The rate at which energy is expended or work is done. In a direct-current circuit or device the power developed is equal to VI, where V is the potential difference in volts and I the current in amperes. In an alternating-current circuit the power developed is equal to VI cosϕ, where V and I are the root-mean-square values of voltage and current and ϕ is the phase angle between them. Cosϕ is the power factor of the circuit or device and the apparent power is the product VI, measured in volt-amperes. The product VI sinϕ is the reactive power. See var.