There is no general rule of thumb for the minimum shelf life of fuels in emergency power systems because the influencing factors vary widely. For example, the quality of the fuel batch, the type of fuel (admixture of biodiesel, additives, etc.), the storage temperature, non-ferrous metals used in the plant and the formation of water all play a role. Experience from the quality testing of fuels in emergency power systems shows that the usability of one fuel can be critical after only 2 years of storage, while another fuel has already been stored in the tank for 30 years and is stable. Only an analytical fuel check could give certainty about the further usability.