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.