The plan submitted by the Northwest Power and Conservation Council said natural gas plants and wind energy could take care of the rest of the demand, and it did not envision new coal-fired plants.
The council said demand is expected to rise at a rate of 1.2 percent a year for the two decades beginning next year.
It said it had identified enough potential in efficient use of power to account for 85 percent of that increased demand.
An aggressive plan for efficiency is the "most cost-effective and least-risky resource available," the council said in a statement.
"The average cost of the efficiency is half the cost of new power plants," it said.
The council of eight members from Idaho, Montana, Oregon and Washington sets policy for the BPA, a federal energy wholesaler based in Portland that provides more than a third of the energy used in the Northwest, generated from by 31 federal dams and one nuclear plant. It sells to more than 140 Northwest utilities and buys power from 7 wind projects.
The BPA is the region's largest supplier of electricity, and its executives are required to act consistently with the council's 20-year plans.
The plans aren't binding on investor-owned utilities, but "I think you will find that they look at it as a bit of a blueprint," said Bill Booth of Coeur d'Alene, Idaho, chairman of the council.
Conservation groups said the council had exercised leadership in setting high goals for energy efficiency but fallen short of what it could have done: outline a plan to wean the region off coal-fired electricity.