Using the four basic parameters and the equation i = C . dv/dt, where “i” is the discharge current, C is the capacitance, dv is the change in voltage and dt is the duration of the discharge, we can calculate the minimum required capacitance assuming zero voltage drop across the ESR. For example a system has a nominal voltage of 2.5 Volts and a drop out voltage of 2.0 Volts. The system must continue to operate for 5 seconds after a mains fail and has an average current demand of 0.5 Amps. In this case the minimum allowable capacitance will be 5 Farad.