Why does Magneto not exhaust himself? Where does he get his energy from?

Perhaps he doesn’t generate the power within himself but controls and shapes the ambient electromagnetic fields that fill the universe, if this is the case he would never run down because he is directing not generating the energy. This would also make him inconceivably powerful considering that everything from microwaves through high energy gamma rays are all just electromagnetic fields of different wavelengths.

Looking at Superman might be illustrative. With his solar powered energy derivation, what we're probably looking at is a rampantly efficient metabolic system, probably on a cellular level rather than a complete systemic level (hence regeneration after systemic death). And more than visible light, it probably runs off of the 'solar wind' and the amount of proton or neutron cast off from the sun's fusion (hence the differences in reaction under a yellow sun or a red sun due to differences in output ratios). So Superman is a hyperefficient solar battery (I can't take credit for that phrasing), using his human-like metabolic processes augmented by a secondary energy source.

I am going to postulate the same for Magneto (and the rest). With Magneto particularly, given his greater power when within the Earth's magnetosphere, it is very likely that he is either in some way absorbing electromagnetic energy at a constant rate, but able to release it with higher apparent efficiency upon concentration. And, much like like a single lightning strike contains about 6% of the energy the world needs in a year after a single strike (which seems like so much more energy when concentrated into appliances, etc.), Magneto could simply have access to such incredible amounts of energy due to the efficiency of his secondary (some may say primary) metabolism that he apparently has unlimited energy, but in actuality just needs so little of it to perform whatever task is at hand that it only seems unlimited (like taking a gulp out of the ocean instead of a sip).

And for the sake of completeness, this still allows people like him to starve, or get thirsty, or need to sleep. Your body creates vitamin D from sunlight - but a whole buttload of vitamin D doesn't compensate for the rest of the amino acids you needs, or for sleep, or for water. So just b/c a superhero has a hyperefficient metabolism in one regard doesn't mean that it compensates for the rest of his metabolism in any other way.

Most depictions of metahumans of which Magneto just happens to be an exceptional specimen of, have no scientifically plausible explanation for how or why they are able to perform such outstanding feats of energy gathering, manipulation or creation with no discernable machinery or applicable technology.

• Most metahumans defy known laws of physics, creating matter and energy seemingly from nothing, often without appreciable side effects such as waste heat or excess light as a byproduct. Superman for example flies without a clearly defined power source, no exhaust, no energy byproducts, and he emits no radiation of any kind (unless he wants to). This is a common trope of the comic industry and is generally accepted without question.

• Magneto's powers were originally explained as a connection and use of the Earth's magnetosphere magnifying his magnetic powers making him capable of doing things most metas simply couldn't imagine, because his powers were augmented by using the much greater magnetic capabilities of the Earth. It made him a fine foil to the X-men making him capable of tackling the entire team single-handedly.

• Yes, this was a stretch, but since most mutant abilities already stretch the realm of plausibility, his was no worse than anyone else's and the explanation of why he was so much more powerful seemed perfectly plausible. Later, they would add to his resume, the fact his powers developed later, which in mutantkind is a kind of super-booster, and that he was an Omega-Level mutant (the unofficial designation of a mutant with the potential to alter or destroy all life on Earth with the use of their mutant power).

Highly relevant StackExchange article: Is Magneto officially an Omega Level Mutant?

• As for why he doesn't tire, it would depend on the writers. Early writers would have him experience fatigue if he fought against a team of metahumans or if he performed a feat of incredible magnetic legerdemaine such as when he turned the Earth's magnetic field into a buffer to reduce Charles Xavier's magnetic powers, or when he stripped the adamantium from Wolverine's skeleton. During the period when he was believed to be dead, his Joseph persona did not have anywhere near the range and power he has today.

But the easiest answer for you is that the Marvel editorial edict paints Magneto with a broad and Magnificent Bastard brush, making him one of the most powerful, relentless and infatigable enemies of the Marvel Universe. Getting tired is for lesser villains. (obligatory TVTropes warning!)

Also See Stack Article answers to:

• Is there a set limit on mutant abilities, where they're deemed too unrealistic?

• Why are X-Men more realistic than Fantastic Four? [closed]

• The X-Factor gene in Ultimate X-Men

When Magneto was originally created back in the 60's (alongside the X-Men), they gave but a brief description of his powers; the ambiguous Master of Magnetism. This would give one a good deal of leeway when it comes to concern of actual application. Magneto's abilities are astounding enough that the Government and a few others built quite a good deal of defensive measure just for Erik Lennsher, a few being plastic-and-porcelain Sentinels, a plastic prison, magnetic disrupters, nonreactive metals, to name a few. If you question is really about the few limitations and extreme abilities of Magneto, then yes, he is rather Godlike in his abilities and output.

But he isn't alone.

Erik Lensherr is an Omega Class Mutant, which is a designation of categories that rate the powers, output, and abilities of a neo-homo sapiens superior. While an Alpha is generally considered first tier in scope, the Omega designation is for Mutants that could quite literally kill the population of the planet, or deal enough devastation to cause so, regardless of circumstances or special instances. Phoenix (Jean Grey) is one such mutant, who is considerably more powerful, as she could conceivably drain a star, and possibly disrupt enough to destroy the galaxy under circumstantial conditions, thanks to the M'kran Crystal. Professor Charles Xavier and Scott Summers (Cyclops) are Alpha Class, which while still incredibly power, do not have the ability to do so under normal circumstances. Yes, Professor X could start a nuclear war with his telepathy, but his power itself could not do so; Magneto can affect the magneticsphere, which would increase solar radiation and genetic mutation. It is also wondered that he could slightly affect Earth's rotation, orbit, and position. Such a feat probably would kill him, but it might be under the scope of his powers.

When many superheroes were created, their powers were more or less limited to the Age they inhabited at the time. Superman and Green Lantern (the original one) had powers that were significantly lesser than they are today. The original Superman couldn't do a tenth of what he can today, specifically because a good many of his powers weren't even thought of or invented yet (such as lasers). The original Green Lantern couldn't affect wood with his ring (sonic screwdriver, anyone?). Nowadays, they're practically Godlike in what they can do, as are their enemies.

So, back to your question.

Several times in the comics (and I think once in the original animated series) Magneto has mentioned that his powers were failing (due to age) but to be revived by no less than the planet Earth itself. Magneto has been resurrected at least once because of such (sometime in the 90's) and if this is truth, then as long as he is within Earth's magneticsphere, it could be possible that his 'power' is boundless. Now Magneto has gotten tired many times, and has weakened as well from overuse, but generally he seems to be the Energizer battery. The only real limitation he seems to have is range; which has been inferred to be several miles (back when he owned an island in the late 70's) to hundreds of miles (when he owned Asteroid M).

I haven't kept up with Marvel in quite a while, but Magneto is a classical villain; goal-oriented, driven, sympathetic, but more than willing to step on toes. His dream of giving Mutants freedom and a place to enjoy it justifies his means of doing some, sometimes brutally. Yet despite his authoritative stance, he isn't an evil man, but I'd stay away from him if you got an electronic pacemaker if I were you (and you were a mutie-hatin' human).