Energy vs. power (rant!)

This is just because every other YouTube video I see about electric vehicles seems to get these two confused, so here goes…

(TL;DR: kWh = energy – either carryied with you (in the case of battery capacity) or used during a journey. kW = power – the rate at which you can convert energy from one form to another (i.e. the maximum rated power of your motor))

Energy (for EV purposes) is measured in kilowatt-hours. It is the amount of energy you have consumed whilst driving, or the total amount of potential “go” (or work you are able to do) that you can store in a battery, hence the “fuel gauge” paradigm within many electric cars.

You can directly convert this energy value in to other units, for example one kilowatt-hour equates to approximately 860 000 calories. Just as food provides you with energy usually measured in calories, charging an electric car provides it with energy measured in kilowatt-hours, and hence the reason your electricity bill is also metered as such.

Power is the rate at which work is done, i.e the rate at which energy is “consumed”. More correctly, it is the rate at which energy is converted from one form to another as energy can neither be created nor destroyed. In the Nissan Leaf for example, its electric motor is rated at 80 kilowatts. If you ran it flat out for an hour, it could convert 80 kilowatt-hours of electrical energy into kinetic (movement) energy, along with some heat as a byproduct of doing so. Obviously you’d have a problem there as you only have 30 kilowatt-hours of energy with you in the battery of a new Nissan Leaf, so hypothetically you could run it flat out for twenty-two-and-a-half minutes, by which time you’d have a flat battery and in all likelihood a speeding ticket.

If you went down a hill and used regenerative braking in order to convert your vehicle’s kinetic energy back into electrical energy at a rate of 20 kilowatts for a minute, you would put one-sixtieth of an hour times twenty kilowatts (1/60) * 20 = one third of a kilowatt-hour back in to the battery pack.

All of this is very simplistic as it doesn’t account for the fact that energy conversions are not 100% efficient (although they are way more efficient than an ICE vehicle turning chemical energy into kinetic energy!), however just remember that the amount of energy you can carry around with you, or the amount you have used is measured in kilowatt-hours (kWh), whereas the rate at which that energy is converted to do something useful is measured in kilowatts (kW).

Rant over 🙂

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