Glossary of Terms

Get to know these helpful heating oil terms and definitions

Annual Fuel Efficiency (AFUE)

Annual fuel utilization efficiency tells you how efficient your boiler or furnace is at converting energy to heat, including the amount of fossil fuel energy consumed per year. The higher the number, generally the more efficient the equipment is. Talk to a reputable installer and fuel dealer and ask for examples of real-world results from customers. Depending on the equipment you are replacing your savings could be dramatic.

British thermal units (Btu’s)

Technically, a Btu is a unit of energy equal to the energy it takes to heat (or cool) one pound of water by 1°F. Btu’s are used to show the value of heating fuels. Fuels with higher Btu’s heat space more quickly.

Coefficient of Performance (COP)

A measurement that shows the efficiency of some types of heat pumps. Readings range from 0 to 4. A COP of 2 means that 1 unit of energy input yields 2 units of energy output.


The flow of heat through solids


The flow of heat through air

Energy Star 

A U.S. Environmental Protection Agency program that promotes energy-efficiency to help consumers save money and to safeguard the environment. The EPA certifies Energy Star products to help consumers make energy-efficiency-conscious purchases.

Fossil fuels 

Finite resources made from decomposed carbon-based plant and animal matter, buried by layers of earth, and formed over millions of years. The three major types of fossil fuels are coal, oil and natural gas.


Stands for heating, ventilation and air conditioning

Maine Public Utilities Commission 

This state agency regulates the monopoly utilities: electricity, natural gas, telecommunications, water and more in Maine.

Natural gas 

A highly flammable fossil fuel comprised almost exclusively from methane

Heating oil

This fossil fuel is a refined form of liquid petroleum. Today’s ultra-low-sulfur heating oil and Bioheat® fuel produce minimal carbon emissions.

Point-of-use heaters 

Another name for space heaters, point-of-use heaters add heat to a certain room or area. Examples include wood stoves, pellet stoves, portable wall heaters and fireplaces.

Programmable thermostat 

A programmable thermostat allows you to adjust your home’s temperature according to your schedule, which is a fantastic way to save both money and energy.


Sometimes called liquefied petroleum gas (LPG) propane is usually compressed and stored in its liquid form. In its natural state it is odorless, but an odor similar to rotten eggs is added to it so people can quickly identify it. It is used for heating homes, powering appliances and as a vehicular fuel, in which case it’s called propane autogas.


Scientists use this rating to identify the effectiveness of insulation. The “R” stands for resistance to the flow of heat through the air—convection—or through solids—conduction. The higher the R-Factor, the slower heat travels through it.

Seasonal Energy Efficiency Ratio (SEER) 

A measurement applied to heat pumps.

Are there more terms you’re curious about? Take a look around Maine Energy Facts or send a message to the Maine Energy Marketers Association (MEMA) here.