Immolation charging does not significantly increase the damage output of the spell, but instead only gives it a slight boost with the larger explosion radius. The damage difference should increase more noticeably per charge level so that doing so is preferable to single-charge spam even vs single targets. The damage doesn't have to result in a 2x or higher, but it should at least be +25% per charge (total 50% bonus for third charge, since first charge is the baseline and therefore no bonus).
