The Stefans were of modest means; his father was a milling assistant and his mother served as a maidservant.

At thirteen, he experienced the revolutionary year of 1848, which inspired him to show sympathy toward the Slovene literary and national movement.

After having graduated top of his class in high school, he briefly considered joining the Benedictine Order, but his great interest in physics prevailed.

Very important are also his electromagnetic equations, defined in vector notation, and works in the kinetic theory of heat.

Stefan was among the first physicists in Europe who fully understood Maxwell's electromagnetic theory and one of the few outside England who expanded on it.

He also researched a phenomenon called the skin effect, where high-frequency electric current is greater on the surface of a conductor than in its interior.