How to calculate the power of a finite length signal?
If the signal $r(t)$ is of finite length (and if integral of its square during its definition interval is also finite) then its average power given by 
$$ \bar{P_x} = \lim_{T \to \infty} \frac{1}{T} \int_{-T/2}^{T/2} |r(t)|^2 dt = \lim_{T \to \infty} \frac{1}{T} \int_{0}^{L} |r(t)|^2 dt = \lim_{T \to \infty} \frac{[\text{constant}]}{T} = 0$$ will be zero...

Btw, you may compute an instantaneous power of the signal, simply as the square of it $|r(t)|^2$...