Many factors influence energy efficiency, including building materials and the type of systems and appliances the home has. Efficiency ratings have been developed to assist consumers in comparing systems and prices. Below are a list of terms and ratings, along with definitions for each one.

In talking about appliances and systems, it is typical to talk about efficiency, which is defined by the amount of energy the system uses to do its work. Different systems have different values that are used to describe efficiency.

**AFUE** (annual fuel utilization efficiency)

A rating that measures the efficiency with which gas and other fossil fuel-burning furnaces and boilers use their primary fuel source over an entire heating season.

**Air infiltration**

The accidental flow of unconditioned outdoor air into a mechanically heated and/or cooled building.

**Btu** (British thermal unit)

A measurement of the energy in heat. It takes one Btu of heat to warm one pound of water by 1° Fahrenheit.

**Caulk**

Caulk is a glue-like material that is used to seal gaps to increase energy efficiency.

**Compact fluorescent lamps** (CFLs)

A CFL is a type of fluorescent alternative to incandescent bulbs that is far more energy efficient.

**Conduction**

Conduction is the transfer of heat through solid objects, such as glass, dry wall, brick, and other building materials.

**Convection**

Convection is the passing of heat to, or from, a solid surface via a gas or liquid current.

**COP** (coefficient of performance)

COP is a measurement of a heat pump’s efficiency (in the heating mode) at a specific outdoor temperature – usually 47°F. For example, a heat pump uses 4,000 watts of electricity to produce 42,000 Btu per hour (Btu/hr) of heat when it is 47°F outside. To determine the COP, you would convert the 4,000 watts of electrical consumption into its Btu/hr equivalent by multiplying 4,000 times 3.413 (the number of Btu in one watt-hour of electricity). Then you would divide your answer of 13,648 Btu/hr into 42,000 Btu/hr heat output. This would show your heat pump to have a 47°F COP of 3.08. So, for every Btu of electricity the system uses, it will produce a little more than three Btu of heat when the outdoor temperature is 47°F.

To calculate chiller efficiency, you divide 3.516 by the number of kilowatts (kW) per ton used by the system. For instance, a chiller that uses 0.8 kW per ton of capacity would have a COP of 4.4 (3.516 divided by 0.8). On the other hand, the COP of a new, more efficient chiller, using as little as 0.5 kW per ton, would be greater than 7 (3.516 divided by 0.5).

**Daylighting**

The process of using natural light to add to, or replace, a building’s artificial lighting.

**Dedicated fixture**

Lighting that has been designed for use with a special type of lamp or bulb.

**EER** (energy efficiency ratio)

EER is a measurement of the energy required by a cooling system to keep the indoor temperature at an outdoor temperature, usually 95°F. The EER value is the Btu/hr of cooling at 95°. For instance, if you have a window air conditioner that draws 1,500 watts of electricity to produce 12,000 Btu per hour of cooling when the outdoor temperature is 95°, it would have an EER of 8.0 (12,000 divided by 1,500).

**Efficiency**

Efficiency refers to the extent to which an amount of work can occur for the least expenditure of effort or fuel.

**HSPF** (heating seasonal performance factor)

HSPF is a measurement of an all-electric air-to-air heat pump’s efficiency (in the heating mode) over an entire season. HSPF is calculated by dividing the number of Btus of heating provided over the entire season by the total number of watt-hours required to operate the system over the season.

**Insulation**

Product that inhibits conductive and convective heat transfer. Some materials are naturally better insulators than others, because they contain more “dead air” pockets. These pockets of trapped gas help to slow the movement of heat. However, if processed properly, virtually any product, including glass, cotton, paper, and plastic can be used to make insulation.

**Internal heat gain
**The accumulation of heat produced by a building’s energy systems, appliances and occupants. Depending on the number of occupants and the type and number of energy systems used during the day, it’s not unusual for internal heat gain to account for 20% of a home’s total summer cooling load.

**Kilowatt** (kW)

A kilowatt is 1,000 watts.

**Kilowatt-hour** (kWh)

The kilowatt-hour is 1,000 watts used for one hour. The kilowatt-hour is the measure most U.S. electric companies base billing. Most residential customers pay 5.5¢ to 6¢ per kWh for their electricity.

**Low-e**

Low-e means low-emissivity. Low-e describes a material that reduces the amount of radiant heat that can be transferred through glass or other translucent window coverings.

**Lumen**

Lumen is a unit of light produced by a light source. It is a measurement used to compare the levels of light by various light sources.

**Payback period**

Payback period is the amount of time it takes for you to receive a return on an investment. In this case, an investment in an energy efficient system produces a return based on the cost savings over time.

**Radiation**

Radiation is method of heat transfer via infrared waves. Radiant heat warms a surface, but does not require air to transfer the heat. For example, all warm bodies radiate infrared energy.

**Return on investment** (ROI)

ROI Is the annual rate at which an investment earns income.

**R-value**

R-value is a measurement of a material’s ability to resist heat transfer. For example, insulation is rated with an R-Value. The materials with higher R-values are polyisocyanurate rigid boards. Fiberglass is a common insulator with an R-value of 3.1 to 3.7 per inch.

**SEER** (seasonal energy efficiency ratio)

SEER is a measurement of how energy efficient a central cooling system can operate over an entire cooling season. SEER is calculated based on the total amount of cooling (in Btu) the system will provide over the entire season, divided by the total number of watt-hours it will consume. Federal law requires central split cooling systems manufactured or sold in the U.S. to have a seasonal energy efficiency ratio of 10.0 or higher.

**Settled density**

Settled density is the amount, or depth, of insulation left after settling.

**Solar gain**

Solar gain is heat that builds up inside a structure after sunlight has entered and turned into heat. Solar gain can cause as much as 50% of the interior heat gain in a home.

**Thermostat setback**

A thermostat setback is a deliberate way to control building energy consumption by controlling the thermostat.

**U-value**

A U-value is the measurement of how much heat can flow through glass, brick, drywall, and other building materials.

**Vapor barrier**

A vapor barrier is a material that blocks moisture from flowing through a wall or other material. They help protect a home or other building against moisture damage.

**Watt**

A watt is a unit of electric power. The amount of power required by electric appliances is expressed in watts.

**Watt-hour**

A unit of electric energy that is equal to one watt used over a period of one hour.

**Weather stripping**

A product designed to seal the cracks that exist between two moving parts or one moving and one stationary part of windows, doors and other movable building components. Weather stripping is used to improve a building’s energy efficiency by preventing the unintentional entry of unconditioned outdoor air.