Okay, this went off from what I was trying to illustrate, so I'll backtrack/confirm a bit.

I agree that servants require mana to exist in the war.

Quote Originally Posted by Buff
Saber's used a ton of mana for Excalibur and also exerted herself fighting those tentacles, but so far Lancer hasn't really shown that he even uses mana in the first place.
Read: Saber vs Lancer is back to the Dock fight even if she is lower on mana than Lancer. Lancer may have more mana, but his Noble Phantasm is "always on", so it doesn't give him an edge. Saber was reluctant to use her Excalibur blast in the earlier fight to not give her identity away, so her inability to fire it soon now doesn't disadvantage her.* If they were having a duel that tested their endurance then their Mana difference could be be pivotal but I do not consider it likely.

* = of course, circumstances have actually changed. Everyone knows she's Arthuria now so she can vaporise shit without trying to hide her identity at the same time.

Quote Originally Posted by Ryll
But some burn mana just by existing.
I have the impression that Berserker burns mana like crazy when he's corporeal, but not when he's tucked away as a ghost.