The Energy Department fired its top conservation expert yesterday, hours after the Merit Systems Protection Board announced that it had rejected an appeal for a third stay of her dismissal.

Dr. Maxine Savitz, deputy assistant secretary of conservation, was notified at 3:30 p.m. that she was being removed from her job effective at the end of the day. The firing was denounced immediately on Capitol Hill, where Savitz and the conservation programs she administered enjoyed wide bipartisan support.

DOE officials had attempted to fire Savitz last October for failing to report to a new assignment in Golden, Colo., but the firing had been held up by two successive stays granted by the merit board at the request of its special counsel.

This time, the merit board said special counsel K. William O'Connor had failed to present a convincing argument that DOE's proposal to transfer Savitz was actually a move to get rid of her.

O'Connor's initial investigation found evidence that the Colorado job was created at the request of Savitz' boss, Assistant Secretary Joseph A. Tribble, who knew Savitz would turn it down. As a member of the Senior Executive Service, Savitz is subject to dismissal if she refuses a job transfer.

But the merit board said yesterday that it was not persuaded that Tribble knew Savitz would decline the transfer. The board said that, although Savitz' personnel file contained a statement that she would not accept reassignment, "Savitz admits that she did not inform Mr. Tribble prior to her reassignment that she was unable to relocate."

Rep. Patricia Schroeder (D-Colo.), chairman of a House Post Office and Civil Service subcommittee, called that rationale "preposterous." The board's reasoning indicates that "so long as a political manager does not bother looking inside a career employe's personnel jacket, that manager cannot be assumed to know what's in the folder," she said.

Rep. Richard L. Ottinger (D-N.Y.), chairman of the House Energy and Commerce subcommittee on energy conservation, noted that Energy Secretary Donald Paul Hodel had promised several members of Congress that he would look into the situation. "It is unfortunate . . . that he has failed to do so and is out of town at the time of disposition," Ottinger said.

Savitz said last year that she believed the transfer was part of the Reagan administration's effort to "do away with all federal conservation programs" and a "punitive" measure for her resistance to budget cuts.

Savitz' pink slip yesterday included an offer of a 30-day appointment as a departmental consultant, which an Energy spokeman said was intended to keep Savitz on the payroll until O'Connor could complete his investigation. An aide to Savitz said she had not decided whether to accept the temporary appointment.

A spokesman for O'Connor said yesterday, "We still intend to conclude our investigation. If there is a need for corrective action, we'll ask for it."