Unique systems, methods, techniques and apparatuses of photovoltaic (PV) string power systems are disclosed. One exemplary embodiment is a PV power system comprising a plurality of PV strings and a floating DC-DC optimizer. The floating DC-DC optimizer comprises a first DC bus rail, a second DC bus rail, a plurality of input legs, each leg being coupled across the first DC bus rail and second DC bus rail, including two input leg semiconductor switches coupled at an input terminal, each input terminal being structured to receive an input current from an end of one PV string of the plurality of PV strings, and an output leg including two output leg semiconductor devices coupled at an output terminal.