Considering the widespread use of effective capacity in cross-layer design and the extensive existence of renewal service processes in communication networks, this paper thoroughly investigates the effective capacity for renewal processes. Unlike prior literature that the effective capacity is approximated with no many insightful discussions, our expression is simple and reveals further meaningful results, such as the monotonicity and bounds of effective capacity. The analytical results are then applied to evaluate the maximum supportable throughput for variable-rate hybrid automatic repeat request (VR-HARQ) systems given a QoS exponent. Numerical results corroborate the analytical ones and prove the superiority of our proposed approach. Furthermore, targeting at maximizing the effective capacity via optimal rate selection, it is revealed that VRHARQ performs better than fixed-rate HARQ with incremental redundancy (FR-HARQ-IR).