The Particle Flow Algorithms adopted for future e+e−e+e− colliders detectors and phase-II CMS upgrade require very high granularity calorimeters to deconvolve the individual contributions of particles in jets. This is especially true for electromagnetic calorimeters (ECAL). For a realistic large detector many technological requirements have to be fulfilled: modularity for industrialization; compact integration of an embedded very front-end electronics featuring large dynamics, low-power and self-triggering; mechanical structure and cooling systems with minimal dead zones. The technological prototype of the silicon-tungsten (SiW) ECAL presented here should achieve all this. 10 layers are produced and tested in beam, while design and optimization studies are ongoing on a variety of simulated key processes to test the performance of the hardware and the algorithms.