The steam fields in the Mayacmas Mountains northeast of Santa Rosa have been the world’s largest producer of geothermal energy since the 1970s. They can meet 60 percent of the electricity demands from the Golden Gate to the Oregon border, according to Calpine Corp., and they’re recharged with wastewater that used to be dumped in the Russian River.

It’s clean, green and renewable energy.

So requiring more geothermal power is a good idea, right?

Not necessarily.

Through legislation and executive action, California has established a high standard for renewable energy as part of the Golden State’s effort to ease the impacts of climate change. By the end of the decade, all utilities must deliver 33 percent of their load via solar, wind, biomass and other renewable power sources, including geothermal.

A bill now pending in the state Legislature would tip the scales in favor of geothermal.

Senate Bill 1139 would require investor-owned utilities such as PG&E and other electricity retailers, including Sonoma Clean Power, to collectively buy 500 megawatts of geothermal electricity from plants that begin construction after Jan 1. The state Energy Commission would decide how much each utility must buy.

That sounds like a potential boost for The Geysers, which is one of two locations in the world with high-temperature dry steam that can be directly used to spin turbines and generate electricity. Calpine and two other power producers operate geothermal plants at The Geysers.

Indeed, Calpine is pursuing a 98-megawatt expansion, and it has a contract to supplement Sonoma Clean Power’s primary electricity supplier. Yet the Houston-based energy company is aligned with consumer groups, the state Public Utilities Commission and other power producers in opposition to SB 1139.

The unusual coalition of opponents is reason enough to take a close look at this bill and its goals.

The lead author is state Sen. Ben Hueso, D-San Diego, and he makes no secret that he’s trying to create demand for plants soon to be built in the Salton Sea area, where geothermal power is generated from saline brines heated by molten rock beneath the earth’s surface.

But at what cost? Opponents say SB 1139 would cost ratepayers $8 billion over 20 years and undercuts California’s renewable energy law. “This does an end run around the competitive process, where all costs and benefits are looked at and evaluated,” Nancy Rader of the California Wind Energy Association told the Los Angeles Times.

A PUC lobbyist told that the state already has identified sources of renewal energy to meet its needs through 2022 and requiring utilities to buy another 500 megawatts of geothermal “would be nothing more than grid congestion.”

Moreover, the bill could hurt Calpine, a large employer and the largest taxpayer in Sonoma and Lake counties. Contracts for 55 percent of Calpine’s production at The Geysers expire by 2017, and saddling utilities with a state mandate to buy geothermal power from plants built after 2014 may prevent them from renewing existing contracts.

Hueso’s bill is awaiting a vote in the Assembly. If it passes, it returns to the Senate for a final vote. The next stop would be Gov. Jerry Brown’s desk. If SB 1139 gets that far, the governor should return it to the Legislature without his signature.