The first objective of this paper is to prove that for the same amount of incident solar irradiation, the amount of useful heat provided by a solar air collector is different during time intervals with stable and unstable radiative regimes, respectively. A simple one-node model is used to analyze the performance of two solar air collectors with similar surface area. One collector has a porous absorber while the other collector has an aluminum U-corrugated absorber. Quantifying the performance of solar collectors during transient operation is more involved than during steady-state operation. Specific instantaneous indicators and variability factors are defined, which show how large is, in average, the relative deviation between the instantaneous and global values of the efficiency and coefficient of performance, respectively. The second objective is to estimate variability factors for different levels of radiative regime stability. Three conclusions are stated. First, when a solar collector performs better on long term than another collector, this is true for all radiative regimes. Second, the instantaneous performance indicators depend on the radiative regime, operation duration and design parameters. Third, the variability factors are smaller (in absolute value) in case of the stable radiative regime than for the unstable radiative regimes. Experimental data confirm that the usual global indicators may be used with reasonable accuracy in case of stable radiative regimes but they should be complemented with variability factors during unstable radiative regimes.