Occupancy Sensor
An optical, ultrasonic, or infrared sensor that turns room lights on when they detect a person’s presence and off after the space is vacated.

Occupied Space

The space within a building or structure that is normally occupied by people, and that may be conditioned (heated, cooled and/or ventilated).

Ocean Energy Systems

Energy conversion technologies that harness the energy in tides, waves, and thermal gradients in the oceans.

Ocean Thermal Energy Conversion (OTEC)

The process or technologies for producing energy by harnessing the temperature differences (thermal gradients) between ocean surface waters and that of ocean depths. Warm surface water is pumped through an evaporator containing a working fluid in a closed Rankine-cycle system. The vaporized fluid drives a turbine/generator. Cold water from deep below the surface is used to condense the working fluid. Open-Cycle OTEC technologies use ocean water itself as the working fluid. Closed-Cycle OTEC systems circulate a working fluid in a closed loop. A working 10 kilowatt, closed-cycle prototype was developed by the Pacific International Center for High Technology Research in Hawaii with U.s. Department of Energy funding, but was not commercialized.


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


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.

Ohm’s Law

In a given electrical circuit, the amount of current in amperes (i) is equal to the pressure in volts (V) divided by the resistance, in ohms (R).

Oil (fuel)

A product of crude oil that is used for space heating, diesel engines, and electrical generation.

One-Axis Tracking

A system capable of rotating about one axis.

One Sun

The maximum value of natural solar insolation.

On-Peak Energy

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

On-Site Generation

Generation of energy at the location where all or most of it will be used.

Open Access

The ability to send or wheel electric power to a customer over a transmission and distribution system that is not owned by the power generator (seller).

Open-Circuit Voltage

The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing.

Open-Loop Geothermal Heat Pump System

Open-loop (also known as “direct”) systems circulate water drawn from a ground or surface water source. Once the heat has been transferred into or out of the water, the water is returned to a well or surface discharge (instead of being recirculated through the system). This option is practical where there is an adequate supply of relatively clean water, and all local codes and regulations regarding groundwater discharge are met.

Operating Cycle

The processes that a work input/output system undergoes and in which the initial and final states are identical.


The alignment of a building along a given axis to face a specific geographical direction. The alignment of a solar collector, in number of degrees east or west of true south.


A discontinuance of electric power supply.


The process by which materials expel or release gasses.

Outside Air

Air that is taken from the outdoors.

Outside Coil

The heat-transfer (exchanger) component of a heat pump, located outdoors, from which heat is collected in the heating mode, or expelled in the cooling mode.


A building element that shades windows, walls, and doors from direct solar radiation and protects these elements from precipitation.


To exceed the design capacity of a device.


A device that converts heat or sunlight directly to electricity, invented by Standford Ovshinsky, that has a unique glass composition that changes from an electrically non-conducting state to a semiconducting state.


Gasoline fuel additives such as ethanol, ETBE, or MTBE that add extra oxygen to gasoline to reduce carbon monoxide pollution produced by vehicles.