Defence they are treated the same way as all other heroes on defence in terms of the mana units per turn (passive) and per tile per cascade.
Can read more on it on this thread (bit to wade through but good info’s): New mana per turn calculation on defense? We investigate Vol 2
Useful explanation from Kerridoc (no numbers unfortunately)
That’s because defense heroes gain mana differently than the offense.
- on offense, a hero only gains mana by matching tiles (unless boosted with a gain-mana buff from, say, Alberich). You gain one tile’s worth of mana when the tile hits a foe, or two tile’s worth if it “ghosts” (doesn’t hit a foe). Certain troops or specials increase the amount of mana each tile is worth.
- on defense, every hero gains a tick of mana each turn. Any hero hit by a tile will gain a chunk of mana; the chunk size decreases the more tiles hit in a particular turn (so a huge cascade may not charge all foes’ mana). Again, mana-boost troops and specials may increase the mana gain.