Power of periodic signal
The average power of a signal $s(t)$ is given by

$$\overline{s^2(t)}=\lim_{T\rightarrow\infty}\frac{1}{T}\int_{-T/2}^{T/2}|s(t)|^2dt\tag{1}$$

If $s(t)$ is periodic, $(1)$ is equivalent to the average power in one period:

$$\overline{s^2(t)}=\frac{1}{T}\int_{-T/2}^{T/2}|s(t)|^2dt\tag{2}$$

where $T$ now denotes the period of $s(t)$.