To determine the WATTAGE of generator you will need for your particular job, multiply the AMPS your tool will use by the VOLTAGE it requires.
15 AMPS X 120 VOLTS = 1800 WATTS
It is always a good idea to have more power than your tool needs so you would want to use at least a 2500 watt generator.
If you would like to determine the total power output in AMPS a particular generator will produce, divide the total WATTAGE of the generator by the VOLTAGE (120 or 240) you will be using.
5600 WATTS divided by 120 VOLTS = 46.66 AMPS