The ratio between the actual and the theoretically achievable annual output of power plant. The theoretical output would occur if the power plant ran continuously throughout the year under ideal operating conditions. However, because the power plant has shutdowns for periodic maintenance or minor repairs, or the external conditions for its operation are not ideal, the power output over the year is less than the theoretically achievable. Renewable energy sources such as wind or photovoltaics have a more significant difference between theoretical and real output because they are weather dependent. For example, the annual coefficient of utilization for wind turbines is usually about 20%.