A power generation plant of any kind carries a Nameplate Capacity, or a Rated Output, which represents the amount of power that it can output, while it is running, in ideal conditions, over some duration. The nameplate capacity is the number that we see in an announcement or a magazine: “New 350MW power plant announced for our area.”

But that number does not represent the amount of power actually generated by the plant, because it is not always available, or because it cannot alway generate its full rated output: to understand why, check out Availability Factor and Capacity Factor.

For most technologies, such as fossil fuel, nuclear generation, or geothermal, the nameplate capacity can be clearly established, based on the engineering of the system, even if it sometimes requires experiments to measure the actual output. For solar photovoltaics (solar PV) and wind power, the calculations are not as straightforward, and may involve decisions by the manufacturer which can skew the nameplate capacity.

Solar PV plants are made of a large number of solar panels, each of which carries a power rating based on the power generated in standard test conditions (STC). Simply adding the power ratings of all the panels in the plant does not (or should not) represent the nameplate capacity. For instance, two identical panels installed at different latitudes, or at the same latitude but with slightly different orientations will generate different amounts of power. Tracking panels, which follow the sun, clearly have much higher generation capacity than static panels. But, in the end, the nameplate capacity of most solar PV systems is captured by the sum of power ratings for all panels, giving a much higher value than what can be truly generated by the system. The value of the nameplate capacity can be compensated by the capacity factor.

Wind farms also experience difficulty in rating their nameplate capacity, since power generation depends upon wind speed. There is no standard test condition used across the industry for these ratings, which means that manufacturer-stated nameplate capacities often cannot be directly compared. In the same manner as for Solar PV, the nameplate capacity of wind farm can be compensated by its capacity factor.