The utilizability or phi-curve method is a design method for some types of solar energy systems that use flat-plate solar collectors. It can be a very useful design tool, for the preliminary design or the design of small systems, cases where methods easy to use and apply are required. In this paper, a simple method to calculate monthly average hourly and daily flat plate collector utilizability is presented. It is based on empirical correlation of phi with independent variables that separate meteorological and collector parameters, making possible quick evaluations of changes in collector design and collector inlet temperature. In general, the method reduces the calculations required to determine phi, and gives good results compared to long-term hourly simulations as well as to existing hourly and daily utilizability calculations methods. (c) 2006 Elsevier Ltd. All rights reserved.