GEOTHERMAL - Geothermal energy harnesses heat from the Earth's core using a pump system that extracts heat from the ground in colder weather for heating and discharges heat from the home in summer, transferring it back to the cooler ground.

SOLAR - Using panels that convert the sun's rays into energy, photovoltaic cells can produce electricity using heat energy to drive a generator. Low-temperature solar collectors use the sun's energy for hot water or space heating.

WIND - Like windmills of the past used to grind grain or pump water, modern wind turbines harness the kinetic energy of the wind to produce enough electricity for utilities when operating in wind farms. Smaller turbines are capable of producing enough electricity to power homes.

WATER - Hydropower uses flowing water to spin turbines that convert energy into electricity. Hydroelectric power accounted for more than 550,000 kilowatt-hours of electricity in Connecticut in 2003.

WOOD - Wood-burning stoves use a renewable form of energy that is capable of heating large spaces and produces less smoke pollution.

BIOENERGY - This refers to the use of any "biomass" derived from organic matter, including wood, municipal solid waste, agricultural crops and waste, aquatic plants and other waste materials, to produce electricity or to be converted into fuels, such as ethanol.

HYDROGEN - This fuel can be produced from other renewable energy sources. It is being looked at primarily to power vehicles.