Omega subgroups of powerful p-groups

Let G be a powerful finite p-group. In this note, we give a short elementary proof of the following facts for all $i\ge 0$: (i) $\exp {\mathrm{\Omega }}_{-}i(G)\le p^i$ for odd p, and $\exp {\mathrm{\Omega }}_{-}i(G)\le 2^{i+1}$ for p = 2; (ii) the index $\mathrm{\mid }G:G^{p^i}\mathrm{\mid }$ coincides with the number of elements of G of order at most $p^i$.