String inverters, microinverters, AC modules, and optimizers use maximum power point tracking (MPPT) to find and use the maximum power available from a PV array, since it varies with sunlight intensity, haze, clouds, sun angle, dirt and debris on the modules, module-cell temperature, and shading. Because MLPEs are paired per module, they avoid the energy penalties incurred when one or more modules in a series string are shaded.
In a string-inverter-based system, shade on one PV module can affect the current in all PV modules in that series string. Bypass diodes in the modules provide electrical detours, allowing current to flow around the string of cells containing the shaded area. But in doing so, that string of cells doesn’t contribute any power. The PV output voltage for that module can drop by 15 to 20 volts—by 25% to 35%. And, if two or three PV modules are partially shaded, voltage can decrease by 60 V or more. That can be enough to turn off some string inverters.
MLPEs all have MPPT circuits, which automatically adjust for changes in PV output current, electronically converting a higher PV voltage at a limited current to a lower voltage at a somewhat higher current. Though less voltage is provided, the current matches exactly that of the other PV modules in the array. This maximizes the available power (power = voltage × current).
Even minor shading can have a big effect on string-inverter-based systems. The MLPEs prevent shade on one module from affecting the output of the others.