Global models are useful in the analysis of fusion reactors due to the facility in computing and presenting the results in terms of comprehensive parameters. A general figure of merit, which encompasses all the relevant tokamak parameters, is introduced in this paper by a convenient normalization of the global power balance equation. In this way, different hypothetical tokamak reactors can be compared in terms of their figure-of-merit value. This criterion is applied to analyse the performance of both ITER-like reactors and a class of newly proposed low-power reactors.