Why Energy is measured in joules, and the rate at which it is used or delivered is power, measured in watts (one watt = one joule per second). In a circuit you calculate it as P = I x E, current times voltage. Resistance, current and voltage each describe one property of the circuit, but only power tells you how fast energy is being consumed.
Watch out Current is tempting because it is also a "rate," but it is the rate of charge flow (coulombs per second, amperes), not the rate of energy use.
Power = energy per time. Watts = joules per second. Rate of energy use means watts.