Horsepower, kilowatts, BHP, and PS explained
kW… no doubt you’ve read it in a car review or heard a YouTuber refer to it in a video car review. What what does it mean? Practical Motoring explains.
MOTORING WRITERS LOUDLY proclaim ‘kilowatts’ or kW in their reviews, but what exactly does it mean? A kilowatt is a unit of power or a rate of doing work and is equal to 1000 watts. A kilowatt is a total unit of power and doesn’t take into account the loss of power from the engine to the wheels via friction in the gearbox and sundry other moving parts that all sap small amounts of power.
But it’s not the only measurement of power where vehicles are concerned. There’s also horsepower which, like kilowatts, is a total measurement of power in the imperial scale of measurement. Then there’s the French version of horsepower, or hp, which is known as CV for Cheval; French for horse. And then there’s PferdeStarke or PS which is German for horsepower.
And, if you’re watching or reading something out of the UK then you’ll have come across Brake Horsepower or bhp. This is a version of horsepower that describes the amount of power produced by an engine after friction losses have been taken into account after testing on a rolling road, also known as a chassis dynamometer. It is generally referred to as being the amount of ‘useful’ power an engine has because it’s a measurement of power, usually but not always, determined after power loss via auxiliary items have been taken into account.
You might also read the word torque when reading a car review or watching a re-run of a TV motoring show. Torque refers to the turning or twisting effort exerted by an engine; think of turning a spanner on a nut – the force you apply is torque, or a drill bit being turned from rest; this results in a ‘moment of torque’.
Does horsepower actually mean horse-power?
Yes, it does. The ‘horsepower’ unit was determined by James Watt in 1783 after he calculated the rate at which Cornish tin mine horses could work. Watt determined that a single horse power worked out as the energy required to lift a weight of 14,982kg one foot in one minute. However, the modern equivalent is the power required to lift a 75kg weight one metre in one second.
Why does Australia use killowatts?
Stay with me. The Standard Association of Australia (SAA) adopted the Systeme International d’Unites (SI) in 1977 which determines the unit of power is best expressed as a kilowatt. One kilowatt is equivalent to 1.34 horsepower. To determine what a kilowatt is from a bhp figure, simply multiply the bhp value by 0.746.
How is power measured?
Generally, power and/or torque are measured via a chassis dynamometer which sees a vehicle tested in full production trim. A chassis dyno applies load to a vehicle’s engine allowing for the accurate measurement of power and torque across the entire rev range and at the wheels. Some dynos are also able to measure output with the ignition turned off.