Do capacitors waste power as heat or whatever on AC?
There would be power loss because with real life capacitors, there are parasitic losses, this means that the capacitor can not act as a pure capacitive load in real life, this is mainly because of the building process/materials/sizes. 

In real life capacitors have an ESL (Equivalent Series Inductance), an ESR (Equivalent Series Resistance), and a Leakage Resistance in parallel with the capacitor which is commonly notated as Rleak.



You would need to know the ESR to calculate power loss. But keep in mind that this is a parameter that degrades with component use.


  As per the industry standards EIA-463-A, MIL-C-62F, under nominal
  operating conditions, capacitors are considered completely degraded
  and not usable in the circuit when it's ESR value reaches 2.8 times of
  the initial ESR value and capacitance decrease in excess of 20% of the
  initial value. 


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