Your sparten can add energy to the 'Energy Boost' reserve (sidabar) by collecting bounties on certain NPC's. You can recieve info about these NPC's by going to the bounty office (explore>shady activities>bounty office) by buying their information. you will then recieve an Event regarding the NPC's location. You need to travel to the same city as the NPC (or wait for them to travel to your area [they move every three hours]).
Upon arriving in the same city as the NPC, go to explore>shady activities and then underneath the link for the 'bounty office' there should be a new link saying;
' (ie. Joe's) hidout'
Click this link and then attack the NPC. If you win the ifght you will recive 200 in energy boost energy, this goes to your stockpile to be used whenever you want to.