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Amperage is the measurement of electrical current. Think of it as a flow of water.

Watts is the measurement of electrical power. It is an indirect relationship between amperage and time (A/t = W). Think of it as the force water carries.

Voltage is a little bit complicated to explain. It is the measure of energy per chage unit. Think of it as the difference in the energies of the positive and negative terminals of a battery.

In order to create work (Watts), you must have a resistor. From Ohm's Law:

V=IR

Ohm's Law demonstrates that for every voltage there is a corresponding current if the resistance is constant. So in a PSU, the resistors in a given rail will determine the voltage (12, 5.5 or 3 volts). The amperage rating is the amount of current flowing to create that particular volatge. Current control is crucial to keep clean power and stable readings. The wattage is the ultimate result of the work performed by the resistors in all rails; higher power ratings indicate the PSU can put out more work because it can handle higher currents.

Hope it is clear enough.