Theres this thing called OHMS law, it defines the relationships between power, voltage, current, and resistance. One ohm is the resistance value through which one volt will maintain a current of one ampere.

Lets say you have 400 Watts, and 14.4 volts of electricity. You would divide the Power by the volts to get the amps, or current. Thats how you would detirmine what fuse a circuit would need.