Power density refers to the amount of power (energy per unit time) generated or consumed per unit area. It is commonly expressed in watts per square meter (W/m²). High power density indicates that a small area can produce or use a large amount of power, which is important in applications like batteries, engines, and solar panels. In the context of energy systems, power density helps compare the efficiency of different technologies. For example, solar panels have a lower power density compared to nuclear power plants, meaning that more land is required for solar energy to produce the same amount of power as nuclear energy.