Oh and of course, the power source is also relevant. USB on a PC usualy can supply 500 milliampere. The Pi uses up to like 3.4 Watts, which at 5V would amount to 680 Milliampere.
People tend to recommend a power source of at least 2 Ampere (at 5V, obviously) though for the Pi itself on its own 1 should be more than enough.