How to calculate capacitor's Power Rating?
Capacitors are usually not rated in watts, it wouldn't make much sense. There are two limiting ratings that you must however consider when designing:


The voltage rating. This one is easy. It is always mentioned in the datasheet, just check you're below. Note they are usually mentioned for DC currents, so if you have AC current as input, take just care to consider the peak voltage, not RMS value.
The ripple current. This one is usually found in the datasheets for capacitors that are used for power supply filtering applications. It is dependant on the ESR of the capacitor. You need to check it if your capacitor sees high AC current flowing through it. Take the RMS value of the capacitor current and check you're below. But if you are filtering a low current signal, you don't need to bother checking this.


Edit: Why the watt rating wouldn't make sense ?

Because it complicates things. The amount of power dissipated by the capacitor is directly dependant on the current through it and its ESR (the voltage across the capacitor pins is not relevant for the power calculation). You usually know what current you apply to the capacitor, but to know what power it dissipates you have to compute ESR*I² (ESR being a characteristic of the capacitor), and check that this value is below Pmax (also a characteristic of the capacitor).

Now, if you can simply check that the current is below a value that is given directly in the datasheet (the ripple current), it is more immediate.