Mix and match the tempos during any given song. It's not as complex as it sounds; you don't have to compose anything as sophisticated as Metallica's 1988 LP " ... And Justice for All." Start out by simply writing the verses at a slow tempo with the chorus ushering in a faster beat. The key lie in having at least one portion of the song go fast --- a hallmark of power metal.
While power metal is known for its aggressive and hard sound, one key component is the incorporation of melody. You can incorporate melody in one of two ways. One method involves the vocal lines; power metal usually features a hard driving riff with an intricate, broad-ranging vocal melody on top of it. The good news is that your singer doesn't necessarily have to have a huge range; as long as a song has growling interspersed with clean singing, it fits into the power metal template.
Power metal's lyrical content usually focuses around fantasy. Exotic realms, historical themes and warriors are common themes in the lyrics. For example, listen to Iron Maiden's "The Trooper." However, real-life subject matter isn't necessarily taboo, either, as evidenced by Pantera's "Suicide Note, pt. II." As long as the latter is worded in broad, ambiguous terms, it falls within the parameters of power metal.
One easy way to create a darker sound is to tune down. The standard guitar and bass tuning is in the key of E, but don't be scared to experiment with E flat, D or even D flat. Keep in mind, however, that the lower you tune, the looser your strings are, meaning it may affect the guitar solos . Also, take your vocalist's singing range into consideration. If he has a high-pitched voice, consider tuning higher.