# How to calculate capacitor's Power Rating?

How to calculate capacitors power rating (in watts)? or is there any ? i am designing a circuit to filter out noise signals i am not sure about power ratings of capacitor(electrolytic) .

• You look them up in the datasheet – PlasmaHH May 4 '16 at 9:51
• Its a tricky one but its related to ESR (equivalent series resistance). I would say focus more on the voltage rating. Capacitors don't really dissipate a lot of real power when used well under their voltage rating. They do though however generate reactive power but that has nothing to do with real power and hence heat. Voltage rating is more important than 'power' rating. – crowie May 4 '16 at 9:55
• @PlasmaHH Actually not. I have never seen a power rating given in watts in a capacitor datasheet. See my answer. – dim May 4 '16 at 10:42
• The power dissipation of a capacitor is only going to be an issue if it has to handle a high ripple current, half the time this is given in the datasheet, so as long as you don't go over that, you'll be fine. If it's only got DC or only very small ripple across it, it'll burn almost no power in the first place – Sam May 5 '16 at 10:00

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.

• thank you for the reply it helped a lot . One thing i want to ask why would the watts rating in capacitor don't make sense what is the reason ? – mayanksagar May 4 '16 at 10:24
• @mayanksagar: I have edited my answer to clarify this. – dim May 4 '16 at 10:35

A capacitor can have an energy rating because it stores energy ($\frac{CV^2}{2}$) so all you really need to know is the maximum voltage that can be safely applied to its terminals hence read the data sheet.

This is all you really need to worry about with "signals". For AC power waveforms and ripple currents there is other stuff to consider and the data sheet usually specifies maximum ripple currents for AC applications.