New software has been developed, adapted to International Standard ISO 9459-2, in which the daily solar domestic hot water (SDHW) system operation is modeled with a heat-limited consumption scheme, fixed by a nominal temperature, (T-dn), and a daily nominal hot water volume production, (V-dn). Solar energy is traced to its end forms; overheating in summer months, remaining in storage to be used the next day, lost through storage tank surface and drawn-off in the useful hot water. Auxiliary heat to comply with nominal settings is also calculated. For commercial reasons it is becoming more and more necessary to have an effective way of classifying SDHW systems. Our proposal is to use as figure of merit, the rate between the hot water produced in one year and the auxiliary heat needed, (L/kW h), calculated when the daily extraction is 2/3 of the storage volume, (V-dn = 2/3 V-s) and the temperature (T-dn) is 60 degrees C. The draw-back of such a procedure results from observing that the value obtained is site-dependent. An alternative classification way is presented, based on a non-dimensional number that is totally site-independent. (C) 2014 Elsevier Ltd. All rights reserved.