Edward explains his sparkles in the Twilight ReVamped & ReWolfed FanFiction Corrective Rewrite Summary! It takes a couple of hours to read through it all. Chapter One: First Sight is written complete 'as a novel' to lay a much better (and less addictive) foundation than Stephenie Meyer - but the rest is a corrective summary written similar to this:
He DOES sparkle in the sun, but it is a fairly-quick, yet gradually-building thing that happens as a reaction to sunlight. BECAUSE he is soul-less and "dark" inside; his ENTIRE BODY is devoid of "life in general" - it craves and consumes sunlight MUCH LIKE his body craves the "soul-essence" of Humans because they have no (or in Edward's case, very little) soul.
In other words, this would be a PHYSICAL result of a Human losing their soul by being turned into a Vampire. The soul is connected to BOTH the body AND the spirit. Since their bodies are lacking in a soul, they are completely lacking in the "life of sunlight" - which, most physical life on this planet needs. A Vampire body will hoard & store sunlight in receptors in his "marbled-leathery" skin (much like a solar panel, someone pointed out, ha). When these receptors start to overload - they sparkle!
It is at THIS POINT that a Vampire is the MOST STRONG - as strong as any newborn (especially if they are also satiated with Human blood)! BUT, it doesn't last very long and it probably depends alot on a Vampire's age and/or how these receptors have been used throughout their lives. These things determine each Vampire's sparkle time; amount of strength they gain; and how long they can stay in the sun.
For instance, an OLD Vampire who has remained a creature of the night their entire life would be incredibly strong (even moreso than your basic newborn because of their older, stronger internal mystical essence) on the few occasions they may need to be in the sun.
An old Vampire who has indulged in a fairly "normal" life in the sun would be a little more consistently-stronger in their daily life because of it; BUT they would have a harder time coming to a zenith during a 'sun battle' if they ever needed to. They would really only be able to count on the strength they are already used to having. I HOPE that makes sense, ha!
Vampires have about 2-5 hours of "gaining/utilizing" sunlight strength. And while they can stay in the Sun for awhile after that, they will become weaker. It could be a form of torture for them - their skin would go beyond sparkling and glow brighter and brighter until the sunlight finally consumes them. It would be an agonizing death for them.
Some of the greatest Vampire battles throughout make-believe HISTORY have been fought at high-noon and lasted less than an hour!
How's that? ;>
Since our Bella is such a sun-lover as a Human, I really don't see her staying away from it much after she is changed into a Vampire (even though she's supposed to, ha!) I always think of Bella when I hear Cheryl Crow's song, "Soak Up the Sun" (and now that I'm listening to it again, her and Edward's love story, in general).
That's Ok, Bella won't need to be worried as much about losing her zenith if she ever needs it. She has another zenith, and besides - she's surrounded by a pretty strong cadre of Vampires no matter what comes up. Also... If Vampires KNOW they have a battle coming up, they can stay out of the sun ahead of time; and doing that could cause them to reach a zenith during a sun battle.
Two Options Available - one on Fanfiction.net without (most of) the extra critique;
the other one continues on this blog with the full Twilight storyline critique.