Air Conditioning and Energy Efficiency

In Boston, air conditioning is needed only for a few months during the year, so homeowners may wonder whether central air conditioning is really needed? Will window units suffice? How much will central air conditioning cost to install, maintain and operate? One big part of the answer is efficiency. Highly efficient central air conditioners may be less costly to operate than fans or other cooling options.

Air conditioners come with energy efficiency ratings that can help you determine how much your air conditioning system will cost. Generally, highly efficient air conditioners are more expensive to purchase but cost less to operate. The seasonal energy efficiency rating (SEER) of an air conditioning unit tells you what you can expect from your air conditioner, and can also help you compare the effective cost of different models over time.

One important measure of an air conditioning unit is its BTU output. A BTU is a British thermal unit, and it is a measure of the air conditioner’s cooling output. Small, room-sized air conditioners have BTU ratings in the neighborhood of about 5,000. Larger room units might have a BTU rating of 8,000-15,000. To compare, a central air conditioning unit might have a BTU rating of 25,000 – 60,000.

The ratings are important because they provide an indication of the square footage a single unit can cool. They can also help you figure out which air conditioners are powerful enough to cool your home, assess different models, and calculate or compare the operational costs.

Generally, efficiency is a measure of what you get out versus what you put in. The SEER, therefore, is a comparison of how many BTU you get out of your air conditioner compared to the electricity it consumes, measured in Watt-hours (W-h). The unit of measure for a SEER is BTU/W-h. The higher the SEER the more efficient the air conditioner is, and the less it costs to operate.

If you have a 5,000 BTU window air conditioner with a SEER of 10.8, your air conditioner will use about 465 watts/hour or .465kW/hr. (5,000/10.8 = 463W or .463kW). If NSTAR charges a summer rate of $0.05/kW hour, your operating cost for the unit will be less than $0.025 per hour. If you operate the unit 8 hours per day, the cost will be about $0.19 per day. An air condition that size will cool off a bedroom and will set you back about $21 for the entire summer. If you rent, don’t spend a lot of time at home, or don’t mind the heat but just want to sleep comfortably at night, a window air conditioner will probably meet your needs nicely, even though it isn’t as efficient as some other cooling options.

Next week, I’ll compare the operating costs of low-efficiency and high efficiency central air conditioners, and you’ll see why high-efficiency central air conditioning systems are really the way to go if you want to cool the whole house. In the mean time, contact Boston Standard Plumbing & Heating anytime for all of your plumbing, heating and cooling needs. We can be reached at (617) 288-2911 anytime!