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Energy consumption for cooling and average temperature

  1. Jul 29, 2014 #1
    Good Morning,

    First, let me preface my question by stating that I do not have a background in physics so my question my or may not be ignorant. I simply have a curiosity and I am having trouble finding any results.

    My question is regarding air conditioners and cooling energy usage. Given an environment where humidity was not a factor, if ambient air temperature was a constant 78 degrees 24 hours a day for 30 days, and the indoor air conditioner was temperature was set to 74, would the same energy be consumed by the air conditioner as another 30 day period with an average temperature of 78 where the temperature fluctuated from the 90's to 60's causing the unit to work harder some of the time, and not at all during other times? This may not be enough information and again I am only looking for a general direction on how to come to a conclusion that can be defensible.

  2. jcsd
  3. Jul 29, 2014 #2
    you're right that it is difficult to be exact with such variables, and some not defined, but imho we only need consider the nature of heat loss (change) between an internal and an external environment. At 90 degrees the Delta T between the internals of the house and the outside is 12 degrees. while at a constant 78 the Delta T is only 4 degrees. If you think of it like filling a tub with the drain uncorked perhaps it will make more sense since at some point the "water" introduced is equal to or less than the "water" leaving and the tub will not fill.

    The drain size is the key and in cooling that drain is Delta T and the insulation between the two.
  4. Jul 30, 2014 #3


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    Staff: Mentor

    If the temperature is in the 60s, is the unit in heating mode?

    In general, an air conditioner is more efficient at a lower temperature difference. If you are suggesting equal total amounts of cooling, the answer to your question still will depend on the shape of the efficiency curve of your AC unit. It will be close to equal though.
  5. Jul 30, 2014 #4
    I think it is being forgotten that we are not cooling (or heating) the outside temperature directly. We are cooling an internal environment that at least in the real world has insulation. If that insulation were extreme the temperature inside would rarely change at all due to outside temperatures. I have seen hi efficiency homes in which the heat from human bodies and running appliances was far more important a factor than outside temps. This is what I meant by undefined terms. The question/problem as posed cannot be answered definitively.
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