Your power, or "Pw", is the amount of magical energy you have:
Dlvl:3 $:43 HP:68(68) Pw:13(18) AC:-3 Xp:8/1295 T:2358
The above status line shows a character whose current power is 13 and maximum power is 18.
Power is consumed by casting spells, at a rate of 5 times the spell's level. When carrying the Amulet of Yendor, you will expend a random amount from 1 to 3 times the norm. If you fail to cast a spell due to failure rate or confusion, you expend only half the energy required for a success. When you have teleportitis, self-teleporting with ^T always consumes 20 power points, but random teleports consume no power.
Power can be regained slowly over time, quickly by quaffing a non-cursed potion of gain energy or with The Mitre of Holiness, and fully by reading a scroll of charging while confused. The potion also increases your maximum power; the scroll does so only if your power is already at maximum when you read it. Eating a newt corpse occasionally raises your current Power by 1-3 points, with the message "You feel a mild buzz"
The rate at which power regenerates over time increases with your Wisdom and Intelligence. If your encumbrance level is stressed, it will not regenerate. The Eye of the Aethiopica causes your power to regenerate at a much greater rate than usual.
If you have energy regeneration (only granted by the carried Eye of the Aethiopica), you will instead regenerate energy every turn.
The amount of energy regenerated each time is between 1 and 1 + (Int+Wis)/15, which for most spellcasters means 1d2 or 1d3.