Energy efficiency is an important consideration for users of all types of HVAC equipment. In order to
compare equipment and systems, there are five ways to measure the energy efficiency of HVAC units,
including air handlers, chillers, roof top units (RTUs) etc. The various methods typically apply to different
types of systems and provide HVAC professionals and customers with an understanding of how a
particular HVAC unit can be expected to perform.
The energy measures this blog will examine are:
Energy Efficiency Ratio (EER)
This is the ratio of cooling output in Btu to the energy in Wh to operate system. EER is typically
measured at fixed indoor (80˚F) and outdoor (95˚F) conditions and it is used for units 10 tons and over.
There are two formulas to determine EER:
EER = Btu of cooling output/Wh of electric input
EER = Btu/hr of cooling output/W of electric input
Here is an example of one of these formulas applied to a system:
AC unit provides 10,000 Btu/hr of cooling and consumes 1,200 watts of electricity
o EER = 10,000 Btu/hr / 1,200 W
o EER = 8.33
The Integrated Energy Efficiency Ratio (IEER) is used to measure part load values (PLV) for commercial
packaged units >65,000 Btu/hr. This measure uses a sliding ambient temperature scale to account for
the variance in loads.
Seasonal Energy Efficiency Ratio (SEER)
SEER is a measure of the total heat removed from a space in Btu during the annual cooling season
divided by electric energy consumed in watts–per-hour. SEER is designed to account for typical
variations in outdoor temperature during the year. The number is higher than EER and it is used for units
10 tons and under, so this is the rating that is typically used for residential HVAC units.
Coefficient of Performance (COP)
COP measures the heat removed from cold space or delivered to hot space and is typically used for heat
pumps. The COP formula to determine the energy required to operate a system looks like this:
- Btu/hr out / Btu/hr in = kW out / kW in
Here is an example of how to determine COP:
- A heat pump delivering 60,000 Btu/hr cooling with a total input of 9,000 W
- COP = (60,000 Btu/hr) / (9,000 W x 3.412*) = 1.95
*1 watt will produce 3.412 Btu per hour (Because units in numerator and denominator have to
be the same, we must use conversion factor of 3.412) .
kW per ton of Cooling
kW per ton of cooling is typically used with larger HVAC units, such as chillers. It expresses the ratio of
electric input in kW to tons of cooling provided. For example:
- Chiller provides 15 tons of cooling and uses 12 kW to operate at full load
- Full-load efficiency of chiller = 0.8 kW / ton
Remember that kW per ton describes full load operation of the HVAC unit, not a partial load. The
Integrated Part Load Value (IPLV) is a weighted average efficiency at part-load capacities. Most often,
IPLV is used to describe chiller performance in kW/ton at various part-load operating capacities, such as
75%, 50%, 25%. AHRI Standard 550/590 sets testing conditions that apply.
Heating Seasonal Performance Factor (HSPF)
HSPF is used to measure cold-season performance of electric heat pumps. It is the ratio of heat output in
Btu during heating season to Wh of energy input. A typical electric heat pump may have an HSPF of 8.65,
and a SEER 11.6.
There are equivalences between these performance measures, so you can compare apples to apples.
The following formulas illustrate the relations between performance measures:
- COP = EER / 3.412 Btu/Wh*
- kWin/ton = 12 / EER = 3.517 / COP
- COP = HSPF / 3.412 Btu/Wh
*1 watt will produce 3.412 Btu per hour
The Air Conditioning Heating & Refrigeration Institute (AHRI) and the International Organization for
Standardization (ISO) have each developed testing procedures for HVAC Performance measures. AHRI
procedures 210/240 measure the performance of packaged units under 65,000 Btu, while procedures
340/360 measure the performance of packaged units greater than 65,000 Btu. ISO has developed
testing standards to measure the performance of WSHPs.