Most portable generators use gas as a fuel to power the engine. Some use diesel, although this is rare and is usually the case with higher output generators. Propane (liquefied petroleum gas) is a popular fuel type due to its clean-burning properties. In addition to being a top choice among environmentalists, there are several other advantages to using a propane generator. You can learn more about propane generators in this article: Propane vs. Gas Generators. In terms of the cost of these generators, gas and propane generators cost about the same, while generators with diesel engines will cost more.
Some generators can run on either natural gas or propane, and these are called bi-fuel or hybrid generators. You can even get tri-fuel generators that can run on natural gas, propane, or natural gas. These generators will cost slightly more than generators designed for one fuel. While the price difference is small, the advantages of using a bi-fuel or tri-fuel generator are self-evident. You have more options, which can be of considerable value during a natural disaster when one fuel is more readily available.