Although it is common to use the terms power and energy interchangeably, they are not the same. Energy is the amount of work that can be done, while power is the rate at which energy is used to do that work. In other words, power is the amount of energy used per unit of time. The two are related according to the following formula:
Power * Time = Energy
As can be seen in the chart below, ultracapacitors, traditional capacitors, and batteries fall in different places on the scale of power versus energy density. Power density refers to how quickly a device can discharge its energy, while energy density refers to how much energy a device contains. Batteries have low power density but high energy density, meaning they have a lot of energy that gets discharged slowly. Traditional capacitors, on the other hand, have high power density but low energy density, meaning they have less energy but it can be discharged more quickly. Ultracapacitors fall somewhere in the middle, with high power density and medium energy density.
Power Density vs. Energy Density