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 (12,000 divided by 1,500). A unit drawing 1200 watts to produce the same amount of cooling would have an EER of 10 and would be more energy efficient.

Using this same example, you can see how efficiency can affect a system's operating economy. First, you'll need to determine the total amount of electricity—measured in kilowatt-hours (kWh)—the unit will consume over a period of time. (A kilowatt-hour is defined as 1,000 watts used for one hour. This is the measure by which your monthly utility bills are calculated.) 

To do this, let's assume you operate your 8 EER window air conditioner—drawing 1,500 watts at any given moment—for an average of 12 hours every day during the summer (1,500 watts x 12 hours). At this rate, it will use 18,000 watt-hours or 18 kWh each day, leading to a total consumption of 540 kWh over the course of a 30-day month (18 kWh x 30 days). At a summer electric rate of 8.51¢ per kWh, it would cost you about $46 to operate that window air conditioner each month (540 kWh x $0.0851)—not including fuel adjustment and state utility tax. At the same time, the 1,200-watt, 10 EER system, consuming 14.4 kWh per day and 432 kWh per month, would cost you about $37, a 20% savings over the less efficient model.

As with EER, a higher SEER reflects a more efficient cooling system.

By federal law, every split cooling system manufac-tured in or imported into the U.S. today must have a seasonal energy efficiency ratio of at least 13.0.

 COP   =        Btu of heat produced at 47°F
                Btu worth of electricity used at 47°F

For instance, let's assume a heat pump uses 4,000 watts of electricity to produce 42,000 Btu per hour (Btu/hr) of heat when it is 47° outside. To determine its COP, you would first 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—13,648 Btu/hr—into the 42,000 Btu/hr heat output. This would show your heat pump to have a 47°F COP of 3.08. This means that, 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.

The second formula is most frequently used to determine chiller efficiency. Using this calculation method, you would divide 3.516 by the number of kilowatts (kW) per ton used by the system. This formula is stated:

                  COP   =        3.516
                                     kW/ton

For example, a chiller that consumes 0.8 kW per ton of capacity would have a COP of 4.4 (3.516 divided by 0.8). On the other hand, a chiller that uses 0.5 kW per ton, would have a COP of 7 (3.516 divided by 0.5).

The higher the COP, the more efficient the system.

 HSPF = Btu of heat produced over the heating season
                watt-hours of electricity used over the season

The higher the HSPF, the more efficient the system.

Most all-electric heat pumps installed in Springfield today probably have HSPFs in the 7.0 to 8.0 range, meaning they operate with seasonal efficiencies of anywhere from 205% to 234%. (To convert the HSPF number into a percentage, divide the HSPF by 3.413, the number of Btu in one watt-hour of electricity.) That means that, for every Btu worth of energy they use over the entire heating season, these systems will put out anywhere from 2.05 to 2.34 Btu of heat. Compare this to electric furnaces, which have nominal efficiencies of 100% (for each Btu worth of electricity used, they put out 1.0 Btu of heat), or new gas furnaces, which have efficiency ratings of about 80% to 97% (for each Btu worth of gas used, they put out 0.8 to 0.97 Btu of heat).

(NOTE: When comparing energy systems that use different primary fuel sources with different costs per Btu, it is important that you understand that higher operating efficiency is not necessarily equivalent to better operating economy. Although an electric furnace might work with greater efficiency than a gas furnace, it might or might not be more economical to operate. That will depend on the prices of electricity and natural gas.)

Virtually all gas forced-air furnaces installed in this area from the 1950s through the early 1980s had AFUEs of around 65%. Today, federal law requires most gas furnaces manufactured and sold in the U.S. to have minimum AFUEs of 78%. (Mobile home furnaces and units with capacities under 45,000 Btu are permitted somewhat lower AFUEs.) Gas furnaces and boilers now on the market have AFUEs as high as 97%.