Horsepower to kW: Understanding the Shift from Engines to Motors

Historically, the output of internal combustion engines has been expressed in "horsepower." Originating from James Watt's steam engine comparisons, this unit includes British horsepower (HP) based on the imperial system and metric horsepower (PS, or Pferdestärke) used widely in Japan and Europe.
1 PS (Metric Horsepower) ≈ 735.5 W
1 HP (Mechanical Horsepower) ≈ 745.7 W
While horsepower remains in common use, the International System of Units (SI) designates the watt (W) as the standard unit of power. Modern specifications increasingly prioritize kW to ensure consistency across electrical and mechanical systems.

Torque represents rotational force, calculated as the product of force and distance. The standard unit is the Newton-meter (N·m). In simple terms, torque relates to acceleration performance--similar to the force applied when pedaling a bicycle from a standstill.
Conversion: 10kgf·m = 98.06N·m

Traditionally, diesel engines offer higher torque at low RPMs compared to gasoline engines, providing superior initial acceleration. Conversely, gasoline engines typically achieve higher maximum RPMs, resulting in higher peak power output (Power = Torque × RPM) and higher top speeds.

Electric motors used in EVs and hybrid vehicles are rated in kilowatts (kW). For perspective, 100 kW is approximately equivalent to 136 PS.

Motor Characteristics: Torque and RPM
Unlike internal combustion engines, electric motors deliver maximum torque instantaneously at zero RPM. This characteristic provides EVs with exceptional initial acceleration. However, as motor speed increases, Back Electromotive Force (Back EMF)--voltage generated in opposition to the current flow--increases proportionally. This acts as a limiting factor, reducing torque at higher speeds. The maximum speed of a motor is determined by the point where the generated torque balances with the load and Back EMF.

Comparing Output: EV vs. ICE
EV performance has evolved significantly. While early mass-market EVs ranged from 100 kW to 160 kW (roughly 136-218 PS), high-performance electric hypercars now exceed 1,400 kW (approx. 1,900 PS), far surpassing the traditional limitations of combustion engines.

As we have seen, the characteristics of the engine and the motor are different. Let's see this difference in terms of driving a car.
Then, let's first consider the acceleration performance from the stopped state. It is important at what timing the maximum torque can be produced. The motor is designed so that the maximum torque comes out at low rotation immediately after starting rotation. Therefore, EVs can be considered to outperform gasoline-powered vehicles in terms of acceleration performance.

On the other hand, the maximum output depends on the performance of the motor. The motor had the characteristic that the load increased as the motor speed increased. Therefore, it can be said that how to reduce this load is the biggest point to increase the motor speed, that is, to increase the output.

When evaluating EV performance, battery specifications are as critical as motor output.

Charging Power (kW): Represents the speed of energy transfer. A higher kW rating indicates faster charging capability.
Battery Capacity (kWh): Represents the total energy stored. "kWh" (kilowatt-hour) indicates how much power can be supplied over time.
Formula: Capacity (kWh) ÷ Time (h) = Power (kW)

The efficiency of a battery is often measured by its energy density--how much energy it can store relative to its size or weight.

Gravimetric Energy Density (Wh/kg): Energy per unit of weight. Crucial for reducing vehicle weight.
Volumetric Energy Density (Wh/L): Energy per unit of volume. Important for packaging constraints. For example, modern 18650-size lithium-ion batteries significantly outperform traditional lead-acid or alkaline batteries in both metrics, making them essential for high-performance automotive applications.