[suicideneil]

Power ( watts/wattage ) = volts x amps.

Most retailers & manufacturers post the maximum amp draw that a motor is capabale of, so just take that figure and multiply it by the number of volts you plan to use. Obviously these are peak/maximium figures, and you rarely see them under normal driving (large amp spikes occur under acceleration normally)- your batteries will determine how much power the system can produce in total, so always get the best you can afford/need (higher C ratings and larger Mah capacity in other words, though 80amps continous is usually enough for the majority of applications).