## 4 Conclusions
For all of them, an economic analysis was performed in order to select the most affordable in economic terms. Net present value and payback time for the proposed systems were evaluated. When not considering the national economic incentives, the stand-alone photovoltaic plants showed a negative cash-flow rate with a consequently negative net present value. Contrariwise, the other analyzed domestic hot water plant plants led to positive net present values varying from 760 \(\upxi\) for the solely pump water heater (payback time = 15.1) to 1902 \(\upxi\) for the solar thermal plant with an electrical heater back-up (payback time = 7.6). If Italian incentives are considered, the net present value is strongly reduced and the values are positive for all analyzed plants. From an energy-saving point of view, the most promising plants include the use of the pump water heater coupled with a proper photovoltaic (100% energy saving) or solar thermal plant (86% energy saving). This fact is verified even if the photovoltaic plant needs an incentive support to be economically attractive. Similarly, the PBT calculated for the pump water heater-based plant shows a value close to the estimated useful life for the plant (14.7 vs. 20 years), if national incentives are not considered. On the other hand, the electric storage water heater with a solar plant, ensuring 66% of energy saving, should have the highest economic advantages in terms of net present value and payback time, even if no incentives are taken into account. Based on the presented results, it would be of interest to test hybrid solar-thermal technologies in order to optimize RES plant support for DHW production by means of an HPWH system. At the same time, new kinds of heat pumps are currently being commercialized. These are split technology pumps with instantaneous production of DHW by means of a wrap-around heat exchanger. Furthermore, they are tested to nominally ensure a larger DHW thermal energy demand (between 19.07 and 24.53 kWh per daily tapping with greatest tapping events equal to 4.42 and 6.24 kWh, respectively) with respect to the here-studied HPWH (11.665 kWh per daily tapping with greatest tapping event in a day equal to 3.605 kWh). Considering that the current research focused on a small island's residential DHW provision, it would also be of interest to test a split HPWH coupled with a hybrid plant producing DHW for micro-communities in islands made up of few families living in small blocks. Data curation, M.B. ([PERSON]) and G.L. ; formal analysis, F.M. ; project administration, M.B. ([PERSON]) and B.D.P. All authors have read and agreed to the published version of the manuscript. The study presented was funded by ENEA (Italian National Agency for New Technologies, Energy, and Sustainable Economic Development) in the framework of the projects \"Technologies for building buildings of the future\". The study presented is partially based on the result of a collaboration between ENEA (Italian National Agency for New Technologies, Energy, and Sustainable Economic Development) and the DII (Department of Engineering) of the University of Palermo in the framework of the projects \"Technologies for building buildings of the future\", Annual Implementation Plan 2016, Simulation and comparison of technologies for air conditioning and domestic hot water installed at the end-users of smaller islands not connected to the national grid in order to reduce energy costs and make the island's electrical system more efficient, and Annual Implementation Plan 2017, Experimental and numerical analysis of solar driven technologies for air conditioning and the production of DHW and ICT systems for control and reduction of electrical loads in the smaller islands not connected to the national grid. **Conflicts of Interest:** The authors declare no conflict of interest. **Nomenclature**