A watt is a measure of power. A thousand watts is a kilowatt, which is expressed as “kW.” “Power” and “energy” seem to be interchangeable, but they are not. “Power” in an electrical system is the amount of electric current that meets the demand expressed by the circuits voltage. “Energy” is the amount of fuel contained within something, or used by something over a period of time. When the term “kWh” is applied to a diesel generator it is actually expressing how much power can be supplied per hour, because it means “kilowatts per hour.” A generator has a gauge that shows its output in kWh. However, if the gauge is broken, you will have to do some calculations to work out how much power it is putting out in an hour.

Check with an engineer or the manufacturer of your generator to find out how much diesel the generator uses per kW produced. You should already have a rough idea how much fuel the generator uses because you need to keep your fuel tank stocked up to meet its demand.

Turn off the generator and fill the fuel tank up to a fixed point on the fuel gauge. For example, if you can fill the tank up so it goes up off the scale, don’t fill it. Fill up to the maximum point on the fuel gauge.

Run the generator for an hour and then turn it off. Check the gauge on the fuel tank to see how many litres it now contains. Deduct the current level from the opening level to find out how many litres of diesel the generator used over the hour.

Divide the number of litres by the figure the manufacturer gave you for how many litres of diesel the generator uses to produce one kW of power. This will tell you how many kW the generator produced in that hour and so will give you the kWh of the generator.