Let $G_{S}$ be a graph with $n$ vertices obtained from a simple graph $G$ by attaching one self-loop at each vertex in $S \subseteq V(G)$. The energy of $G_{S}$ is defined by Gutman et al. as $E(G_{S})=\sum_{i=1}^{n}\left| λ_{i} -\fracσ{n} \right|$, where $λ_{1},\dots,λ_{n}$ are the adjacency eigenvalues of $G_{S}$ and $σ$ is the number of self-loops of $G_{S}$. In this paper, several upper and lower bounds of $E(G_{S})$ regarding $λ_{1}$ and $λ_{n}$ are obtained. Especially, the upper bound $E(G_{S}) \leq \sqrt{n\left(2m+σ-\frac{σ^{2}}{n}\right)}$ $(\ast)$ given by Gutman et al. is improved to the following bound \begin{align*} E(G_{S})\leq \sqrt{n\left(2m+σ-\frac{σ^{2}}{n}\right)-\frac{n}{2}\left(\left |λ_{1}-\fracσ{n}\right |-\left |λ_{n}-\fracσ{n}\right |\right)^{2}}, \end{align*} where $\left| λ_{1}-\fracσ{n}\right| \geq \dots \geq \left| λ_{n}-\fracσ{n}\right|$. Moreover, all graphs are characterized when the equality holds in Gutmans' bound $(\ast)$ by using this new bound.