Energy density vs power density

This is closely related to energy vs power.

Energy density and power density are similar to energy and power, except that their values relate to volume. Gasoline has a higher energy density than wood, and a capacitor has a higher power density than a battery.

Energy density

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If a system has a high energy density then it is able to store a lot of energy in a small amount of volume. 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. 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.

Power density

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If a system has a high power density, than it can output large amounts of energy based on its volume. For example, a tiny capacitor may have the same power output as a large battery. Because 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, let's imagine we're out camping. Evening has come, and it's time for S'mores, so we need a fire. What do you use to start your fire? Naturally, you use kindling. It's high surface area-to-volume ratio means that it burns quickly - a high power density. Thing is, once your fire is going, you don't want to use kindling anymore, because it burns too fast. You switch to logs because they have have a high energy density. A single log that you place in the fire burns well for a long time.