Recently it was brought to my attention that I may not be as careful as I should be about the difference between kilowatts and kilowatt-hours. Yes, it’s true. Wash my mouth out with soap and slap my hands. However, our use of kilowatt-hours in the sizing and capacity of electric vehicle batteries does lead to some interesting problems.
For one, people who are used to gasoline or diesel powered vehicles are used to measuring capacity in volume – gallons, liters, barrels, hogsheads, etc. They then measure speed in time units like “miles per hour.” Herein lies the difficulty. In electric vehicles we measure the capacity of the battery in kilowatt-hours. This makes sense to engineers. Engineers build cars. The problem is, everyone else gets a little confused about why batteries are measured in hours. Then, the conversation gets even muddier when we talk about charging. Here lies the error in my ways. Let’s say you have a charger that peaks at 10kw of power. For simplicity, we’ll say it provides exactly this much power all the time no matter what. So, 10kw for 5 minutes, or 10kw for 5 hours. We obviously need a way to turn the actual amount of energy it provided into something more concrete. Enter the kilowatt-hour. A kilowatt-hour is the amount of energy transmitted or work done. It’s equal to 1 kilowatt for 1 hour continuously. So, with kilowatt-hours we can accurately measure how much energy is coming and going from a battery pack.
It is also 3.6 mega joules. Well now, that’s another term nobody outside of engineering knows. But, the joule doesn’t *seem* like it has a time factor in it. It does indeed include a time but you don’t see it. You see, a joule in turn has its own definition. A joule is 1 amp passing through a resistance of 1 ohm for one second. That’s really intuitive! But, hidden inside, a joule is based around one second which is partly why 1 kilowatt-hour is so many joules. But, since joule doesn’t say “seconds” or “per second” when written out, it can seem to the average consumer to be a meaningless unit and thus different capacities can be discussed without worrying about time. Is a battery pack 237MJ or is it 306MJ? Obviously 306MJ is better, right? Yes, it’s certainly bigger. We can completely ignore the MJ part. In fact, for 99% of everyone using the car, MJ could just as easily stand for Michael Jordans and everything would work out the same.
My error was in describing a 10kw charger as providing 10kwh of charge per hour. The engineers have probably at this point had their heads pop off and fire is belching from their necks. 10… kilowatt hours…. per… HOUR?! That’s so dumb! Yes, it’s dumb. But, your average person sees the capacity of their battery more like a unitless value. Sure, a Tesla Model S might have an 85kWh battery but no one knows what a kilowatt-hour is. Their usual exposure to this term comes with their electricity bill. And, there too, it might as well just be a unitless figure that gets multiplied by a price. And so, we could reframe my statement in mega-joules if we wanted. A charger rated at 36MJ per hour will in fact take 8.5 hours to charge a 306MJ battery. Now, the extra “hour” in the statement I made is missing. Instead we have joules which themselves are time dependent but since we don’t see that it feels better. Now the statement would be “A 36MJ charger will provide 36MJ per hour.” In fact, we could coin a new word – flergle. The flergle is exactly equal to 1 kilowatt-hour. Now a pack that is 85 flergles will take 8.5 hours to fully charge if the charger is rated at 10 flergles per hour. The actual result is exactly the same. To the average person who just wants to charge their car, this makes perfect sense and hand waves away all of the discussion as to why a battery is rated in something that includes the word “hour.”
But, the engineers of this world are right about this still. It is important that words matter. Definitions matter. Precision is not worthless, it is something to aspire to. And so, they are right that kilowatt hours per hour sounds really, really dumb. They would say it’s not a real thing. But, perhaps the basic idea behind it still illustrates a valid point. Batteries are measured in a unit called “kilowatt-hours.” Charging time is measured in minutes and/or hours. Chargers have a peak charging rate. That rate is kilowatts. The actual accumulation of those kilowatts is kilowatt-hours. I would assert that measuring batteries in kilowatt-hours is somewhat confusing to the average person. But, that’s what we’ve got. And so, it is important to remember the difference between a kilowatt and a kilowatt-hour. This is especially true if you have engineer friends. They will thank you.