A watt is a unit of power that is defined as joules per second. It's basically a measurement of how much power a device is using. It's calculated by the voltage delivered to the device, times how much current the device uses: Power (watts) = Voltage (volts) * Current (amps) RMS stands for root mean square. It's used in measuring volts and watts to determine an AC (alternating current) waveform's equivalent in DC (direct current). When working with a sine wave, the RMS value is equal to the Peak value, times 0.707.