To refill your energy meter, use your lightning bolt to charge objects that can drain energy such as batteries and light posts. Then, after it is fully charged, drain it. If the object does not explode your energy meter will be refilled.
Shock a pedestrian, then walk up to them. Lock on, then repeatedly press SQUARE to restore your power.
After you kill a person, shock their body with your basic lightning bolts that require no energy. Press L2 while they are sparking to absorb the energy and recharge.