Gasoline generators are generally the most common and can be very inexpensive. Many however, are not of the highest quality and may not provide a very long service life. When buying a gasoline generator for all but the most temporary aplications, try to select one with a pressurized lubrication system and an overhead valve train. These models will last longer and be more fuel efficient than their lawnmower engine type counterparts. Some of the high quality gasoline powered units will provide 1000 hours or more of dependable service.
Natural gas or LP powered generators typically last longer than their gasoline powered counterparts. Several high quality, low rpm generators are on the market that should provide several thousand hours of operation without major repairs. A primary consideration for this type of generator (or any generator) is the economical availability of fuel. In most parts of the country natural gas is cheap and readily available, but in some areas this is not the case. For example, in most of Alaska operating this type of generator could prove twice as costly as operating a similar gasoline fueled powerplant.
Diesel generators are arguably the most expensive and noisy of the lot, but the high quality low rpm models offer unparalelled longevity and fuel economy. Initial cost is typically higher than gasoline or natural gas units, but because of their longevity the investment is typically worthwhile. Special care should be taken with diesel gensets to operate them near 70% of their capacity most of the time to achieve the longest possible service life. Quality diesel gensets can provide power over a service life of 10,000 + hours.