Buck Converter Output Power
If you multiplied a sinewave by a DC value you get another sinewave with a different amplitude. This is equivalent to multiplying a sinewave current by a DC voltage. The average power (V x I) is zero. That's what power is - it's an average of V x I.
What this tells you is that current taken from a battery (for example) that is sinusoidal (or has sinusoidal content), creates no average power i.e. the average of \$V \times I_{SINE} = \text{zero}\$.
Now it can be one sinewave or any number of sinewaves or any shape of signal you want because, the pure AC content of the current multiplied by the DC voltage is always having an average value of zero.