Seems we have always been told that, if we want to save money, we should set our thermostat up when we aren't home to save money during the summer. This concept also applies in the winter, but I'm focusing on summer for this discussion.
I was thinking about this. If I keep the thermostat set, say 76F all day, the air conditioning cycles on and off all day, obviously using energy. If I set it higher, like 80F while I'm gone, the AC stays off most the day as the house slowly warms up. Problem is, when I get home, and set the air back to 76F, the AC runs, and runs, and runs; trying to cool the house while it is 100F outside.
So, the obvious question is, do I save enough electricity during the day to make up for the long run-time after I get home. Or, is it actually better to just hold 76F all day?
How the experiment will work: I picked two days, which have basically the same weather forecast. 6/6/2011 and 6/7/2011. Lows in the upper 70sF and highs near 100F. Yes, it is early in the season for us to have temps this high, but that shouldn't impact the data. Both days have light winds and clear skies. So, they are pretty close for comparison purposes.
On 6/6/2011, I set my thermostat for 76F in the am and left it there all day and into the evening. For 6/7/2011, I set the house to 80F starting at 7am. I do keep my house at 76 when I'm home. It is a touch warm, but trying to save $$. I figure 80F is a good 'away' temp. We have pets in the house and if I let it go to 85 or 90F, the AC would obviously have a harder time to cool to 76F when I got home.
To read the elecritity usage, I'm using my smartmeter reports to see the hourly electrical usage. Some of you are thinking that this shows whole- house electrical costs, not just the AC. That's true. I have set the lights and things the same for both days (off) so the really only power use is my baseload. My baseload includes things like the refrigerator, DVR, etc. Since the baseload is the same for both days, We can just focus on the total usage. I did consider that if the house is hotter, the refrigerator would in theory work harder to keep my food cold. Well, we are only talking 4 degrees here. I'm not going to sweat (ha) the details here.
BTW, according to the government, I should save 1 to 3 % for every degree I set my thermostat up.
The Results
Energy used 6/6/2011 from 8am to 6pm: 14.7 kWh. My cost is $0.12 per kWh. so I paid $1.74 to let my AC hold 76F all day.
Energy used 6/7/2011 from 8am to 6pm: 11.2 kWh. I paid $1.35 in electricity from 8am to 6pm by letting the house heat up to 80F and then cool to 76F starting around 4:45pm. If you are wondering, the house was back to 76F by 6pm. Also, it appears the AC stayed off all day, which means my house didn't reach 80F until late in the day. Considering I work 4 days a week on average, I should save $6.56 per month. Doesn't sound like much, really. I tried to think of negative consequences of setting the temp up, but can't really think of one.
Botton Line... I DID save money by setting my thermostat up 4 degrees F during the day. Your numbers would likely be different due to differences in weather, home construction, etc.
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