For a continuous-time signal $ Energy = \int_{t_1}^{t_2} \! |x(t)|^2\ dt $
Over an infinite period of time $ Energy(\infty) = \lim_{T \to \infty} \int_{-T}^{T} \! |x(t)|^2\ dt $ $ \lim_{x \to \infty} f(x) = L $
