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Connecting kilowatt to an EV’s power and charging speed
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The higher the kilowatt, the faster energy is consumed or produced.
PHOTO: AFP
Why is kW used to describe an electric vehicle’s (EV) power output as well as charging speed?
Kilowatt or kW is the power rating for any machinery, including electric motors and fossil fuel-powered engines. It is the unit of measure of how fast energy is produced or consumed.
If two people of the same size, weight and health run for, say, 400m in a race, they both consume the same energy over the distance. However, the one who reaches the finish line in a shorter time has more power.
Work done, or energy per unit time, is power. Mathematically, power in watts is energy (in joules) divided by time (in seconds).
The principles are the same in the context of EV chargers. What is injected into the battery by the wall box power supply cable is electrical energy (joules), and the rated power (kW) tells you how fast the energy flows.
Whether AC or DC, a 100kW outlet delivers the same quantum of electrical energy, which is twice as fast as one that is rated at 50kW. In other words, 100kW in one hour is equivalent to energy consumed at 50kW over two hours.
The standard unit to measure electricity consumption is kilowatt-hour (kWh), which in the example above would amount to 100kWh.
In Singapore, domestic, commercial and industrial electrical energy consumption is billed on cumulative kWh.
For homes, electrical energy consumption is now regulated at 31.91 cents per kWh.
EV charging at public stations is a little more expensive, ranging from about 42 cents per kWh at AC stations, which vary from 3.7kW to 22kW, to as high as 60 cents to 85 cents per kWh for the DC fast-charging stations, where power outputs range from 50kW to the latest 480kW.
So “filling” 50kWh of charge into the battery of a typical mid-sized EV using an 11kW AC supply takes about five hours and costs less than $25. For the same car, a 150kW DC supply would need less than 30 minutes and cost around $35. Charging times may vary depending on the car’s battery health and, often, the condition of the station’s power supply unit.
In any case, electricity charges are calculated purely on kWh of energy consumed and not on the plugged-in duration. It is also important to remember that charging power is automatically controlled and limited by the EV’s on-board charger, regardless of the station’s power capacity.
Most hybrid-EVs are limited to 3.7kW AC input, while the latest EVs typically accept 100kW to 350kW DC, depending on make and model.
