HARTFORD, Conn. – Improved energy efficiency will help keep electricity use flat in New England in the next decade, allowing customers to save on utility bills and power companies to scrap costly transmission upgrades, the grid operator said last week.

The region’s six states spent $1.2 billion from 2008 to 2011 to boost energy efficiency, ISO-New England told reporters in a briefing on Wednesday. Spending on energy efficiency is expected to increase to $5.7 billion from 2015 to 2021.

The Holyoke, Mass.-based grid operator said energy efficiency has more than doubled since 2008 in an annual auction to win commitments from generators and others for power available three years from now. The result is that electricity use previously projected to rise by 0.9 percent annually between 2012 and 2021 will instead be flat.

The increased efficiency also will help utilities save money by skipping transmission upgrades. ISO said the region can defer 10 transmission upgrades that earlier studies showed were needed to ensure reliability. Deferring the upgrades will save an estimated $260 million, ISO said.

In 2010, New England’s six states sponsored more than 125 energy efficiency programs offering financial incentives to promote efficient electrical devices, ISO said.

Stephen J. Rourke, vice president for system planning at ISO, said replacing incandescent lighting with compact fluorescent bulbs is the “easiest and least expensive” way to cut energy use. Savings are multiplied as apartments, office buildings, factories, schools, hospitals and other large energy users install efficient heating and air conditioning, he said.

Seth Kaplan, vice president for policy and climate advocacy at the Conservation Law Foundation, a regional environmental group, said the ISO’s report shows that efforts to conserve energy work. He said the group welcomes the ISO report.

“This is a very big deal,” he said. “The system needs to know what the demand is.”

Nationally, demand for electricity is leveling off as residential power use falls, experts say, reversing a long upward trend. More efficient lighting and electric devices are partly credited for the change. New homes also are being built to use less electricity and government subsidies for home energy savings programs help older homes use less power. Rourke said the weak economy also has contributed to reduced electricity use.

New England used about 130,000 gigawatt-hours of electricity last year. One gigawatt-hour can serve about 1 million homes for one hour.