You did not say what model Yamaha. I just did a quick search and for the model the EF2000iS- it is rated for 1600 watts (max. 2000 watts) and 13.3 amps (max. 16.7 amps) @ 120V AC.
The welding machine:
120 volts, 60 Hz, 20 amps, single phase input
Welding Output Range: 60-120 amps AC
amperage...