For a continuous-time signal
$ Energy = \int_{t_1}^{t_2} \! |x(t)|^2\ dt ............. (1) $
Over an infinite period of time
$ Energy(\infty) = \lim_{T \to \infty} \int_{-T}^{T} \! |x(t)|^2\ dt = \int_{-\infty}^{\infty} \! |x(t)|^2\ dt .......... (2) $
If Equation 2 converges, Energy is finite.
If Equation 2 diverges, Energy is infinite.
$ P(\infty)=\lim_{T \to \infty} {\frac{E(\infty)}{2T}} = 0 $
