I'll use English units and let you convert to SI. Most small DG sets will consume about 0.312 pounds of fuel per horsepower per hour. Diesel fuel weighs about 7.1 pounds per US gallon on average. This should get you close enough. Load puts a tremendous magnetic force inside the generator (alternator) and it takes more power to overcome this resistance to rotation. Like going up a hill in your car.
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If it eats, it's going to be trouble!