ONsite is a system that provides 110 volts of power from DTAP batteries. If you are coming from the video or movie industry you are probably familiar with DTAP. If you are coming from the stills industry, DTAP is a port often used on V-Mount (and Gold mount) batteries to supply a second power port aside from the main connector. This family of batteries packs a lot of power in a small form factor. you usually use a V-Mount battery to power a big camera, a monitor, some lights and more accessories; they are mighty.