Cream probably invented the power ballad with "Sunshine of Your Love". I'm willing to concede if there were others before Cream. Of course Zepplin did some power ballads, also, but they were shortly after Cream or do I have that wrong, also :)?
should have read " invented the modern rock power ballad..."
After reading some about the definitions by critics of a power ballad, they are varied. However, they all seem to agree on a slow tempo that can build to a moderately fast tempp "Stairway To Heaven". The lyrics are about love/lost love, angst, loneliness, emotional strife, life confusion and all are melodic.
Google for yourself if you disagree. However, my intent of this article was what us rockers consider power ballads. Guess I should have been more specific.
In case no one mentioned above: Love Hurts by Nazareth.