Capacitors store energy then give it back once required. A perfect capacitor is nearly lossless on DC power because you only fill it once then it keeps energy in it until you discharge it so no power loss to mention, but on AC the capacitors will be charged then discharged all the time which in my theory seems to waste power, is that true? If it is true then how can I measure that loss for a 250V 8000µF capacitor connected to 170V AC power line?
Electronic – 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.
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