While a lot of attention is paid to the design of consumer electronics like mobile phones, various other domains have been neglected so far when it comes to user experience. In this paper a user-centered design approach for designing the user interface of a controller for solar thermal plants and heat exchanger stations 8211; called smart sol 8211; is described. The design process is characterized by the cooperation of user experience designers on the one hand and engineers and programmers on the other hand.