Easy way to calculate the resistors power rating is:
P = I squared x R
For example:
If you require say 1500mA (I) and you know your resistor is 1ohm (R) then:
1.500 squared x 1 = 2.25W's
You can even shorten the equation because your multiplying the squared current value by 1 which is the same the squared current.
So for a 1ohm circuit you could say:
Power = I squared.
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