How long do capacitors last if not used?

Electronic capacitors that sit unused on a shelf—commonly known as New Old Stock or spare replacement parts—typically have a storage lifespan of five to ten years, provided they are kept in a cool, dry, and climate-controlled environment. Over time, even without electrical current flowing through them, the internal electrolytic chemical paste and dielectric film inside uninstalled aluminum electrolytic capacitors degrade gradually, and storage in hot, humid garages can accelerate this chemical breakdown. When old, unused capacitors are finally installed after years of sitting in storage, the internal dielectric layer may fail to re-form properly under voltage, leading to immediate short-circuiting or rapid failure. You can verify the reliability of a stored spare capacitor by testing its microfarad value and leakage current with a specialized meter before installation.