As pyro stated W=IV. Every fan has a voltage and amps written behind it, just multiply the two and you are set (watts= amps*volts; watts = amps^2*resistance). Note that there are fans out there that can consume almost 30-50watts (Delta fans) so be careful not to use those. I have had fans burning out fan controllers because of high wattage but fear not many many consumer fans are well below 5watts. Try to get fans that have good static pressure and cfm, remember most 38mm fans will be higher wattage.