Steven Swanson and Richard Arnold installed the last set of solar wings on the ISS late on Thursday during a six-hour spacewalk to bring the total number of solar panels to eight.
The solar array will provide the electricity to fully power scientific experiments and support the station's expanded crew when it is increased to six in May.
Two more spacewalks are planned for the current Discovery mission on March 21 and March 23
The Discovery lifted off from Florida's Cape Canaveral on Monday and docked with the ISS at 17:20 EDT on Wednesday (21:20 GMT Tuesday).
The shuttle delivered a crew of seven to the ISS, including Japanese Aerospace Exploration Agency astronaut Koichi Wakata, and also brought with it the space station's fourth and final set of solar panels.
Wakata, the ISS's first long-duration Japanese resident, replaced U.S. astronaut Sandra Magnus, who will return to Earth on Discovery after more than four months aboard the space station. The Japanese astronaut is scheduled to return to Earth during the next shuttle mission, STS-127, scheduled for June 2009.
NASA is planning to wrap up the current Discovery mission on March 25 and the shuttle is scheduled to land on March 28 at 13:42 EDT (17.42 GMT).
