Diesel generator consumption formula
For the best estimate of fuel consumption, you should use the power in kilowatts (kW), not apparent power in kVA. Most DGs have three-phase output for which power is equal to voltage X current X square root of 3 X power factor. Since it is difficult to measure power factor (phase difference between voltage and current) you should use a three-phase wattmeter.

The DG manufacturer may assume that the average load power factor is 0.8 or higher. You could assume that pf is 1.0 and say that power = voltage X current, but that would make your fuel consumption estimate high. You could measure the line to neutral voltage and the current for each phase and then add the three power calculations to estimate power.

Say that you have a 10 kVA generator that has a 3-phase, 400 volt output that has connection points to provide three single phase 230 volt supplies. Each supply would be rated 3.3 kVA and be capable of 3300/230 = 14.3 amps. If you connect an assortment of lighting and heating appliances that draw 8, 11 and 13 amps, you would have 8 X 230 + 11 X 230 + 13 X 230 = 7360 watts or 7.36 kW. That would be about 3/4 load, so the fuel consumption would be about 3 liters per hour.