San Francisco took a major step toward public power Tuesday when the Board of Supervisors gave initial approval to a five-year contract with Shell Energy North America to provide 100 percent renewable power to San Franciscans willing to pay a premium.

The 8-3 vote provided a veto-proof majority for a program that will effectively break Pacific Gas and Electric Co.’s decades-old monopoly on the consumer power market in its headquarter city. It also lays the groundwork for city-owned renewable power production.

“The long-term goal is to really do our own generation,” said Ed Harrington, the outgoing general manager of the San Francisco Public Utilities Commission, who delayed his retirement to see the proposal brought before legislators.

The plan comes eight years after the city began setting up a community choice aggregation program, which allows municipalities to choose alternative electricity providers. Former Supervisor Tom Ammiano, now an assemblyman, began pushing for public power, a touchstone issue for many on the city’s political left, 14 years ago.

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