Watts cannot be calculated using the number of volts alone. Amperage is required, too.
To find how many watts are in 120 volts, one need to know the formula that relates power (P) in watts to volts (V) and current (I) given in amps. The formula for this relationship is that power equals volts multiplied by amps, or P = V x I.
So, if the question is how many watts are in 120 volts that delivers a current of 2 amps, then the answer is 240 watts using the formula.
Watts, amps, volts and ohms are the units utilized to measure electricity.