A failed transformer was to blame, according to a 49ers spokesman.
The power went out about 25 minutes before the 5:40 p.m. scheduled kickoff.
Power was restored by about 80 percent within about 15 minutes.
City engineers were working to switch the power over to the backup power supply.
The power first started to come back on in the press box and the scoreboard ring around the stadium about 15 minutes later, drawing cheers from Steelers and 49ers fans.
The 49ers went on to beat the Steelers, 20-3.
The Associated Press contributed to this report.