Tellegen's theorem is valid for **any linear network**. This includes both electrical and non-electrical networks, as long as the network elements are linear and obey Ohm's Law.
In essence, Tellegen's theorem applies to:
1. **Electrical Networks**: Circuits composed of resistors, capacitors, inductors, and linear dependent sources.
2. **Mechanical Networks**: Systems involving linear springs and dampers.
3. **Thermal Networks**: Systems involving linear resistive thermal elements.
The theorem states that for any linear network, the algebraic sum of the power across all branches is zero. This principle holds regardless of the configuration of the network, provided the elements remain linear and the network is in a steady state.