I stopped using Tri-disaster (well, typically throw out 1 maybe 2, then once any of them die I stop).

So energy drain is only a dps increase if you are bone dry on mana, or about to be.

So I would say at 20% or lower mana, start using solely energy drain instead of fester.

And also drop thunder from your rotation (except when you are going to contagion). This should result in an almost mana neutral position.