Even under the powerful thrust of 20-20-20 measures, solar thermal systems are experiencing a slow-down in their development in most EU countries. One reason is the growing competition with photovoltaic (PV) systems; another one is their traditional confinement to sanitary hot water (SHW) production. In order to widen the use of solar thermal collectors (STC), they should also be able to contribute to space heating and cooling and become multi-purpose STC. This paper addresses the issue of optimal sizing of multi-purpose STC systems. The criterion adopted is based on maximization of Net Present Value and has been applied to some Italian cities and in Pakistan with diverse weather conditions. Results show that optimal thermal collector areas per peak cooling demand (Ac/Pc) can be conveniently expressed as a function of Peak Heating to Cooling Ratio (Ph/Pc). The optimum Ac/Pc varies between 3 and 5 m2/kWc and decreases with increasing Ph/Pc. The paper also analyses and compares a STC-based system with a multi-purpose PV-based heat pump system using a traditional one (boiler and compression chiller) as reference. Results show that steadily decreasing prices have made PV systems more favorable, even without consideration of public subsidies. Finally, the limit cost of STC in order to reach the same cost effectiveness of PV systems is calculated.