However, the current ripple in the PFC’s output capacitor has components at the switching frequency (say 70kHz). …but also has a significant component at the twice line frequency (100Hz), due to the PFC operation.

Since electrolytic capacitors have a higher ESR at 100Hz than 70khz, is it really very pertinent to actually do a FFT on the capacitor ripple current so that the power dissipation of this 100Hz component with the (increased) ESR can be found?

(When measuring the capacitor ripple current RMS on the scope over a 10ms period, it obviously just spits out an IRMS measurement, and doesn’t tell you what is the IRMS of the actual 100Hz component.)