O
Ohm (Ω)
The unit of electrical resistance in which an electromotive force of one volt maintains a current of one ampere.
Oil Equivalent Ton (OET)
Unit of big amounts of energy. For instance, OET is adopted in energetic balances or statistic estimates and it is equal to the energy produced by the burning of a ton of oil. As the calorific value of crude oil is equal to 41.860 kJ/kg, 1 OET = 41.860 -10³ kJ.
Open accumulators
Accumulators with slots for the expulsion of the gases produced during the recharge in the surrounding environment (usually, hydrogen and oxygen).
Open circuit voltage (Voc) [V]
The maximum voltage produced by an illuminated photovoltaic cell, module, or array with no load connected. This value will increase as the temperature of the PV material decreases.
Operating point
Current and voltage produced by a PV device under charge.
Operative conditions of functioning
A PV device works in “operative conditions” with a 20 °C room temperature, 800 W/m² radiation and 1 m/s wind force.
Orientation
Position of a light-catching surface with respect to the directions of cardinal points N, S, E, W; its unit is called Azimuth.
Overcharge
Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.
