What is start-up voltage?

The inverter needs a minimum voltage from the panels to power its own circuits and begin synchronising with the grid. In the morning, as sunlight increases, the string voltage rises; once it exceeds the start-up voltage, the inverter switches on and starts producing. In the late afternoon the reverse happens.

The start-up voltage is not the same as the MPPT operating range. An inverter may "wake up" at a fairly low voltage but only deliver its normal output once the voltage enters the MPPT range. Both values are listed in the datasheet.

A string's voltage is roughly the voltage of one panel multiplied by the number of panels, and it falls as the panels heat up. If a string has too few panels, the inverter will start late in the morning, shut down early in the afternoon and may disconnect in cloudy weather or on very hot days.

When designing, calculate the string voltage for both the hottest and the coldest day: enough panels to keep the voltage above start-up and within the MPPT range, but not so many that a cold winter morning pushes it above the maximum input voltage.