I think the formula in this page is not updated. In Racial trait, you can read:

Arcane Torrent : Activate to silence all enemies within 8 yards for 2 seconds. In addition, you gain 10 Energy, 10 Runic Power or (5 + Level) Mana for each charge of Mana Tap currently affecting you. 2 min. cooldown.

But the formula showed in this page states:

The formula is roughly 2.17 * level + 9.136

The above formula results in 88-114 percent more mana than the official one. The formula was added in Feb/2007. —The preceding unsigned comment was added by Fsomalia (talk · contr).