I have a 14kVA diesel generator which has run about 600 hours total at an average load of maybe 40%. During this time is has gone through about 1400litres of fuel which is about 370 US gallons. I was told when I bought this unit that it would use about 1 Imperial gallon per hour at full load so I guess my consumption about matches that.
The diesel is rated at 25 to 30 horsepower depending on the operating rpm; 14kVA is equivalent to 18.7 hp at 100% efficiency so I guess my engine is putting out around 20 hp assuming a generator efficiency of 90%.
So if you were running a 50 hp diesel at its rated output I guess it would go through about 2 gallons an hour maybe a bit more.
If you have natural gas at your building you might consider a gasoline engine with a natural gas conversionit could be less expensive in the long run. And quieter...do you know how noisy a diesel is? I do.