A rating used to measure heroes' added skill points, stats, level, evolution, and ascension. Increasing any of these will increase their power rating.
It can be used as a measure of how relatively strong that hero is, however, it doesn't guarantee that he or she will win against a foe with lower rating. It's mostly a cosmetical number serving only as an indicator of relative power, as an intimidating factor in Arena, and as a matchmaking factor for Gauntlet.
During an evolution, enchantments disappear which may cause a loss in power. This is intended and in turn, opens up new item slots and enchanting slots to allow you to usually go to higher power rating than before.
1 point in a skill raises a hero's power by 4.