# Energy density vs power density

Energy density is the amount of energy in a given mass (or volume) and power density is the amount of power in a given mass. The distinction between the two is similar to the difference between Energy and power. Batteries have a higher energy density than capacitors, but a capacitor has a higher power density than a battery. This difference comes from batteries being able to store more energy, but capacitors can give off energy more quickly.

## Energy density

If a system has a high energy density then it is able to store a lot of energy in a small amount of mass. A high energy density does not necessarily mean a high power density. An object with a high energy density, but low power density can perform work for a relatively long period of time.[1] An example of this type of energy storage is a mobile phone. Its power will last most of the day, but to recharge the device, it must be connected to another power source for an hour or more.

Figure 1. This demonstrates the relationship between energy density and power density. For example, Fuel cells will have very high energy densities, with relatively low power densities.[2]

## Power density

If a system has a high power density, than it can output large amounts of energy based on its mass. For example, a tiny capacitor may have the same power output as a large battery. However, since the capacitor is so much smaller, it has a higher power density. Since they release their energy quickly, high power density systems can also recharge quickly. An example application of this type of energy storage is a camera flash. It has to be small enough to fit inside the camera (or cell phone) but have a high enough power output to light up the subject of your photo. this makes a system with a high power density ideal.

## An example

To better understand energy density, consider people lighting a fire while out camping. Evening has come, and it's time for S'mores, so it is time to build a fire. Naturally, the fire is first lit with kindling. It's high surface area-to-volume ratio means that it burns quickly—a high power density. Once the fire is going, kindling isn't a good fuel choice anymore, because it burns too fast. Now the fire burns better with logs because they have have a high energy density. A single log burns well for a long time.