Regardless of how a power plant generates electricity, all plants can be compared
on the basis of the maximum amount of power they produce, how efficiently they
produce that power, and how much power they actually do produce. The maximum amount
of power that a plant produces is known as the plant's capacity and is typically
given in units of megawatts. There are in fact three types of capacity for a power
plant. One is the plant's Nameplate capacity. This is the capacity determined by
the manufacturer for the power plant's electric generator and represents the
maximum output that the generator can produce without exceeding its thermal design
limits. The second and third types of capacity for a power plant are its Net Summer
and Winter capacities. These are determined by an actual performance test under
operational conditions and indicate the maximum load the plant's generators can
support during the height of the summer and winter months. These particular
capacities are affected by the temperature of the cooling water for thermal
electric power plants, the temperature of the ambient air for plants with
combustion turbines, and the seasonal characteristics for water flow and reservoir
and storage for hydroelectric plants. The efficiency of a power plant is generally
the fraction of the energy consumed by the plant that is output as electrical
energy. This fraction is unitless and always less than one. However, the
efficiencies of thermal electric power plants and combustion turbines are also
often expressed in terms of heat rate. Heat rate is the amount of energy in the
fuel consumed by the plant over a period of one hour, divided by the amount of
electrical energy output by the plant in that hour. Note that this ratio is always
greater than one and has units of MMBtu per megawatt hour. The benefit of
expressing efficiency this way is that it provides a direct measure of how much
fuel is required to produce a megawatt hour of electricity. Returning to
efficiency, the most efficient types of power plants are generally hydroelectric
and tidal power plants, followed by thermal electric power plants, particularly
combined cycle plants. Among the least efficient are nuclear power plants and many
renewable generators including solar and geothermal plants. An important difference
with the renewable plants however is that their energy input is free. Which brings
us to the last measure of a power plant, which is how much power the plant actually
produces. This measure is known as the plant's capacity factor and is given by the
actual amount of energy the plant produced over a period of time, as typically
measured in megawatt hours, divided by the amount of energy the plant could in
theory have produced if it was run continuously over this period of time at its
Nameplate capacity, also in units of megawatt hours. So, like efficiency, capacity
factor is a ratio that is less than one and is unitless. However, do not confuse
capacity factor with efficiency. Rather than providing a measure of how well the
plant converts input energy into electricity, capacity factor measures how
continuously a plant typically runs. Factors that affect the actual run times of
plants include how often they need maintenance, how often they are called on to
actually generate electricity, and in the case of renewable generators, how often
the natural resources are actually there. Such as how often the sun shines, or how
often the wind blows. For example, while the efficiency of a nuclear power plant in
2014 averaged 30 percent, such plants had a capacity factor that year in excess of
90 percent. In other words, nuclear power plants were up and running almost all the
time. Conversely, solar PV generators in 2014 had a capacity factor of less than 30
percent. This is because PV plants only produce electricity when there is
sufficient sunshine, and they only produce their peak capacity of electricity for a
brief period in the day or even year when the sun is at its highest point in the
sky.
