  This paper deals with the old yet unsolved problem of defining and evaluating
the stored electromagnetic energy - a quantity essential for calculating the
quality factor, which reflects the intrinsic bandwidth of the considered
electromagnetic system. A novel paradigm is proposed to determine the stored
energy in the time domain leading to the method, which exhibits positive
semi-definiteness and coordinate independence, i.e. two key properties actually
not met by the contemporary approaches. The proposed technique is compared with
two up-to-date frequency domain methods that are extensively used in practice.
All three concepts are discussed and compared on the basis of examples of
varying complexity, starting with lumped RLC circuits and finishing with
realistic radiators.
