Active Solar Energy System:  A solar application that uses electrical or mechanical equipment (typically pumps and/or fans) to assist in the collection and storage of solar energy for the purpose of heating or cooling (buildings, liquids or gases) or to make electricity.

Building Integrated Photovoltaic (BIPV): A term for integration of photovoltaic cells into the building envelope, typically replacing conventional building materials.  BIPV may be used in vertical facades, skylights, and increasingly as roofing shingles.

Degree Day: Term referring to the number of units (degrees) that the average outdoor temperature falls below or exceeds a base value (usually 65 degrees F) in a day. Each degree that the mean daily temperature is above the base value is called a cooling degree day (CDD) unit.  Each degree that the mean daily temperature is below the base value is called a heating degree day (HDD) unit.  The monthly CDD and HDD values are simply the sum of the individual CDD and HDD units for the days in the month.  Degree-days are reasonably good indices of the heating and cooling requirements of buildings.

Electrical Grid: The network of privately and publicly owned transmission and distribution facilities from which most homes and businesses in the US get their electricity.

Energy Efficiency: Using less energy/electricity to perform the same function.  A device is energy efficient if it provides comparable or better quality of service while using less energy than a conventional technology.  Weatherization of buildings and Energy Star® appliances and office equipment are examples of efficient technologies.

Grid Tied PV System: A solar electric system connected to the utility ad capable of selling power back to the utility, depending on utility approval and on having equipment specifically designed for that purpose.

Insolation: Incident Solar Radiation, a term denoting the amount of solar radiation (all wavelengths – direct, diffuse and reflected) that strikes the ground.  The basic unit of measurement is the Langley, a unit of heat energy equivalent to one calorie falling on one square centimeter of surface.

Inverter: A device used to convert DC energy to AC energy  It often includes the ability to condition the power (balance the high spikes and low troughs in the AC power curve).  Critical computer network or application servers, or even personal computers, are often connected to inverters for protection from power outages, brown outs or power surges.  If the AC power fails, a DC battery can be connected to the inverter to provide energy for a short period of time.  That energy is converted to AC current and allows time to shut down the computer safely in an emergency.  The same technology on a larger scale is used to change DC power, from a PV array or battery pack, to AC power to run lights and appliances in a home.

Kilowatt-Hour (kWh):  A measure of the amount of electrical energy used or produced determined by the rate of energy consumption (production) times the amount of time over which it was consumed (produced).  This is the standard unit of billing for electrical energy.  If a 100-watt light is on for ten hours it has consumed 1000 watt hours or one kWh or energy.  In 1989, a typical California household consumed 534 kWh in an average month.

Load: Refers to any device consuming energy.  Usually expressed in units of watts or amps.

Net Metering: A method of accounting for energy produced on a customer’s site that is sold back to the local utility.  It requires utilities to give full retail credit for the electricity sold back to them.

Passive Solar Building: A carefully designed system that uses orientation to the south, proper window sizing, seasonal window shading, extra insulation and properly sized internal mass to supply most of its heating and cooling requirements.  The building’s shell collects or reflects the right amount of heat needed for the season simply because it has the proper form in relation to the sun.  Backup heating or cooling is only needed on extremely hot or cold days.  Radiant heat assures comfort and quiet for the building’s occupants. Day-lighting reduces the need for artificial light.  The building embraces the natural rhythms of the days and seasons.

Passive Solar Energy System:  A solar heating, cooling or electrical generation system that uses no external mechanical power to move the collected solar heat or electricity.  Instead it uses the natural heat transfer processes (radiation, conduction and convection) and the “greenhouse effect” to collect, distribute and store useable heat or coolness without the help of mechanical devices (pumps or fans).  These systems have few moving parts and are therefore considered “passive”.

Photovoltaic (PV):  The ability to generate a current or voltage when exposed to visible light or other electro-magnetic radiation.  “Photo” means light, “voltaic” means electricity.

Photovoltaic Array: A number of PV modules connected together.

Photovoltaic Cell:  A semiconductor that converts light directly into electricity.

R-value: A unit of thermal resistance used for comparing insulating values of different material.  It is basically a measure of the effectiveness of insulation in stopping heat flow.  The higher the R-value of a material, the greater its insulating properties and the slower the heat flow through it.  The specific value needed to insulate a home depends on climate, type of heating system and other factors.

Renewable Energy (R-E): Energy produced from virtually inexhaustible resources, resources that constantly renew themselves or that are regarded as practically inexhaustible.  The include solar, wind, geothermal, hyrdro (water power) and wood.

Super Insulation: An energy conservation strategy where a large amount of insulation is added to the shell of a building.  Combined with other efficient and solar design practices, it eliminates the need for mechanical heating and cooling, and thus pays for the extra cost of the insulation.