What is a BTU?

BTU stands for british thermal unit.  1 BTU is equal to the amount of energy required to heat 1 lb. of water by 1 degree Fahrenheit.

What is EER?

EER stands for Energy Efficiency Ratio.  The Energy efficiency ratio is the cooling output of the unit (BTU) divided by the power input to the unit in Watts (W).  An example calculation of EER would be a 12,000 BTU unit requires 1,500 Watts of power to operate.  12,000 BTU / 1,500 W =  8 EER.

An EER of 8.5 is the minimum efficiency acceptable by the national appliance standards.

For more information on EER requirements see the Energy Star Web Site.

Why select a window air conditioning unit?

If there is a small area to cool a window air conditioning unit will be more economical. The window unit will cost less up front and cost less to run than a central air system.

Why is it important to pick the proper size?

Air conditioners work by removing both heat and humidity from the air.  Selecting a window unit that is too small will cause the unit to run continually and will not properly cool the room.  Selecting a unit that is too large will be less effective than a properly sized unit.  Dehumidification of the air is necessary to achieve proper comfort cooling.  A unit that is too large will quickly cool the room and shut off.  The short cycles on and off lessen the unit's ability to effectively dehumidify the air.

Larger units cost more to run and generally turning a larger unit on and off will be less efficient than running a properly sized unit for a longer period of time.

How does a Window Air Conditioner work?

Air conditioner's remove unwanted heat and humidity from the indoors and exhaust it to the outdoors.  The window ac unit pulls the indoor air over a heat exchange that is filled with cool refrigerant.  The heat from the indoor air moves to the cool refrigerant cooling the air.  The refrigerant is then compressed and sent to another heat exchanger that has outdoor air moving across it.  The hot refrigerant from compression gives it's heat to the ourdoor air, which exhausts the heat to the outdoors.

Which unit should I pick, higher or lower EER?

This is a common dilemma for window air conditioner shoppers.  Window-ac.com will break down a sample case and show you how to analyze the effect of the EER.  Generally, the unit with the higher EER will be more expensive due to having higher efficiency components and larger heat exchanger surfaces.

Window AC unit 1:  12,000 BTU / 8.5 EER / $250                    12,000 BTU / 8.5 EER = 1,412 W

Window AC unit 2:  12,000 BTU / 10.2 EER / $3 0 0                   12,000 BTU / 10.2 EER = 1,176 W

Assume electricity at $0.10  / kWh.

Assume that the expected usage of the window air conditioning unit is 4 months of the year.  The difference in input watts to the ac units is 1412 W - 1176 W = 236 W.

4 mo. x 30 days /mo.  x 10 hr/day = 1200 hr

{(1200 hrs x 236 watts) / (1000 watts/kW)} x $0.10/kWh = $28.32

So, to make up the initial cost difference of the higher efficiency unit it would take approx. 2 years of the expected usage of the air conditioner.