.

What happens to my extra solar power if nobody buys it?

If there is no buyer and the grid is oversupplied, some solar gets curtailed, meaning it is dialed down and goes unused. The imbalance market exists partly to reduce how often that happens.

Is the Western EIM the same thing as CAISO?

No. CAISO is the nonprofit that runs California's grid and operates the market. The Western EIM is the real-time trading platform CAISO runs for utilities across the West, including many outside California.

Why does the fog matter to any of this?

Fog delays morning rooftop output in neighborhoods like the Sunset and Richmond, then production surges after burn-off. That sharp midday jump is part of what makes a real-time, five-minute market useful.

References