Start time based on warm standby (preheated or operated in the last 12 hours).

A regional transmission organization in the United States is an electric power transmission system operator that coordinates, controls, and monitors a multi-state electric grid.

A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid.

Refuse-derived fuel is a fuel produced from various types of waste such as municipal solid waste, industrial waste or commercial waste.

A refrigerant is a working fluid used in the refrigeration cycle of air conditioning systems and heat pumps where in most cases they undergo a repeated phase transition from a liquid to a gas and back again.

Refractory metals are a class of metals that are extraordinarily resistant to heat and wear.

A refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures.

In optics, the refractive index (also known as refraction index or index of refraction) of a material is a dimensionless number that describes how fast light travels through the material.

The refracted ray or transmitted ray corresponding to a given incident ray represents the light that is transmitted through the surface.

Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves.

The reflected ray corresponding to a given incident ray, is the ray that represents the light reflected by the surface.

The reflectance of the surface of a material is its effectiveness in reflecting radiant energy.

Refined coal is the product of the application of a coal-upgrading technology that removes moisture and certain pollutants from lower-rank coals such as sub-bituminous and lignite (brown) coals and raising their calorific values.

Reduction is the gain of electrons or a decrease in the oxidation state of an atom, an ion, or of certain atoms in a molecule (a reduction in oxidation state).

Redox (reduction–oxidation) is a type of chemical reaction in which the oxidation states of atoms are changed.

Recursion is a method of solving a problem where the solution depends on solutions to smaller instances of the same problem.

A process to ensure that a system is producing the highest performance possible.

A performance indicator or key performance indicator (KPI) is a type of performance measurement. KPI's evaluate the success of an organization or of a particular activity (such as projects, programs, products and other initiatives) in which it engages.

Per mille is an expression that means parts per thousand.

Pipes that feed water to water turbines in a dammed run-of-the-river hydroelectricity plant.