Generators are usually chosen based on their maximum and rated power. To calculate an appliance or equipment's wattage, see below.
Watts= Volts X Amps
Amps= Watts / Volts
Another rule of thumb is that 1 HP equals 746 watts of electricity. So when you have totaled your entire watts needed, divide by 750 and this should be the approximate HP of your generator.
Please keep in mind the RPM, revolutions per minute, determine HP and most manufacturers require the generator to be run at a certain RPM. Decreasing RPM will decrease electrical output.