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 the above equation converges,
$ \lim_{x \to \infty} f(x) = L $
