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What Is Capacity Factor?

M. McGee
M. McGee

Capacity factor is a metric used to describe the output of a power generating system. This factor is essentially a ratio between the power generator’s actual output and its output at full capacity. While this factor can mean several things, the overall use is as a measurement of the reliability of the generator; the closer the ratio is to 1:1, the more stable the power system. Since ratios are often difficult to use as comparisons, the factor is often converted into a percentage — high being good and low being bad. Capacity factor is wildly variable based on the type of generation system, from natural gas at just over 10% to nuclear at over 90%.

There are several different methods of measuring the output of a power generator. In most cases, each of these figures attempts to close in on one specific area, often to the exclusion of all others. For example, the total output of a system can easily be compared to the total output of another but neither of these figures takes other factors such as input product, energy lost to processing or involved manpower into account. Capacity factor is another such measurement; it is concerned with exactly two different numbers and no others.

A Power generating system's output determines its capacity factor.
A Power generating system's output determines its capacity factor.

The first number used to determine capacity factor is the generator’s maximum power output. This the amount of energy generated by the plant when operating at full capacity. This amount is generally under the actual output capacity of the system, but maximum power output assumes that there is no adverse wear generated on any part of the system. The second value is the actual power output of the plant over a specific amount of time.

These two numbers are expressed as a ratio of actual output: potential output. In order to make the ratio more useful, it is generally converted into a percentage by dividing the actual output into the potential output. For example, if a plant had the ability to produce 100 units of power but only actually produced 60, it would be expressed as 60:100 or 60%.

Since one plant operating at 50% capacity for the entire period or a plant operating at 100% capacity for half the period would both be 50%, it is difficult to find a hard use for capacity factor. The general use is as a way of determining overall reliability in the power system. If a plant is constantly under-producing or having shutdowns, it is often a matter of strained infrastructure or equipment. This means that, should the power really be needed, the plant will likely fail to meet the needs of its users.

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    • A Power generating system's output determines its capacity factor.
      By: Gianluca Rasile
      A Power generating system's output determines its capacity factor.