The present invention relates generally to photovoltaic systems. The present invention relates more particularly to photovoltaic roofing systems in which photovoltaic elements are disposed upon roofing elements such as shingles, tiles, shakes or slates. For example, in one embodiment, the present invention provides a photovoltaic roofing system for use on a roof deck, the photovoltaic system comprising: a first plurality of horizontally-arranged roofing elements disposed on the roof deck, each having a headlap region and a butt region having a bottom end; a bus bar batten system extending across the headlap regions of the first plurality of roofing elements, the bus bar batten system comprising one or more rigid supports and an electrical interconnection system; a first plurality of photovoltaic elements, each disposed on a butt region of one of the first plurality of roofing elements, each operatively engaged with the electrical interconnection system of the bus bar batten system; and a second plurality of horizontally-arranged roofing elements disposed on the roof deck and covering the bus bar batten system and the headlap regions of the first plurality of horizontally-arranged roofing elements.