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Electrical (electric) power equation: Power P = I × V = R × I2 = V2 ⁄ R where power P is in watts, voltage V is in volts and current I is in amperes (DC). If there is AC, look also at the power factor PF = cos φ and φ = power factor angle (phase angle) between voltage and amperage.
Do you need me to do the math for you, 300W / 120V = 2.5A