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Ohm — A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.

One-Axis Tracking — A system capable of rotating about one axis.

Open-Circuit Voltage (Voc) — The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing.

Operating Point — The current and voltage that a photovoltaic module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.

Orientation — Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.

Outgas — See gassing.

Overcharge — Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.