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

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

Geothermal ยท Analysis

The Geysers: the Bay Area's baseload steam, kept running on wastewater

North of the bay sits the largest geothermal field on Earth, the one renewable feeding San Francisco that runs day and night. Treated sewage piped uphill is what keeps its steam coming.

Filed October 17, 2026 By the Clean Energy SF editors Reading time: 6 minutes

I. How steam becomes your baseload

If you get your electricity through CleanPowerSF, the city's public power program, your monthly statement tells you a slice of your power is renewable, but not which renewable or where it sits on a map. Most of that slice is solar and wind that runs only when the sun or the wind cooperates. A smaller, steadier piece comes from a place you can nearly see on a clear day from the right San Francisco hilltop: the steam fields in the Mayacamas range, north of the bay.

That place is The Geysers, the largest geothermal field in the world, spread across the ridgelines of Sonoma and Lake counties. It is the one renewable on the grid serving San Francisco that runs around the clock, in fog or heat, which is why engineers call it baseload (power that is always on, as opposed to power that comes and goes with the weather). Here is the part that surprises people: the steam spinning those turbines has been thinning for decades, and the fix is sewage.

II. The mechanics, and why no fuel is burned

The Geysers is not geysers in the tourist sense. It is a dry steam field, meaning wells drilled thousands of feet down hit rock hot enough to flash water straight to steam, and that steam rises on its own to spin turbines at the surface. Nothing is burned. The heat is just there, left over from magma cooling under the Mayacamas, and it does not care whether the coast is socked in.

Calpine, the company that operates most of the field, runs on the order of a dozen or more power plants along those ridges. At last public report the field's net output sits well below its original nameplate, very roughly in the range of 700 to 900 megawatts depending on the day and the season, down from the peak it reached in the late 1980s. That power flows into the system run by the California Independent System Operator, or CAISO, the nonprofit that balances supply and demand for most of the state, including the lines that feed San Francisco.

III. The wastewater that refills the reservoir

A steam field is not bottomless. Pull steam out faster than rain and groundwater seep back in, and the pressure drops, which is exactly what happened here through the 1980s and 1990s. The operators needed water to put back, and they found it in a problem two counties were already trying to solve: what to do with treated wastewater that would otherwise be sent to creeks and the Russian River.

Santa Rosa built a pipeline, roughly 40 miles of it, to send treated effluent (cleaned sewage water, not drinking water) uphill to The Geysers, where it is injected deep into the rock to make fresh steam. Lake County runs a similar line from its side. Together they deliver something on the order of ten to fifteen million gallons a day at last public report, turning a disposal headache into refilled generation. It is one of the larger water recycling projects of its kind anywhere, and it is a big reason the field has not declined faster.

IV. The honest math on a declining field

Recharge slows the decline. It does not reverse it. The rock holds a finite amount of heat, and while the injected water keeps steam flowing, the long trend of the field has been gently downward for a generation, and most public assessments expect it to keep drifting that way. So the honest framing is not growth. It is a very long, managed plateau propped up by wastewater.

For a San Francisco ratepayer, that matters in a specific way. Firm renewable power that runs at 3 a.m. in a fog bank is rare and valuable, and there is not much of it on this grid. To see how much of the CleanPowerSF mix leans on geothermal, the program publishes an annual power content label, and the state tracks the field's output separately. Neither number will be a round headline figure, and anyone who quotes you one to the exact megawatt is guessing.

V. Worth watching this month

1. On CAISO's public Today's Outlook page, watch whether the geothermal line stays roughly flat from midday into the middle of the night, which is the whole point of baseload and is routine.

2. CleanPowerSF and the state release an annual power content label showing each program's renewable mix, usually in the fall, and that is the number actually worth waiting for.

3. Any Calpine announcement about repowering an old unit or adding capacity at The Geysers is worth a look, though most news out of the field is routine maintenance.

4. The City of Santa Rosa periodically reports how much recycled water its Geysers pipeline delivered, a routine figure that quietly tracks the field's health.

5. Occasional federal geothermal lease activity through the Bureau of Land Management can signal long-term plans for the ridges, and it matters only on the rare months it moves.

Frequently asked questions

Are there actual geysers to visit at The Geysers?

No. It is a dry steam field, where wells tap steam from hot rock deep underground. There are no erupting geysers at the surface.

Does geothermal count as renewable on my bill?

Yes. California counts geothermal as an eligible renewable, so it shows up in the CleanPowerSF renewable share. It is one of the few renewables that runs around the clock.

Does injecting wastewater cause earthquakes?

The area has frequent small quakes, and injection is associated with some of that minor seismicity. Operators and state agencies monitor it, and large damaging quakes from it have not been the pattern, but it is watched closely.

Will The Geysers run out?

Not abruptly. The field has declined slowly for decades, and recharge extends its life, but the long trend is downward. It is best thought of as a managed resource, not a growing one.

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