From first principle, the diesel engine can run very hard for a short period of time. This is called the STANDBY time. The PRIME POWER is when the engine runs at load factor not exceeding 80% power for any 24 hour period. Then there is the continuous rating which is BASE LOAD. This causes problems when you order a STANDBY generator at say Engine Gross Power of 1437kWm Net Engine power of 1395kWm which gives 1325kWe. Where m = mechanical and e = electrical. For the same set up, PRIME POWER Gross = 1309 kWm, Net = 1267kWm and 1204kWe. Finally BASE LOAD Gross 1047KwM Net 1005kWm and 955.2kWe. You think you are buying a 1325kW unit and are actually buying 955kW.
The Torque is based on speed. As the speed is fixed to give 50Hz the torque is now only determined by the electrical load. Torque = 9.55 x P x 1000 / n in RPM so if your maximum power is 10kW your torque is 63Nm. So the torque will not give you power or control the speed.
Effect of speed variation on power output. The two are not related directly. The governor controls the speed. The aim is to keep the speed constant. An increase in the demand of electrical energy transfers to a demand in mechanical energy on the shaft. AS the energy supply to the engine is fixed by the governor being in a set position, the shaft slows down. If the shaft slows due to the power demand, the governor opens to raise the speed to set point again. The reverse happens for a reduction in electrical power demand. The power demands on the electrical side controls everything. If the power demand rises, the current increases and the voltage reduces. The AVR (Automatic Voltage Regulator) will then inject more current into the rotor and the voltage rises again to the nominal voltage.