To understand why wattage is the best metric for charging, you only need a tiny bit of basic science. The formula for determining electrical power is incredibly simple: watts equals volts multiplied by amps (that's W = V × A for you math heads). Voltage represents the water pressure, or how hard the water is being pushed through the system. Current, which is measured in amps, represents the volume of water flowing through the pipe at any given moment. Just make sure you aren't charging your phone wrong to begin with.