A preliminary usefulness metric is defined. The metric is intended to characterize the energy efficiency of light sources in a more comprehensive way than the luminous efficacy of a source by taking selected spectral aspects of human centric lighting into consideration. The presented version of the metric is a combination of well-established measures of color quality, brightness and the circadian effect derived from the spectral power distribution of the light source. The metric includes a limited application dependence: it yields different values for interior and exterior applications, static and dynamic as well as relaxing and activating light source spectra. Light source categories (A-G) with preliminary category limits were also computed in the two-dimensional diagram associated with the metric. The metric should be considered as a basis for further discussions and not as a final solution.