  We prove an analogue of Yau's Caccioppoli-type inequality for nonnegative
subharmonic functions on graphs. We then obtain a Liouville theorem for
harmonic or non-negative subharmonic functions of class Lq, 1<=q<\infty, on any
graph, and a quantitative version for q > 1. Also, we provide counterexamples
for Liouville theorems for 0 < q < 1.
