 In June 1967, American bombers hit North Vietnam's hydroelectric plants and reduced North Vietnam capacity to generate electricity by 85%, accordingly to McNamara's calculations.At the same time, he also calculated that the annual amount of electricity generated in North Vietnam was equal only to a fifth of the electricity generated every year at the Potomac Electric Power Company's plant in Alexandria, Virginia, making the destruction of North Vietnamese power plants meaningless to the outcome of the war as the amount of electricity generated was so small.He also calculated in 1967 that over the last two years, American bombers had inflicted damage on North Vietnam equal to about $300 million while at the same time, Rolling Thunder had cost the U.S. Air Force about 700 aircraft shot down over North Vietnam whose total value was about $900 million, making the bombing campaign uneconomical.