Most fans used in computers are intended to run on 12VDC. The fan label will typically specify the design voltage as well as design amperage and/or watts. For heat sink fans, the design amperage or wattage can be an important factor. The highest power fans presently available often draw more current than motherboard fan headers are intended to supply. This has resulted in many people blowing their fan headers.
It is simple to relate fan voltage, amperage, and wattage. For DC devices, power (watts) equals volts multiplied by amps. If a fan label indicates an amperage of 0.48A, it will draw 12 * 0.48 = 5.76 watts at design speed.
Fan labels from an 80 mm and 60 mm fan. The 80 mm fan normally runs on 12 volts DC and draws 0.13 amps. This is 12 * 0.13 = 1.56 watts. The 60 mm fan normally runs on 12 volts DC and uses 2.8 watts. This is 2.8/12 = 0.233 amps.
Key Point: You must verify the rating of your motherboard before plugging in a powerful fan.