How many terminals does the device have? Are we talking about a two terminal device (like a resistor, capacitor, inductor) or a four terminal device (like a transformer).
The "ideal" transformer has no power loss as the input power equals output power. This is because the primary and secondary voltages and currents are natuarally scaled to give you the ratio. For example, a 2:1 transformer will have a half the voltage at the secondary (compared to the primary) but *twice* the current at the secondary (again compared the primary). In a real transformer there are losses, for example, the resistive losses in the copper wires that are used to construct the windings. Another thing about a textbook (ideal) transformer is that it passes all frequencies -- so the voltage and current transformation by the turns ratio is valid at all frequencies (even at dc). In reality, this is not true! The transformer will not pass dc and high frequencies (so it acts like a bandpass filter).
If you can clarify your question a bit, I may be able to help you further.
Cheers,
v_c