  We present a counterexample to a lower bound for the power domination number
given in Liao, Power domination with bounded time constraints, J. Comb. Optim.
31 (2016)725-742. We also define the power propagation time, using the power
domination propagation ideas in Liao and the (zero forcing) propagation time in
Hogben et al, Propagation time for zero forcing on a graph, Discrete Appl.
Math.160 (2012) 1994-2005.
