When current varies, as it does in AC circuits,
where t is time and P is the instantaneous active power being converted from electrical energy to heat. Far more often, the average power is of more interest than the instantaneous power:
where "avg" denotes average (mean) over one or more cycles, and "rms" denotes root mean square.
These formulas are valid for an ideal resistor, with zero reactance. If the reactance is nonzero, the formulas are modified:
where  is phase difference between current and voltage,  means real part, Z is the complex impedance, and Y* is the complex conjugate of the admittance (equal to 1/Z*).
For more details in the reactive case, see AC power.