Robert Bosch photo
by Jacques Gordon
We’re accustomed to thinking about an engine in terms of horsepower and torque, but the power of electric motors in hybrid and electric vehicles is reported in kilowatts. Why the difference and how do these power ratings compare?
First of all, here’s the simple conversion: 1 horsepower (hp) equals 0.7457 kilowatts (kW), or 1 kW equals 1.337 hp. You can do the math yourself (hp x 0.745 or kW x 1.34) or you can use an online conversion chart. Here’s the one I use:
The reason these power measurements are different is because they’re different kinds of power.
One kilowatt is 1,000 watts, a unit of power named for Scottish engineer James Watt. He didn’t invent the steam engine, but he patented several improvements to the Newcomen steam engine that was being used to pump water out of mines. Back then, mine pumps were often powered by what was called a “horse engine,” a treadmill for horses with a power take-off shaft. Watt (in collaboration with others) determined that one horse could lift 550 pounds (of water) one foot in one second, and that became the official definition of one horsepower.
Notice that horsepower is a calculated number, and the factors are weight, distance and time. Weight and distance are combined when we measure torque; a force rotating around a shaft. When an engine forces a shaft to turn, we measure the pounds of force generated perpendicular to and one foot away from the center of that shaft. It’s a direct measurement of foot-pounds (ft-lbs) of torque. To get horsepower, we need to know how many foot-pounds per second the engine is generating. The formula is torque x rpm divided by 5252. That last number is a constant factor that results from converting rpm (speed of rotation) to feet per second.
Horsepower is a measurement of mechanical force. The power of an electric motor is described by measuring the electrical energy it is designed to consume.
When we measure electric current, we’re measuring the flow rate of electrons through a circuit. One amp is equal to a specific number of electrons flowing per second (6.241 × 1018 electrons, if you must know). That flow rate depends partly on the electrical pressure pushing electrons through the circuit, known as voltage, and on the resistance in the circuit opposing the flow of electrons. Increasing either the voltage or amperage increases the amount of electrical power in the circuit. To make it easy to calculate and work with electrical power, we simply multiply volts x amps and call the result watts. So 12 volts flowing at 10 amps produces the same 120 watts of power as 120 volts flowing at 1 amp. The more watts available in a circuit, the more resistance it can overcome. If the resistance is a motor that’s 100% efficient (no unplanned electrical resistance), then a 7.5 kW motor is able to convert 7,500 watts of electrical power into mechanical work. Generating 7.5kW with a battery is a tall order, but that’s a story for another day.
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