Three different settings, namely, offline power control, online power control, and power control with look-ahead, are considered, corresponding respectively to the cases with non-causal, causal, and partial non-causal knowledge of the energy arrival process. A complete characterization of the optimal offline power control policy is presented. In the online setting, the focus is placed on the greedy policy, which is optimal in the low-battery-capacity regime, and universally near-optimal policies, which include the maximin optimal policy, the fixed fraction policy, the two-piece fixed faction policy, and the locally fixed fraction policy. Finally, power control with look-ahead is introduced to bridge offline and online power control, the entire spectrum of optimal policies is characterized for Bernoulli energy arrivals, and the extension beyond the Bernoulli case is also discussed.