Water cooling continues to be an attractive solution for mitigating thermal effects in photovoltaic (PV) panels. However, a technically viable and economically effective design of a cooling system for utility-scale solar power plants already operating is still in its early developments. In this context, this work not only presents the design and characterization of a water cooling kit system specifically conducted for already existing and operating PV plants, but also takes into account various aspects of its potential as a commercial-scale product. One considers not only water and energy losses in the cooling system, optimizing it for a real industrial application, but also economic aspects as the investment cost, durability and maintenance have been taken into account. In this specific context, the cooling kit was designed to minimize its cost and also the amount of water used. An analytical model is also presented and validated to predict the PV temperature of a cooled and non-cooled panel, all based on meteorological data at the desired installation place. Prototyping and testing were done on a 20 kW PV plant functioning in a roof of an industrial building in Lisbon (Portugal). Experiments showed how cooling/heating thermal time constants of the PV panels and water ON/OFF time cycles affect PVs' efficiency. A large amount of test data was produced and analyzed, showing how the cooling kit can increase annual energy production up to +12%.