Scientific reading
Choose the boundary and output goal
A device draws input energy and transfers some of it into the intended useful form. Other transfers can include thermal energy, sound or other forms, depending on the system. A complete simplified steady-run ledger writes E_input=E_useful+E_other, provided the machine has no net stored-energy change during the selected interval and no unlisted energy flows.
Calculate input and useful power consistently
For the same positive interval t, mean input power P_in=E_in/t and mean useful output power P_useful=E_useful/t. The difference is P_other=E_other/t, so P_in=P_useful+P_other under the stated accounting assumptions. Do not compare powers measured over unrelated time intervals without explaining why that is physically meaningful.
The word useful is a design choice
Useful does not mean energy appears from nowhere or that every other transfer must be thermal. A lamp is judged by the light it provides for an intended task; a heater may treat thermal energy as useful. Identify the actual function before assigning output categories. An ideal model may assume 75% useful output, but a real conversion efficiency requires measurements.
Example: two identical time windows
A synthetic device receives 400 J over 5 s and provides 300 J useful energy. Other transfers account for 100 J. Average input power is 80 W; useful power is 60 W and other-transfer power is 20 W. All values refer to the same 5 s window.