Conservation of Power in Filters
When considering power a filter block must have a load resistor (that's where power is delivered). Delivering power to a filter block's input side also involves a source resistor.
The filter block itself may include resistors, and/or lossy reactive components. The simplest case to consider is where the filter block only includes reactive (L, C) components:







simulate this circuit &ndash; Schematic created using CircuitLab
 The filter block of reactive components cannot dissipate real power but it can temporarily store or transfer power amongst its neighbouring reactive components. Power is either delivered to Rload or it is refused by its input. Any real power accepted at the filter block's input side must be (eventually) delivered to Rload.

Referring to a filter block as a reflection attenuator reinforces this point of view. Power refused at the filter's input side can be considered a reflection of power back to the signal source where it would be dissipated by Rsource.