The end goal of all renewable technologies is the generation of electricity. Electricity is a secondary form of energy—an energy carrier, in contrast to primary sources like natural gas and solar—and is neither renewable nor nonrenewable. From its generation at power plants, electricity is distributed to residences and commercial venues by a web of transmission lines known as the grid.
Photo: Union of Concerned Scientists, Inc
The size of a renewable power plant is often reported in terms of its electrical generation, so electricity is an important embarking point to discuss renewables. Most frequently, power plants are described by their electrical capacity, typically in megawatts or kilowatts. These are units of electrical power, or electrical energy per unit time. Household electric bills delineate usage in terms of kilowatt-hours, which is the standard unit of energy.
Kilowatts, megawatts, and gigawatts differ in scale by a factor of a thousand, and each connotes a scale relevant to a particular level of generation. Projects on the order of kilowatts are small-scale, typically residential, or for local commercial or municipal use. Megawatt-scale projects are medium- to large-scale power plants that generate for broad (utility-scale) public consumption. Gigawatts may be very large-scale power plants or the size that may be consumed by entire states. Rhode Island power plants and renewable energy facilities generate 2,000 megawatts, or 2 gigawatts, of electrical capacity.
An important consideration when discussing power plant generation is the nameplate capacity and capacity factor of the plant. Nameplate capacity, typically expressed in megawatts, is the maximum rated output of a generator. The capacity factor denotes the actual proportion of power that is generated. This is noteworthy because different types of energy have different capacity factors which may be hidden if a plant is only described by its nameplate capacity. For instance, nuclear has a typical capacity factor around 90%, coal and natural gas have capacity factors around 50%, and wind and solar have capacity factors between 20% and 35%. This implies a twofold difference in actual generation between a nuclear facility and coal or natural gas facility, and a nearly fourfold difference between nuclear and wind or solar, even if all plants were reported as having the same nameplate capacity. This distinction is important when comparing the contributions of energy sources to actual consumption.