kWkg

kWkg, commonly written as kW/kg, is a unit of specific power that expresses how much power, in kilowatts, is available per unit of mass, in kilograms. It is defined as P/m, where P is power (in kilowatts) and m is mass (in kilograms). This metric is used to compare propulsion and energy‑storage systems in terms of power density rather than total power or stored energy alone.

The value of kW/kg depends on what is being measured and how the device is massed. For motors and generators, it indicates how quickly mechanical energy can be delivered from a given mass. For batteries, ultracapacitors, or other energy-storage devices, it indicates the rate at which electrical energy can be supplied relative to their mass. The practical usable power is also constrained by thermal limits, efficiency, and cooling, so the measured kW/kg often reflects peak or sustained operating conditions rather than a single universal figure.

Converting to other units, 1 kW/kg equals about 0.607 horsepower per pound, since 1 kW is 1.341 hp and 1 kg is 2.2046 lb.

Typical ranges vary widely by technology and application. High-performance electric machines can achieve several kW/kg, while conventional internal-combustion engines are often around 0.1–0.3 kW/kg. Battery packs and related energy-storage systems generally offer lower power density, typically below a few kW/kg, though high-rate configurations can reach higher values. kW/kg is a key design consideration in fields such as electric vehicles, aerospace propulsion, and portable power systems.