Karna's prana consumption

Question

Throughout the fate universe, there are many strong servants. And one of them is Karna, who is said by Nasu to be on par with Gilgamesh.

But it is a fact that he consumes so much mana that an ordinary master can't handle it. A basic mage can't even keep his mana burst(flames) activated continuously for 10 seconds. Why does this happen?

The reason I'm asking this is simple. We have Gilgamesh, who uses thousands of noble phantasms (and that is his main offence) and still doesn't drain his master. This one may not seem like a good example (since he uses hate of Babylon) so let's look at another case;

Artoria(saber). She was summoned in fuyuki ALIVE. She is different from other servants this way. Since she was summoned like this and not in a servant container as how it should be, she had no choice but to use mana burst to match in strength with other servants. And even though she was using mana burst again and again, Shirou could handle it.

Why is it like this? Does this mean that Karna alone is like this? Will his prana consumption decrease if he summoned in archer or rider class?

Answer 1

Gilgamesh is not using hundreds of NPs. He's using the Gate of Babylon, just opening a portal, shooting a projectile and then retrieving it, it's actually very cost effective. The only weapons he can actually release their True Names are Ea and Enkidu.

Source from Fate Complete Material III, on Gilgamesh: Tactics

Gilgamesh battles by shooting the originals of all Noble Phantasms like bullets via Gate of Babylon. The tactic yields excellent destructive power to magic energy efficiency. Since every Noble Phantasm is a bullet, the real magical energy used is merely the activation cost of Gate of Babylon. Because he possesses the originals of all Noble Phantasms, he can assault the weakness of any Servants. As a result, Gilgamesh has the advantage over most of the Servants in combat.

Artoria doesn't even have a proper path with Shirou, she's using her own prana reserves to use her skills. So no, Shirou is not sustaining her Mana Bursts.

Karna's Mana Burst (Flame) is just very high-maintenance. Extremely powerful as well, the dude is a fucking demigod, easily Heracles tier, who also has terrible prana consumption, Illya is probably the only one who could sustain Karna, much like she did with Herc. This last bit is pure speculation.

Answer 2

Paulnamid's answer covers your mistake about Gilgamesh, and briefly mentions what you've got wrong about Artoria. But to go into more detail on Artoria...

Artoria has the heart of a dragon. Or, depending on who you ask, her heart is a dragon. She can as such store and produce massive amounts of prana all on her own, and does so as naturally as (and via) breathing. If a mage is a minor machine at this, she's a massive factory. The problem for her is that she needs something of a starter supply to activate the core, and it's this she was originally in short supply of in F/SN. She starts out with no supply of energy from Shirou, at all. So there's no issue with Shirou withstanding her Mana Bursts, as he was never involved in that in any way. She was initially relying entirely on her stored reserves, and used up something like a fifth of what she started with in her first encounter with Berserker. But eventually, in the suitable route, Shirou gets his circuits connected to Artoria's with Rin's help, and her draconic magic core activates (well enough for her to achieve what she does for the rest of the war, anyway)*. Depending on which version of the story you have digested, you might see Shirou encounter a dragon in this sequence, and the core light up with a prismatic light as it activates with Shirou's energy. After this point, Shirou is essentially only providing the fuel necessary to keep Artoria's core running, and the core's production covers her mana burst.**

The distinction then is that, as a Servant, Karna has no intrinsic ability to produce his own prana supply, at least not at the level necessary to sustain everything he can do. Artoria, however, has a prana factory intrinsic to her very being (though it may need a jump start).

_{*In the Unlimited Blade Works route, Shirou is instead connected to Rin, who has a heart of dolphins or something. Rin then fuels both Artoria and Shirou.}

_{**You may be asking yourself, "why doesn't Artoria just feed her own core the energy needed to kickstart it? She's got a lot more than Shirou does for most of this time." The answer there doesn't seem to get spelled out as explicitly, but the short of the matter is that she was never trained in any of the magecraft arts, and she has no real understanding about how to use her core and her absolutely amazing draconic magic circuits properly. But if she had been... So she has difficulties really activating the core on her own, and it's the direct hookup of her core to Shirou's supply that enables it to finally get going.}