1 hard point energy gives 2 mana, and 2 mana is 2 mana. The amount of mana this nets, depends on your bo level. Or actually, your amount of %increased mana. Add SoJs, arach hoto etc on top of your % from bo.
Items that has +to energy (and +to vita) will not be enhanced by %inc mana(or life). Same goes for life/mana per level (e.g. shako), sadly.