The average power dissipation in a pure capacitance in ac circuit is  

Average power in ac circuits is given by  $\mathrm{P}={\mathrm{V}}_{\mathrm{rms}}{\mathrm{i}}_{\mathrm{rms}}\mathrm{cos\phi }$
For pure capacitive circuit $\mathrm{\phi }={90}^{\mathrm{o}}$  so P = 0