When considering very large single-tower heliostat fields, the attenuation and spillage losses are growing with the distance to the central receiver. Therefore multi-tower configurations are modelled and investigated to find the thermo-economic optimal size of transition from single-to multi-tower plants. First a method of building a multi-tower layout is defined, second a criterion is set up to select which receiver an individual heliostat is instantaneously aiming at, and third the thermo-economic performance of a three- towers heliostat field is compared to that of an equivalent single-tower heliostat field. Subsequently, a sensitivity analysis and a thermo-economic optimisation are performed both on single- and multi-tower configurations to find the two energy/cost trade-off curves and their intersection, which gives the thermo- economic optimum transition size from single- to multi-tower plants.