Why do cells use fat and starch for long-term energy storage instead of ATP molecules? ATP is used for long-term storage, while fat and starch are used for immediate energy. ATP is used for short-term energy and to build molecules of starch and fat. Fat and starch are unstable and can be stored short-term, while ATP molecules are stable and stored long-term. Fat and starch are stable if used as energy immediately, while ATP is used as long-term storage.