That depends on where you bought your generating set and where you live. Your location determines the frequency and the voltage of the mains.

1. 110-120 volt, with a frequency of 60 hertz (North and Central America, West Japan)
2. 220-240 volt, with a frequency of 50 hertz (rest of the world, apart from exceptions)
3. 110-120 volt, with a frequency of 50 hertz (Tokyo, for instance)
4. 220-240 volt, with a frequency of 60 hertz (Manila, for instance)

Exception
You can find (construction) machines in the United Kingdom with 115V, these machines have yellow plugs.
 
You only have to make sure that you use the right plug once you know the right voltage and frequency.