Each ability costs a certain amount of power, which can be found on the Skill window by hovering the cursor over the ability icon. When an ability is used, the player's power supply is depleted by the power cost of the ability. An ability can not be used if the player does not have the required power.
Your maximum power is determined by your level and the number of attribute points spent on increasing the Body and Mind attributes. It can be further increased by some of the modifiers on equipped armor and weapons.
Power recharges of its own accord both in and out of battle based on your regeneration rate and will completely replenish if left to do so. Additionally, power can be restored through the use of Power Chargers.