When an AC voltage U is put onto a loss less capacitor C the capacitor will require a certain reactive power Q (current 90 degress out of phase with the voltage):
$Q=UI = \omega C U^2$ in VAr (Only reactive power, no real power)
I can also determine the reactive power strating from the stored energy in a capacitor:
$E= (1/2) C U^2$
With $ Q=dE/dt $ using Fourier transformer to go to the frequency domain: $ Q=\omega E$
Filling in the equations results into: $Q=(1/2) \omega C U^2$
You can now see my problem. Deriving the reactive power of a capacitor from voltage / current / capacitance results in factor 1/2 difference compared to deriving reactive power derived starting from energy stored in a capacitor.
Can you help me with what I'm missing or doing wrong.