[baremetal]

radioactive decay.

[jeffbee]

In other words it isn’t renewable, it’s just very long-lived.

[connicpu]

There's no such thing as infinite energy in this universe. There's enough deuterium in the oceans to power humanity at our current pace until well after the sun expands and makes earth uninhabitable, but that is still a limit where we'd run out.

[colechristensen]

So is the Sun.