Specific energy is a measure of the energy stored per unit mass in an energy storage system, such as a battery or fuel. It's expressed in watt-hours per kilogram (Wh/kg). This metric is crucial for applications where weight is a critical factor, like electric vehicles and portable electronics. A higher specific energy indicates a lighter device for the same energy output. For example, lithium-ion batteries have a higher specific energy than lead-acid batteries, making them more suitable for lightweight applications.