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RADIATION — The sun’s energy that comes to earth in the form of direct, diffuse, and reflected rays.

— The transfer of heat through electromagnetic waves, without heating the air between objects.

RADIOTELEPHONE A two-way radio system that enables use of a regular telephone but with radio instead of wires.

RATED CAPACITY The manufacturer’s specification for the amount of charge that may be stored in a battery, commonly expressed in amp-hours at a specific rate of discharge.

RATED WATT The manufacturer’s specification for power output of a generating device. In most cases, this is not the most accurate measure to look at, since it predicts output only for ideal circumstances.

RATE OF CHARGE The amount of energy per unit time that is being added to the battery. Rate of charge is commonly expressed as a ratio of the battery or cell’s rated capacity to charge duration in hours. Example: A C/20 rate on a 100 AH battery would be 5 amps, the capacity of the battery divided by 20.

REGULATOR A device that prevents overcharging of batteries by controlling the charge cycle, and usually adjustable to conform to specific battery needs. Regulators do not step the voltage down, but control the rate of charge so the battery stays at a specified voltage. Also called CHARGE CONTROLLER.

RENEWABLE ENERGY Flows of energy that are regenerative or virtually inexhaustible from natural ecological cycles. Most commonly includes solar (electric and thermal), biomass, geothermal, wind, tidal, wave, and hydro power sources.

RESISTANCE Refers to how well a material conducts a flow of electrons, measured in ohms (Ω). Resistance is the property of materials to impede a flow of electrons through the material. All materials have some resistance. Those of low resistance are known as conductors, while those of high resistance are known as insulators. The unit used to measure resistance is the Ohm (Ω).

RESISTOR A device with a known amount of resistance used in electrical circuits.

REST VOLTAGE The voltage of a fully charged cell or battery that is neither being charged or discharged.

RF (radio frequency) Any radiation of a frequency that may be received or radiated by radios. Common usage: RF interference (RFI); refers to the interference of radio frequency radiation with the operation of devices or appliances such as radios, televisions, computers, etc.

RMS Root mean square; defines a time averaged value of a varying sinusoidal parameter, such as AC voltage, amperage, or wattage. The square root of the average of the squares of a set of numbers.

ROOF MOUNT A PV or solar collector rack intended to be installed on a roof. For PVs, its elevation angle can be fixed or seasonally adjustable.

ROTOR The blades of a wind generator, shaped to spin when exposed to wind, harnessing the wind’s energy.

RUNNER The part of a hydro turbine that accepts the water and turns its energy into rotating motion.

R-VALUE The measure of a material’s resistance to heat flow. The higher the R-value, the greater its insulating capabilities.