BTU: British Thermal Unit. A measure of energy. Equal to the amount of energy required to raise one pound of water by one degree Fahrenheit. Every fuel has a specific BTU content. For example, #2 oil contains approximately 140,000 BTUs per gallon, one Therm of natural gas is 100,000 BTUs, one kWh of electricity is 3,412 BTUs.


SEER: Seasonal Energy Efficiency Ratio is the measure of efficiency by which the cooling process of air conditioners and heat pumps is rated. A higher SEER number equals greater energy savings. The SEER rating of a unit is the cooling output in BTUs (British thermal unit) during a typical cooling-season divided by the total electric energy input in watt-hours during the same period. It has units of BTU/watt-hr. SEER is calculated with the same indoor temperature, but over a range of outside temperatures from 65 to 104 degrees F. Today, U.S. regulatory agencies require all new products to have a 13.0 SEER rating or better. Typical Range 13-21.

EER: Energy Efficiency Ratio is the ratio of output cooling (in Btu/hr) to input electrical power (in watts) at a given operating point. EER is generally calculated using a 95F outside temp and an inside (actually return air) temp of 80F and 50% relative humidity. EER is typically used for window AC units but is also used for Central AC units. EER = COP x 3.412. Typical Range 8-15.

HSPF: Similar to SEER except for heating. Heating Seasonal Performance Factor is used to measure the efficiency of air source heat pumps. It is the ratio of BTU heat output over the heating season divided by total seasonal watt-hours of electricity used. It has units of BTU/watt-hr. The higher the HSPF rating of a unit, the more energy efficient it is.  Typical range: 7-13

COP: Ratio of useful energy output to the amount of energy input. Used as a measure of the energy efficiency of air conditioners, heat pumps, and other cooling and heating devices. COP equals heat delivered (output) in BTUs per hour divided by the heat equivalent of the electric energy input (one watt = 3.412 Btu/hour). (EER divided by 3.412). The higher the COP, the higher the efficiency of the equipment. It’s essentially tells you if you use X amount of energy in a Heat Pump you get COP * X out. Typical range for an air-source heat pump is 2-3, while a ground source heap pump will get around 4-5. For reference, electric resistance heat has a COP of 1 or about 100% efficient, what you put in you get out.

AFUE: a thermal efficiency measure of combustion equipment like furnaces, boilers, and water heaters. The AFUE differs from the true ‘thermal efficiency’ in that it is not a steady-state, peak measure of conversion efficiency, but instead attempts to represent the actual, season-long, average efficiency of that piece of equipment, including the operating transients. It is a dimensionless ratio of useful energy output to energy input, expressed as a percentage. For example, a 90% AFUE for a gas furnace means it outputs 90 BTUs of useful heating for every 100 BTUs of Natural Gas input (where the rest may be wasted heat in the exhaust). A higher AFUE means higher efficiency.