My friend and I are having a debate on an age old summer question. Is it
more efficient to leave the A/C on all day and keep my apartment cold, or
turn it up and cool my hot apartment when I come home?
My belief is that the apartment gets hot due to the greenhouse effect and
the A/C should be on all day, so it cools intermittendly.
His belief is that it's wasting energy to cool all day.
What is the answer to this question?
Frankly it is not all that simple. In reality it can be cheaper to
leave it on all day. It may be cheaper to turn it down during the day.
(turning it off and opening the windows is a killer unless you have very low
Ignoring the fact that it is more expensive to cool (remove the same
units of heat) when the temperature difference is high (Hot out side) than
when it is cooler outside, then it would always be cheaper turn it off or
down anytime possible as that would reduce the total number of heat units
(BTUs) that it would need to move.
However most people would want cool it down in the afternoon when it is
hot outside, it may be a wash or it may cost more or less depending on the
temperature difference and the efficiency of the unit.
The other factor is a practical one. If your unit is properly sized, it
will not have enough extra capacity to cool you home down quickly after
being off most of the day. If it were larger, then it would be oversized
and it would not be able to control the humidity no matter when you turned
it on or off. (Note: in some parts of the country where it is very hot and
dry, this would not be true so a larger unit might be in order IF it did not
loose all the gains from the temperature/efficiency difference.
In short there is no simple answer. You are both right. ;-)
Do you have access to your electric meter? If so why not do a test on
similar days? One day do it one way the next the other go out read the
meter at say 8 pm and compare results, then you have the proof you need
to win the debate or the proof you need to stop debating!!
Post the results here too...
My supplier, Reliant Energy, in my city does not vary the rates
depending on the time of day. Therefore it is always a tradeoff of
comfort versus cost.
So in my case and probably yours, it is ALWAYS cheaper to set back or
turn off the AC. The argument that it takes longer to bring the
house back down to set temperature and thus it is cheaper just doesn't
But a properly designed AC will take several hours to bring the
temperature back down to set level. That's the comfort factor
mentioned in the tradeoff mentioned in the first paragraph. Mine
takes about 3 hours to bring inside temp down to 78 from 85 on a 95
On Wed, 04 Aug 2004 16:22:12 -0400, Vito Cavallo
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.