Specific power
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In engineering, specific power, also known as power per unit mass or power density, refers to the amount of power delivered by an energy source, divided by some measure of the source's size or mass. It is used to compare various power sources by a common figure representing their suitability for a particular role.
- Area: Wind turbines are often compared using a specific power measuring watts per square meter of turbine disk area, which is pi times the length of a blade squared. This measure is also commonly used for solar panels, at least for typical applications.
- Volume: When measuring a heat engine, specific power is typically given in horsepower per cubic inch of engine displacement, or alternately kilowatts per litre. The result offers an approximation of the peak-power output of an engine. This is not to be confused with fuel efficiency, since high-efficiency often requires a lean fuel-air ratio, and thus lower power density. A modern high-performance car engine makes in excess of 1.65 hp/in³ (75 kW/L).
- Mass: When comparing batteries, specific power refers to the power-to-weight ratio, measured in kilowatts per kilogram (generally, W/kg). Fuels are usually measured in terms of energy density, since the available power is usually not a limitation. A high-octane fuel actually burns more slowly, and due to engine dynamics this can actually allow an engine to produce more power. In high performance vehicles, the specific power determines the vehicle's acceleration. The specific power on a mass basis is also used to characterize solar panels for use in aerospace applications where this parameter is a critical for an efficient launch.
