In regions with limited coal and/or natural gas supplies, utilities often make use of petroleum as a fuel for thermal power plants. In some regions utilities utilize fuel-switching units to provide backup in the event gas supply is unavailable or prohibitively expensive. Unlike natural gas, petroleum has the advantage that it can be stored onsite in tanks. The technology used in petroleum generation is similar to natural gas with a few changes to account for physical characteristics of the different fuel. Thus operational characteristics of petroleum units are similar to natural gas units. The major drawbacks to petroleum units are that fuel is often significantly more expensive and environmental impacts are greater than their natural gas counterparts. In fact, some areas do not permit petroleum generation due to air quality concerns.