![doomsday kills superman doomsday kills superman](https://static1.srcdn.com/wordpress/wp-content/uploads/2021/02/Superman-Doomsday.jpg)
Doomsdays was a genetically-engineered creature that the Titans made to be their trump card, giving him the ability to get use to damage and power, and thus become more immune to it. Superman basically initiated a Kamakazi run on Doomsday. This let him trash the Green Lantern Corps (who may have an unlimited source of power through the power of Will) though doesn't stand up to SBP. Exiled during the Infinite Crisis, had a suit constructed to store yellow solar rays to prevent the possibility of depleting his stored energy. Take Superboy Prime who by, all intents and purposes, is equal to Superman in every way. While he wasn't necessarily 'killed' (as many of his clones/likenessess have equally survived such drastic beatings) Superman isn't an immortal. Superman's ability to convery solar energy to fuel his abilities proved ultimately to what caused his defeat to Doomsday. This explains why Superman has abilities at a nighttime zone on Earth when the sun isn't out, as the reflected rays off the moon and the miniscule amount from the star in the galaxy are not enough to fuel his abilities.
![doomsday kills superman doomsday kills superman](https://oyster.ignimgs.com/wordpress/stg.ign.com/2013/06/superman-doomsday.jpg)
While there hasn't been a real limit of how much he can store, it is a finite amount.
![doomsday kills superman doomsday kills superman](https://static1.srcdn.com/wordpress/wp-content/uploads/2016/08/doomsday-dinosaur.jpg)
Superman, as we know it, is a solar battery of sort he can store the yellow rays of our sun and converts it to become the fast/strong/whatever that he is.