In terms of electricity, the marginal cost is usually directly related to the cost of fuel and the efficiency of the power plant used to produce the next kilowatt‐ hour. The marginal cost may also include a very tiny increase in costs associated with maintenance of the power plant. Renewable energy resources which may have higher percentages of capacity value include geothermal plants, biomass plants, landfill gas generation facilities, hydroelectric plants, and even solar energy projects. Another factor affecting the economic competitiveness of various generation technologies is the expected useful life of the power plant. Ad valorem taxes, or property taxes, can be a significant component of operating expenses for any electric generation plant. Federal, state, and local governmental entities may offer incentives to encourage construction of some types of electric generation and may require quantification of environmental externalities in order to discourage construction of other types of generation.