Power density is the core measure controlling the speed and range you can get with a given vehicle. Power density controls the quantity of electricity you can store within a given space, and is measured two ways: (For more about these units see http://visforvoltage.org/book/ev-batteries/13232)
Energy Density and Power Density are two ways to measure the speed and range you can get with a given vehicle. Each vehicle has a maximum weight it can carry, and a maximum volume or size it can carry. Together those form a budget into which you fit the people, cargo, drive train, electronics, and battery pack.