COST OF OPERATION
Natural gas and propane generators are the most expensive to operate. They also come with a very high installation cost and have over five times the maintenance requirements.
ENGINE LIFE
The life expectancy of a generator natural gas or propane engine is under 1,000 hours, while diesel is 20,000 hours or more.
FUEL SAFETY
Natural gas and propane are incredibly flammable. Both must be delivered using underground pipes.
Diesel is safer to store and is only combustible under high pressure and temperature.
SUPPLY
Natural disasters such as floods or earthquakes can disrupt the delivery of natural gas or propane. Diesel is readily available during times of emergency. Bio-diesels are also an alternative fuel that can be used. There is no alternative fuel for gas-powered generators.
ENERGY
Since natural gas, propane, and gasoline contain less energy than diesel fuel, you need a lot more of it to produce electrical power. This makes it the most expensive fuel for power generation.
Diesel fuel contains the most energy, and you use a lot less of it in comparison. The fuel savings diesel offers, quickly make up for the higher initial cost of a diesel-powered generator.
Natural gas and propane are suitable for heating and cooking but not as fuels for engines. In an engine, the majority of the fuel energy is lost as heat. Less than 30% of the energy ends up producing power to turn a generator. Diesel engines have an efficiency of up to 50%.
ENGINE TECHNOLOGY
Engine efficiency is directly related to its compression ratio. Diesel engines produce the highest compression ratio since its fuel has a high flashpoint. This means it can only be easily ignited under high temperature and pressure. The result is a very powerful engine design.
New emission regulations have resulted in even higher diesel engine efficiencies than ever before. Diesel engines today are 99% cleaner than they were just ten years ago. With the increased efficiency and removal of sulfur from it (since 2007), diesel generators now produce near-zero emissions of nitrogen oxide (NOx), hydrocarbons (unburned fuel), and Particulate Matter (PM).
It would take 60 engines today to produce the same amount of pollution one engine did a few short years ago.
NOISE
To remain small and so that they can produce enough power, gas engines usually run at very high speeds. Most turn at 3600 RPM, a speed your car does not even do on the highway. It is the main reason they are very loud and don't last long. Diesel produces less noise running at a slower 1800 RPM.
Diesel engines have also come down significantly in size. New compact designs make them a popular choice for power generation worldwide.