1.Volts is the unit of measurement for potential difference while Watts is a unit of measurement for power.
2.You can get wattage by multiplying the voltage with the maximum current the power source can deliver.
3.It is more difficult to get a watt reading than a voltage reading.
4.Volts only describes a portion of the power source while Watts can give you a more realistic measurement.
5.Power supplies and batteries often show how many volts it provides but not how many watts.