A team of pathophysiologists at the University of Milan has found that climbing stairs or walking for short bursts allows people to consume 20% to 60% more energy than if they do the same activity nonstop for the same distance. In their study, published in the Proceedings of the Royal Society B, the group asked volunteers to walk on a treadmill or climb stairs while also monitoring their oxygen intake.
The paper behind this meagre article talks about 10-240 second bouts. The 20-60% figure is for 30sec bouts.