Question:
Is it better to leave the central air condition on when you're going to
be away for, say, 8 hours, or should you turn it off during that time,
turn it on when you get home, and then it runs continuously for about
1.5 hours while your house goes from 85+ degrees to about 78? Which
method conserves more energy and thus saves dollars?
Assume that in the first case that the house is already cooled to about
78 and that the air conditioner only has to kick in about once every
half hour or so for ten minutes to keep it at that temperature. Which seems
like a lot, now that I'm thinking about it. Anyway. Could I just measure
the time it runs during the 8 hour period & compare that to the 1.5 hours
it runs continuously, thus assuming whichever is longer is worse?
Answer:
Pretty much, yes. The power consumption will vary somewhat with the
load on the compressor, which in turn varies with the outside temperature;
hotter = higher condenser pressure = higher compressor load = more power.
However, you can probably ignore this.
If your A/C is running 10 minutes every half hour, that's 1/3 of the
time. Over 8 hours, it is 2 hours 40 minutes. This is 1 hour 10
minutes more than the 90 minutes you say it needs to run when you
leave it off while you're away, so you're saving about 44%.
The warmer your house is, the less heat leaks in from the outside.
Letting it warm up reduces its heat gain and cuts the amount of
heat you eventually have to pump back outside. Until you get home,
you probably don't care what temperature it is inside.
