The Department of Energy's inspector general reported yesterday that its Washington headquarters loses 126,000 pieces of mail a year - a stack 1,270 feet high, more than twice the height of the Washington Monument.

J. K. Mansfield who based his calculations on sample mailings this year, said the amount of mail lost would almost certainly be worse if incorrectly addressed mail were counted. In the samples, all addresses were verified before mailing.

Mail delivery time is nothing DOE can brag about either. On the average, it takes 2.25 days for all mail to get from one to another of the department's seven Washington offices.

Apprised of the problem, William S. Heffelfinger, DOE's director of administration, took bureucratic counter-measures.

In a memorandum attached to the inspector general's report, Heffelfinger said major actions had been taken, including a notice to all employes "calling their attention to the necessity for correct addresses on internal mail and informing them how to adress mail."

Heffelinger also reported the development of a "users' Short Course" to teach the clerical staff proper procedures. An aide said the course takes about an hour and emphasizes use of proper titles and abbreviations and, for messages to the outside world, ZIP codes.

There also were more conventional measures in Heffelfinger's list, such as hiring extra workers in the mail room, and "adjustment in administrative procedures to minimize the disruption caused by personnel relocations." Asked what that means, one employe said, "Nobody moves."

With all systems go, Heffelfinger said he hoped to deliver all correctly adressed mail within eight working hours between office buildings and within four working hours inside the same building. He set no deadline for such progress.