I don't know how genset diesels compare with the big Detroits, but a Series 50 or 60 DD uses about 0.3 to 0.35 gals/hr per horsepower. I'm guessing the genset engines are in the same ballpark.
One HP is the same as 746 watts. That means if the generator is about 75% efficient, you'll need about 1-1/3 HP per KW output. If you're loaded at the full 10 KW, that's about 13-14 horsepower, and at 0.35 pounds of fuel per horsepower hour you're looking at about 4.9 lb/hour or 3/4 gallon per hour.
Diesel weighs about 7 pounds per gallon.
At 5 KW, you'd expect to burn close to 3/8 gallon per hour. Given the figures you cited, I suspect the smaller diesels drink more fuel per HP-hr. Whatever the fuel/hp-hr is, the method of calculation is still the same.
Hot dang, I think I ain't a newbie anymore!