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OFF-PEAK The period of low energy demand, as opposed to maximum or peak demand.

ON-PEAK Energy supplied during periods of relatively high system demands as specified by the utility.

OHM (Ω) The unit that quantifies a material's resistance to electron flow.

OHM’S LAW Basic formula defining the relationship between voltage, amperage, and resistance. Commonly stated as E = I x R, or Voltage = Amperage x Resistance.

OPEN CIRCUIT When an electrical circuit is interrupted by breaking the path at one or more points, stopping the electrons from flowing. A light switch opens an electrical circuit when it turns off the light.

OPEN CIRCUIT VOLTAGE (VOC) The maximum possible voltage across a PV array, module, or cell. The voltage across the terminals of a photovoltaic cell, module, or array with no load applied when the cell is exposed to standard insulation conditions, measured with a voltmeter.

OPEN LOOP SYSTEM A fresh water or "direct" solar hot water system, generally for use in freeze-free climates.

ORIENTATION Placement according to the directions N, S, E, or W.

OUTGASSING The emission of gasses by a material. See also GASSING.

OVERCURRENT Current that exceeds the rated current of the equipment or the ampacity of a conductor, resulting from overload, short circuit, or ground fault.

OVERCURRENT DEVICE A safety fuse or breaker designed to open a circuit when an over current occurs.