Measuring energy in kilowatt hours is nothing new. It’s likely been written on your electricity bill for years. For electric vehicles, the core concepts of kilowatt hours, or kWh, all still apply but we use them to talk about batteries in a slightly different way.
A Quick Refresher: Energy
A kilowatt hour is the measurement of an EV battery capacity, or size. To fully explain what that number is telling us, we need to understand each part of it.
What is a watt? It’s a unit of power. And power measures energy over time. So, if you’re carrying something heavy up a flight of stairs, energy is what it takes to carry it at any given moment, and power is what it takes to keep carrying it for 10 minutes.
What is a kilo? Kilo is the prefix that means thousand. A kilowatt is 1,000 watts. To get kilowatts from watts, you simply divide watts by a thousand. A watt is pretty small so most household electrical uses are measured in kilowatts.
Once you understand watts and kilowatts, the kilowatt hour part is deceptively simple. Say you have ten 100 watt light bulbs in your house. Each one is 0.1 kilowatts, and all together, you have 1 kilowatt. A kilowatt hour is - quite literally - how much energy is needed to keep all your lights on for an hour. If you swap out those 100 watt bulbs with 25 watt bulbs, you need only 0.25 kWh to run them for an hour, and the single kWh you had needed to power your ten 100-watt bulbs will now power your lights for four hours.
Since power is the rate of energy per time, multiplying watts by time (an hour) gets you back to the raw energy measurement, which the EV world calls capacity.
Energy in Car Batteries
To sum it up, a 60 kWh battery can generate 60 kW sustained over one hour. If you only use 15 kW per hour, you get four hours of energy. If you use more power, say 120 kW, your battery will only last for a half hour. This is why an EV battery rated at a certain kWh has a fluctuating duration. The power needs of your car will determine its duration.
Higher kWh batteries tend to accommodate longer vehicle ranges but the two are not linearly correlated. You can see that by comparing these popular electric vehicles and their corresponding battery sizes and estimated ranges.
Vehicle efficiency, or how well it turns energy into miles, ultimately determines final range. For now, let’s save efficiency and MPGe for another day.