  PQ-type adjacency polytopes $\nabla^{\rm PQ}_G$ are lattice polytopes arising
from finite graphs $G$. There is a connection between $\nabla^{\rm PQ}_G$ and
the engineering problem known as power-flow study, which models the balance of
electric power on a network of power generation. In particular, the normalized
volume of $\nabla^{\rm PQ}_G$ plays a central role. In the present paper, we
focus the case where $G$ is a join graph. In fact, formulas of the
$h^*$-polynomial and the normalized volume of $\nabla^{\rm PQ}_G$ of a join
graph $G$ are presented. Moreover, we give explicit formulas of the
$h^*$-polynomial and the normalized volume of $\nabla^{\rm PQ}_G$ when $G$ is a
complete multipartite graph or a wheel graph.
