The mower you describe would likely have a "nominal" 3 AMP charging system. Subtract a bit for the carb fuel solenoid.
IF it had lights, they are likely powered by a separate AC lighting circuit. The way to tell is-
IF the lights only work when the engine is running, you have the circuit above. IF they will light when the engine is OFF, you would have a higher capacity DC system. Anywhere from 9 AMPs to 25? Unlikely though, on a smaller riding mower.

The higher capacity systems will have a Voltage Regulator. Typically 2 Yellow wires (AC) input and 1 Red wire output (DC). It's mounted to the engine.

This post was edited by bill_kapaun on Sun, Mar 31, 13 at 20:54

You are not going to get enough AC from the engine's charging system to do anything for you.

Walt Conner

Unless he riggs the PTo to generator???