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Context: The energy-limited (EL) atmospheric escape approach is used toestimate mass-loss rates for a broad range of planets that hosthydrogen-dominated atmospheres as well as for performing atmospheric evolutioncalculations. Aims: We aim to study the applicability range of the ELapproximation. Methods: We revise the EL formalism and its assumptions. We alsocompare its results with those of hydrodynamic simulations, employing a gridcovering planets with masses, radii, and equilibrium temperatures rangingbetween 1 $M_{\oplus}$ and 39 $M_{\oplus}$, 1 $R_{\oplus}$ and 10 $R_{\oplus}$,and 300 and 2000 K, respectively. Results: Within the grid boundaries, we findthat the EL approximation gives a correct order of magnitude estimate formass-loss rates for about 76% of the planets, but there can be departures fromhydrodynamic simulations by up to three orders of magnitude in individualcases. Furthermore, we find that planets for which the mass-loss rates arecorrectly estimated by the EL approximation to within one order of magnitudehave intermediate gravitational potentials as well as low-to-intermediateequilibrium temperatures and irradiation fluxes of extreme ultraviolet andX-ray radiation. However, for planets with low or high gravitationalpotentials, or high equilibrium temperatures and irradiation fluxes, theapproximation fails in most cases. Conclusions: The EL approximation should notbe used for planetary evolution calculations that require computing mass-lossrates for planets that cover a broad parameter space. In this case, it is verylikely that the EL approximation would at times return mass-loss rates of up toseveral orders of magnitude above or below those predicted by hydrodynamicsimulations. For planetary atmospheric evolution calculations, interpolationroutines or approximations based on grids of hydrodynamic models should be usedinstead.

A critical assessment of the applicability of the energy-limited approximation for estimating exoplanetary mass-loss rates