Glossary: O

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Off-line UPS

A UPS type which feeds power to the load directly from the utility and then transfers to battery power via an inverter after utility drops below a specified voltage. The delay between utility power loss and inverter startup can be long enough to disrupt the operation of some sensitive loads. Also called a standby UPS.


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

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.


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

Online UPS

A UPS in which the inverter is on during normal operating conditions supplying conditioned power to the load through an inverter or converter that constantly controls the AC output of the UPS regardless of the utility line input. In the event of a utility power failure, there is no delay or transfer time to backup power.

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 (OCV)

The voltage across the cell/block or battery terminals with no load applied. The maximum possible voltage across a PV array, module, or cell with no load.

Open Loop System

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


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


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


Any point on a wiring system where current is taken to supply electrical power for a load.

Output Enable Delay

The delay between the start of primary side switching and the start of output current walk in. This is in two parts: a fixed hardware delay controlled by the secondary side control circuits and an adjustable delay controlled by the microprocessor. During this period the output voltage is at the minimum of approximately 40 V.

Over-Voltage Shutdown (OVSD)

A protection method that will shutdown any rectifier module with an output voltage over a preset maximum value.


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.


A condition in which the load wants more from the power source (such as a UPS) than the power source has been designed to supply.


Over-Voltage Shutdown: A protection method that will shutdown any rectifier module with an output voltage over a preset maximum value.