the Energy functional of a PDE : $-\triangle u +u|u|^{p-1} =f$

Question

how can I show that the Energy functional of a PDE : $-\triangle u +u|u|^{p-1} =f$ in $\Omega$

$u=0$ in $\partial \Omega$, is : $$ E(u)=\int_\Omega\Bigl(\frac12|\nabla u|^2+\frac1{p+1}|u|^{p+1}-f\,u\bigr)dx. $$ Please :)