A fuel cell electric generator designed for back-up in the absence of network electricity supply. The generator comprises a fuel cell stack, means for supplying the stack with a first and a second reagent flow comprising, in turn, pressure reducing means, and a manifold body to communicate with the stack said first and second reagent flows and at least a flow of coolant fluid via a respective coolant loop. The manifold body comprises inside chambers for the mixing of said reagent flows with corresponding re-circulated product flows and a coolant fluid expansion chamber within which said pressure reducing means of said first and second reagent flows are positioned at least partially drowned by said coolant. Method for the start-up and shutdown of the generator, and a method for detecting the flooding of a fuel cell and a method for detecting the presence of gas leakages in the generator are also disclosed.