The performance of an illuminated PV array or system is characterized by the system’s I-V curve.  To harvest power from the system the voltage must be adjusted in order to maximize the power.  This function is called Maximum Power Point Tracking (MPPT).  MPPTY is typically performed by the inverter or a DC-DC converter.  A control system is used to find the maximum power point (MPP), which changes with varying irradiance and cell temperature.  Array utilization is a measure of how well the MPPT can remain at the MPP and conversely how much energy is lost from errors in this MPPT function.  Most models assume that the MPP can be held at all times.

As PV systems become more prevalent, utilities will begin to require inverters to operate the arrays off the MPP in order to help support grid operations.  Current performance models cannot represent performance under such conditions.