Let me start by briefly going over the following two questions: how does a generator use fuel and what's so special about diesel? Any electric generator produces electricity by rotating magnetic field in its stator. The rotational force that spins the field is produced by an engine that burns fuel. It is the main component that determines the type and basic properties of a generator.
Technically speaking, a diesel engine (DE) is a compression ignition reciprocating engine. The fuel in DE is ignited within a cylinder without a spark. Like a gasoline engine, it is an internal combustion engine that typically use a four-stroke or two-stroke cycle. The main difference is diesel fuel is injected into the combustion chamber after the air has been compressed, and the high temperature caused by the compression ignites the mixture. This is in contrast to a gasoline devices, in which the fuel is injected into the air as it is drawn into a cylinder, and ignition is initiated by a spark. Since in DE only air is compressed, and fuel is injected at the end of the combustion process, compression ratios can be much higher without threat of detonation. This greatly increases the device efficiency.

The term "diesel fuel" (DF) generally refers to any fuel for a compression ignition engine. Most commercial diesels is made from petroleum and is often called petrodiesel. There are also non-petroleum types of such fuel, such as biodiesel. Crude oil naturally contains some sulfur, a portion of which is converted to sulfate particulates in the exhaust and contribute to so-called particulate matter (PM) emission. To reduce PM and to enable some advanced emissions control devices for better air quality, many countries set the limits to sulfur content of DF. Particularly, in U.S. starting in June, 2006, EPA required at least 80% of the on-highway DF to be Ultra-Low Sulfur Diesel (also called ULSD or S15), with maximum sulfur level of 15 parts per million (ppm). Gas pump might still sold DF with 500 ppm sulfur until December 2010. By 2014 DF for all categories of equipment in US had to be ULSD. Note that in European Union and Japan ULSD means less then 10ppm sulfur.

Will your old engine work normal on ULSD? Likely it will -- it may just have a small reduction of efficiency. Conversely, newest engines (built after 2007) with advanced pollution-control technology designed for ULSD, may be damaged by fuel with high sulfur content.


DGs are widely used both as a prime energy source in places where grid is not available and as an emergency backup power source for grid connected buildings. Depending on the size, they can be used for small loads, such as electric tools, and for large loads like factories, commercial buildings, and hospitals. However, because of the high cost of standby diesel systems, they are used primarily in industrial and commercial applications. Private homes more often use natural gas or propane models that you can get for less. Diesel standby systems are widely used as an emergency backup source in hospitals, fire and police stations because of their shortest warm up time. For example, the standard NFPA 99 for health care facilities requires the genset to pick up the load and meet the requirements of the emergency system within 10 seconds after loss of main power. DG is probably the only type of gensets that can reach rated power within ten seconds. This type of devices is also used in remote locations in hybrid off-grid solar systems as an auxiliary power generator. Portable diesel generators are especially popular on jobsites because typically they have 2-3 times longer life than gasoline-powered models. High-power stationary DG may cost around $1000-$2000 per kilowatt. Mid-sized portable diesel gensets typically run for $200-500/kW. As a rule, the higher wattage the lower cost per watt.
The chart below summarizes basic advantages and disadvantages of diesel generators.



The most reliable and maintenance-free of all types of emergency backup generators: no ignition systems, carburetors, nor spark plugs (just regular oil and filter changes);

Lowest fuel consumption (typically it would burn half the amount of gallons that would be required for a same-size gasoline generator);

Can be stored in large tanks (up to 2 years with fuel preservatives), it is less flammable than other fuels and is the safest one to store.
The most expensive type;

The gas pumps may not operate and the fuel may not be available during blackouts;

Contains paraffin waxes that become solid at freezing temperatures- needs low temperature operability additives;

High emissions (such as nitrogen oxides - a smog-forming pollutant) and unpleasant odor. According to WHO, diesel fumes are carcinogenic.

In conclusion, diesel-powered gensets are more reliable and have higher efficiency than other gensets, but they are also the most expensive and the most polluting of all types. Such devices are used primarily in industrial and commercial applications.