Estimate how long a generator runs on a full tank at a given load.
Run time is tank capacity divided by burn rate, and burn rate depends on load in a way that is not proportional. A generator consumes fuel roughly in line with the power it delivers — about 0.27 litres of diesel or 0.4 litres of petrol per kilowatt-hour at full load — but engine efficiency falls away at part load because friction and pumping losses are nearly fixed. A set running at a quarter load therefore burns considerably more per kilowatt-hour than one at three-quarters, which is why the received wisdom is to size a generator so it sits between fifty and seventy-five percent loaded. Running one nearly empty is wasteful; running one at full rated output continuously is hard on it, since most portable sets rate that figure for surge rather than for hours.
Fuel burn and run time
Output kW = rated kW x load / 100; part-load penalty = 1 / (0.55 + 0.45 x load fraction); litres per hour = output kW x specific consumption x penalty; run time = tank / litres per hour
Generators produce carbon monoxide, which is odourless and lethal. Never run one indoors, in a garage, or near windows and air intakes, and never refuel a hot or running engine. Follow the manufacturer's instructions and local electrical regulations for any connection to building wiring.
Because much of the engine's consumption goes on overcoming its own friction and pumping losses, which barely change with load. At twenty percent load a set can burn nearly twice as much fuel per kilowatt-hour as at seventy-five.
Between fifty and seventy-five percent of rated output. That is where specific fuel consumption is lowest, the engine runs at a healthy temperature, and there is still headroom for a motor starting up.
Because propane has less energy per litre than liquid petrol or diesel. It burns cleaner and stores indefinitely, which is why standby installations favour it despite needing a larger volume.