When doing light work the i5 already sips at the power so just not needed. Others have experimented in capping the maximum CPU power at the higher-power levels but for light work the 13" MBP already has it sorted. Keeping the work light is the trick and browsers such as Firefox and Chrome really eat the battery so sticking with Safari when away from power does make a big difference. I also find a big difference with media players - picking the right one can also help.