The objective of this article is to explain the differences between power and energy and explain why these concepts are important for electric vehicles.

For an introduction to Amps, Volts, watts and watt-hours see this wiki article.

Power
If you are familiar with cars, you are probably familiar with the concept of power. Power is what gives the car acceleration and what maintains a car at a given speed. Mechanically power is the product of torque and rpm, so while torque can be multiplied through gearing (by lowering rpm), power is independent of gearing

Now on the electrical side of things power it is important to know that Voltage times Amps = Watts. This is only a measure of instant power, this is the amount of power that flows though an electrical circuit, this does not tell us how long this power can be used. Power is measured in metric units as watts (W) or more commonly kilowatts (1kW=1000W) or horsepower in the imperial units (1hp = 746W). Technically there is no difference between power produced by an ICE or an electric motor, though because the torque curves are so different just comparing the peak power of the two motors isn't sufficient. The peak power of an EV is generally decided by the controller, as long as the batteries and motor can handle that power. For example a Zilla LV 1k can produce 1000 amps at 156V, which is 156kW or ~210hp.

Energy
Energy is what gives an electric car range. It is the product of power and time (E = P x t) or the product of voltage and amp-hours. Energy is stored in the batteries as chemical energy and is converted to kinetic energy by the motor to make the car move. The measurement used for measuring total electrical energy in a battery pack is called Watt-Hours (Wh) or sometimes Kilowatt-Hours (1kWh=1000Wh). Watt-Hours is a measurement we can use to figure out how much energy we have, i.e. how long can we sustain a given level of power.

If a vehicle takes 10kW to travel 55mph, and the owner wishes to do so for two hours then the energy required to do so is power x time or 10 x 2 = 20kWh. So the owner would need to size the battery pack to contain 20kWh in order to travel that distance.

For more information on how to use these concepts to size your battery pack see this article.