You didn't say so, but I assume we're talking about the elevator rising by 1 story.
The work it has to do is (force) x (distance) = (15,000 N) x (3 m) = 45,000 Joules.
Power = (work) / (time) .
Power = (45,000 Joules) / (50 seconds)
Power = 900 joules/sec
Power = 900 watts
Power = about 1.206 Horsepower