What is a kilowatt hour and why is it used when using battery capacity instead of battery weight?
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A watt is 1 amp times 1 volt. A watthour is one amp times 1 volt times one hour. This is a measurement of power over time. If your phone charges at 5V and 2A then that is 10 Watts. A 10 watthour battery can charge that phone for one hour. Weight of the battery has nothing to do with how much charge it can hold. Typically kilowatt hours are used in appliance power. The power meter on the side of your house measures your power usage in kilowatt hours.
A battery’s weight is entirely irrelevant to its capacity. A lead acid battery can weight 5x the amount of a lithium polymer but put out the same power.
A kilowatt hour is the amount of power the battery can supply measured in time.
Short answer: it’s a measurement of how much electricity the battery can produce before it’s “empty”. Weight is not really relevant. It’s like if you compare a 1 liter plastic bottle and a 1 liter glass bottle. The glass bottle will weigh much more but it will still only hold one liter of liquid.
A kilowatt hour is the amount of energy stored in a battery. We use it to measure how much energy the battery can give, instead of looking at the weight of the battery. This is because the weight of the battery doesn’t tell us how much energy it has inside.
It finally clicked for me when I thought of it like this (this may honestly confuse you more, but it at least helped me understand);
Think of Kilowatts being like “Speed”, and think of Kilowatt Hours being like Distance. To call it a “Kilowatt Hour” would be like calling distance a “Miles-Per Hour… Hour”. Like, if you were going to say you drove 30 miles, you decided to say “I drove 30 miles-per-hour hours.”
So in simplest terms, the Kilowatt Hour is the battery’s capacity (quite literally like the distance/range of an EV). And the number of Kilowatts determines how fast that capacity is depleted.
For example, an EV with a capacity of 70 kWh would get depleted after 3.5 hours at 20 kW.
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