Generator petrol budgeting
Engineering in Emergencies' chapter on generator sizing explains clearly that a generator is rated for a peak apparent power output(kVA) and so power factor of loads must be considered when sizing.
The book also provides a ballpark figure for fuel consumption - 0.3 litres per kWh.
Is a generator's fuel consumption dependent only/primarily on real power out? So though we may have to buy a 10kVA generator to run 5kW at a power factor of 0.5, such a set up would (fundamentally) be expected to use more or less the same amount of petrol as a 5kW load at a power factor of 1?
And does 0.3 litres per kWh seem reasonable for today's generators? A reasonable figure for both petrol and diesel?
The fuel consumption will be a function of the actual power demand of the generator, and various machine efficiencies. Hence, following start up the fuel consumption will be proportional to the load.
Actual fuel consumption will vary between manufacturers and models, and manufacturers will publish fuel consumption under various conditions for each model in a brochure, which you should consult as part of the decision making process.
Diesel engines are inherently more efficient than petrol, so there will be differences in consumption between the two types, and I cannot give a single generalised figure.
You should try to compare brochures from a couple of manufacturers. Try comparing Cat, Lister Petter and Lombardini. You also need to consider the need for de-rating the engine based on installation and location. Again information on this topic will be presented in brochures.
Diesels will generally give better fuel economy than petrol (gasoline) units.
Fuel consumption varies PRIMARILY by kW - real power.
Another big issue is percentage of unit rating. Typical published fuel consumption is at full or near full loading, Very lightly loaded engines will have much poorer fuel consumption per kWHr as mechanical losses remain about the same, since RPM doesn't change with loading %. So fuel consumption isn't fully proportional to loading.
Generators/Alternator ratings of kVA is based on amp current limits in windings (keeping temperature within limits of insulation). But kW rating is dependent on available horsepower of prime mover. Output frequency is dependent on RPM of prime mover and number of poles in the alternator design.
Generally generators are rated at 0.8 power factor, but within their kW limits can safely power even at unity power factor, as winding current is lower at unity power factor. So overloading on kVA will show as alternator section too hot and shorten winding life. Overloading on kW will tend to drag down engine RPM, this can happen due to high altitude or high ambient temperature giving a reduction in available horsepower (clogged air filters also!).
I've got a document of items to consider when using generator as a prime power source.
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