You can basically sum up power metal as "all metal descended primarily from the innovations of Judas Priest and Iron Maiden (plus maybe Deep Purple, though usually without any keyboards), but only minimally defiled by the innovations of thrash and later, uglier noise." It is, at root, an early-'80s sound -- in fact, some people use the phrase "New Wave of British Heavy Metal" interchangeably, though that was an era more than a genre, and plenty of archetypal power metal wasn't British at all. (Lots of it was German, and Manowar and Riot even came from New York!)
Alternately, here's Canadian critic Martin Popoff's definition, in the glossary to his indispensable 1997 book The Collector's Guide to Heavy Metal: "Really heavy bands that are neither predominantly speed, thrash or necessarily OTT... most power metal bands pride themselves on their playing ability and can lean toward progressive at times. Most techno-metal bands are also power metal and vice-versa, but there are exceptions... power metal means dense and considerably heavy, the most pummeling and truly weighty of metallic alloys." ("OTT" means "Over The Top," i.e. really, really fast; "techno metal" means Yngwie Malmsteen, not Prodigy. He also lists several thrash and doom bands that neither I nor Wikipedia -- which adds all-important requirements about lyrics focusing on "fantasy and mythology, camaraderie and hope, personal struggles and emotions, war and death, or combinations of the listed themes" -- would consider PM. But I shall leave them out, so as not to confuse the minions.)
Anyway, in the past decade or two -- and especially, perhaps, in the past year or three -- there seems to be a power metal resurgence, not to mention a resurgence of what might even be called power pop metal (which has nothing to do with power pop, per se). And that's what this Cheat Sheet focuses on: bands that nobody heard about before the '90s, at the earliest. (Blind Guardian, probably the oldest band here, actually put out two albums in the very late '80s, but they didn't chart anywhere -- even Japan! -- until 1990.) And the vast majority of these bands formed much later than that. So: not only no Priest or Maiden, but no recent albums by Accept, Angel Witch, or Manilla Road, who are actually all still quite good. They'll just have to wait for some other time, but for the 20 bands and albums below, the icy mountaintop awaits!