The daily AC performance ratio ($PR_{AC}$) is defined in IEC 61724 as:

$PR_{AC}=\frac{Y_{fAC}}{Yr}$ ,

where:

• $Y_{fAC}$ is ths the AC system yield defined as the measured AC energy produced by the PV system in the day (kilowatt hours per day) divided by the rated power of the PV system.  The definition of this rating is not specified in IEC 61724, but is defined here as DC power rating at STC conditions (1000 W/m2, cell temperature of 25 °C, and AM1.5 spectrum).
• $Y_{r}$ is the plane-of-array insolation ($\frac{kWh}{m^{2}}$) divided by the reference irradiance (1000 W m-2).  $Y_{r}$ is in units of time.

The $PR_{AC}$ can also be evaluated at other time intervals (e.g., hourly, monthly, etc.).  One of the shortcomings of this metric is that it is sensitive to temperature variation and when plotted over a full year typically exhibits lower values in the warm parts of the year and higher values in the colder times of the year.