Geothermal energy systems, not to be confused with ground-source heat pumps (see below), produce energy in regions with specific geological conditions where heat escapes from the earth’s core at temperatures high enough to create steam that can be used to power a turbine. (Such conditions are uncommon in the eastern United States.)

The Gravity-Film Heat Exchanger (GFX) is a technology that uses the heat from hot water going down drains to preheat water in a hot water tank.

Graywater is untreated household wastewater (see below) that has not come into contact with toilet waste, including water from sinks, showers, dishwashers, and washing machines. Some local codes may count dishwasher and kitchen sink wastewater as blackwater.

Green is an adjective now commonly used to describe something or someone that is environmentally conscious or friendly, sustainable, or has positive environmental attributes, effects, or objectives.

Green power is a synonym for renewable energy or renewable energy credits.

Green –e is a program established by the Center of Resource Solutions to both promote green electricity products and provide consumers with a rigorous and nationally recognized methods to identify those products.

Greenwashing is the unjustified appropriation of environmental virtue by a company, an industry, a government, a politician or even a non-government organization to create a pro-environmental image, sell a product or a policy, or to try and rehabilitate their standing with the public and decision makers after being embroiled in controversy.

Ground-source heat pumps (GSHP), often mistakenly called geothermal heat pumps, should not to be confused with geothermal energy systems (see above). A GSHP circulates liquid through an underground loop, bringing warmer or cooler temperatures to a building depending on the season. Because it uses the constant temperature (45-50 degrees Fahrenheit) of the earth at 5-6 feet below ground, heating and cooling costs are reduced. A GSHP uses electricity for the pump, but the system is 300-400% efficient, and the payback period is generally 3-5 years. These can be installed anywhere.

Gypsum is a mineral used in drywall. Mining of gypsum disrupts habitats, uses energy and causes pollution. Synthetic gypsum is made with fly ash (see above), a byproduct of manufacturing and energy generating processes, which reduces the need for mined material. Recycled gypsum drywall is made with reclaimed drywall and waste from drywall manufacturing.