What is the approximate temperature of a solar-mass black hole from Hawking radiation?

A solar-mass black hole has a Hawking-radiation temperature of about 60 nanokelvins.

Hawking radiation is a quantum effect associated with black-hole horizons. For a nonrotating, uncharged black hole, the temperature decreases as mass increases. A black hole with the Sun’s mass is therefore extraordinarily cold, far colder than the 2.7-kelvin cosmic microwave background.

The 60-nanokelvin estimate describes the idealized radiation temperature, not the temperature of material in an accretion disk. Gas orbiting a black hole can become millions of degrees hot and emit X-rays, while the black hole itself remains extremely cold by comparison.

Because the cosmic background is warmer than a stellar-mass black hole, such a black hole would absorb more background energy than it emits today. Evaporation becomes important only after the universe cools dramatically, and the lifetime of a solar-mass black hole is vastly longer than the universe’s current age.