Huh. I've been wondering why Harry has so much juice. He makes it a point to say that in terms of fuel tanks, he's got a huge one. For one reason or another, I never connected that to his birthday of Halloween, where power can be stolen—or, maybe, transferred. Is it possible that Margaret arranged a transfer of power from something to Harry as he was born—maybe even some of her own power? I'm thinking about this like passing on furies in the Codex Alera series, which is likely wrongheaded, but I think there might be something there.
Alternately, could a birth power a ritual the way a sacrifice could?