For future spacecraft, energy storage flywheels could replaced the traditional electro-chemical batteries as the main energy storage system. But, flywheels are nor simply a "plug-in" alternative to batteries. Noting the torque/momentum capability of flywheels, it is even more promising to use the flywheels both for electrical power and attitude control. Such a flywheel-based system is investigated from the ACS point of view. First, a generic analysis of the different flywheels configuration is presented. Then, GEO and LEO applications are evaluated.