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One Door Destroys the Benefits from Hundreds of CFL Light Bulbs

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One Open Door Cancels Benefits of 72 Homes Using CFL Light Bulbs

The energy wasted by a set of open double doors cancels out all the benefits of 72 homes upgrading to energy efficient CFL light bulbs. Here are my assumptions and calculations. How do you calculate the energy used by an air conditioner to cool an outdoor space? It is not easy. There is no generally accepted formula that I know of. Short of doing a controlled experiment, it is difficult to make an accurate estimate.

Vendors of air conditioners provide calculations to help you size units for your room. But none of these calculations support outdoor applications. Apparently, air conditioners are not designed to cool an outdoor area. Imagine my surprise?!

The closest I found was a calculation provided by a vendor of heaters*1 for outdoor applications, such as for heating a construction site. So I assume that the energy used to cool an outdoor area is the same as the energy used to heat the same outdoor area by the same temperature.


Energy required to heat an outdoor area*1:
Temperature increase: 5.6C (10F)
Area: 12 x 8 ft.
Height: 8 ft.
Result: 24,000 BTU/hr = 7,031 Watts

Therefore, it takes 7,031 Watts to cool an outdoor area of 96 square feet.


Reality Check

To see if the above result is reasonable, we can use an air conditioner vendor's calculation for indoor cooling. The 7,031 Watts calculated above for cooling an outdoor area should be larger than the calculated results for cooling an indoor area. And it should be larger by a reasonable amount, not by a factor of 100 for example.

We will use the calculation provided by The Computer Support Group *2 to calculate the size of air conditioner required to cool a room of particular size.

Energy to cool an indoor area*2:
Area: 100 square ft.
Room insulation: None
Sun exposure: Yes
Kitchen: No
Result: 8,250 BTU/hr = 2,417 Watts

Since the estimate for cooling the outdoor space is roughly 3 times greater than the estimate for cooling an indoor space of equal size, I conclude that 7,031 Watts is a reasonable estimate for energy required to cool a 12 ft. by 8 ft. outdoor area.


How Many CFL Light bulbs?

Next, how to relate this to CFL light bulbs?

For a 100 Watt bulb, the CFL equivalent uses only 27 Watts*3. Thus 73 watts are saved for every CFL bulb used in place of the old style 100 Watt bulbs.


So, our outdoor area which takes 7,031 Watts to cool, consumes the same energy as saved by

7,031 Watts consumed/ 74 Watts saved per CFL = 96 CFLs.


Next, although the air conditioning in retail stores are used throughout business hours, the bulbs in a residence are used only during the evening hours. I assume air conditioners are used 12 hours a day (9am-9pm) and residential light bulbs are used only 4 hours a day (7pm-11pm). That's a factor of 3.

So, the energy consumed by air conditioners is equivalent to the energy saved by 96 CFLs * 3 = 288 residential CFLs

Next, how many bulbs in a household? Although the typical household has many bulbs, they are not all used all the time. I assume the typical household has the equivalent of 4 bulbs that are used all evening.

So, the energy used by air conditioners to cool an outdoor area of 12 ft. by 8 ft. is equivalent to the energy saved by 288 residential CFLs / 4 residential CFLs per household = 72 homes.


References

1. http://www.heatershop.com/btu_calculator.htm
2. http://www.csgnetwork.com/acroomsizecalc.html
3. http://en.wikipedia.org/wiki/Compact_fluorescent_lamp





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