Since you said it is an Onan but not the model number I can't tell you. But that's as simple as running the model number on the internet. There is also a metal tag riveted to the generator which gives all the specs.
For instance mine is an "Onan MDJE 3cr", and if you search that on the web, up pops the service manual for Onan's MDJA - MDJF and inside is the spec ratings for each. MDJE 3cr is a 7.5 KW generator.
Careful . . .I Have a Generator and I'm not afraid to use it !