Albert Barnes Commentary Revelation 16:8

"And the fourth poured out his bowl upon the sun; and it was given unto it to scorch men with fire." — Revelation 16:8 (ASV)

And the fourth angel poured out his vial upon the sun. This means toward the sun, or so as to reach the sun. The effect was as if it had been poured upon the sun, giving it an intense heat, and thus inflicting a severe judgment upon men. This corresponds also with the fourth trumpet (Revelation 8:12), where it is said that the third part of the sun was smitten, and the third part of the moon, and the third part of the stars. For the general meaning of this symbol, see Barnes on "Revelation 8:2".

The idea is that a scene of calamity and woe would occur as if the sun were made to pour forth such intense heat that men would be scorched. It cannot be supposed that the sun would be literally made hotter, or that the exact nature of these calamities would be that men would be consumed by its rays.

And power was given unto him. This refers to the sun. The meaning is that a calamity would follow as if such an increased power were given to its rays.

To scorch men with fire. Literally, And it was given him to scorch men with fire; that is, with heat so great that it seemed to be fire. The Greek word—kaumatisai—meaning to burn, to scorch—is used in the New Testament only in Matthew 13:6, Mark 4:6, and Revelation 16:8-9, in all which places it is rendered scorch and scorched. Compare, however, the use of the word kauma, in Revelation 7:16 and Revelation 16:9; kausis, in Hebrews 6:8; kausoō, in 1 Peter 3:10, 12; and kausōn, in Matthew 20:12, Luke 12:55, and James 1:11.

The notion of intense or consuming heat is implied in all the forms of the word; and the reference here is to some calamity that would be well represented by such an increased heat of the sun.