We want to cool the bedroom at night. We have a portable AC unit. Let’s say we want the room to be 72 degrees at bedtime. Without AC, the room is 80 degrees. Is it cheaper/more efficient to keep the room at 72 a few hours before bedtime, with the AC on low and cycling on and or to blast the room an hour or so before bedtime?
Cool the room down when you need it cool. Keeping it cool all day is just wasting energy. It takes the same amount of energy to cool the room down slowly or quickly. Besides, it’s probably hotter during the day meaning you’re expending even more energy.
This being the Dope, I will wait to see if there are conflicting answers before collecting on my bet
This is exactly the scenario in our house. We turn on the window unit about an hour before going to bed. Then we turn it off in the morning. Works great.
It may seem like a waste of energy to turn your A/C on and off, but doing so actually saves you a fair amount of money, Amann says. “Air-conditioning systems run most efficiently when they’re running at full speed,” she explains.
So while your unit might make more noise initially cooling a space down from 80 to 75 degrees, running all day at a less powerful speed requires more energy overall.
Since this is factual, let’s move it to GQ (from IMHO).
It’s cheaper to cool the room down when you need it than to keep it cool all day long.
The reason for this is that the amount of heat that transfers in and out of the room depends on the temperature difference between the room and the outside. Once the room reaches equilibrium at 80 degrees, then the heat transfer basically stops.
If you run the AC all day long, then the room won’t reach equilibrium and heat transfer will continue all day long. Since more heat transfers into the room all day long only to get pumped back out by the AC, you expend more energy keeping it cool.
The same principle works in reverse in the winter. It’s cheaper to leave your house unheated during the day and heat it up when you come home. The main problem with this though is that if it gets too cold in your house the water pipes will freeze and burst, so if you don’t have a programmable thermostat then you’ll just want to set the thermostat down to a temperature where the pipes won’t freeze (but don’t turn the heat completely off). If you have a programmable thermostat, you can program it to let the temperature drop during the day while you are away then turn on the heat so that the house heats up to a comfortable temperature just before you get home.
There is a conflict, but only if you have a heat pump, or something that has a similarly inefficient high power heating/cooling mode. Taking the winter example, if your programmable thermostat lets the house get too cold before turning on, the heat pump can go into high power heating mode, which is horribly inefficient and will cancel out all of your efficiency savings. A lot of newer heat pumps have settings that can work with your programmable thermostat to prevent this from happening.
If electricity costs the same across the entire day, then it’s most efficient to cool a room right before you need it. However, if electricity prices vary significantly and your house is well insulated, then it can often pencil out to be more efficient if you overcool the room when prices are cheap and let it come up to your desired temperature.
Often, this takes the form of running the AC extra cold in the mornings and then relying on the stored cool air to keep the house cold during the middle of the day.
If you want to go down a geeky rabbit hole, Technology Connections has a great video covering the topic:
The real answer is the usual one. It depends.
Heat inflow is proportional to the temperature difference. So hot out and cool in means that more energy flows in for the AC to pull out.
But with a warm room there is the heat already in the room, not just in the air but in the furniture, ceiling, floor, and walls. Perhaps one of the more overlooked aspects of room comfort is the apparent feel of the room temperature due to the temperature of the surfaces. They radiate heat, and it can matter. Just getting the air cool may not be enough. One of the more miserable things you can do is find your bed has heated up during the day to the point where sleep is impossible in it.
But the dominant unknown is the insulation of the roof and walls. Effective enough and once you cool the room you will need almost no additional energy to keep it cool. Poor enough and the AC just spends its time pumping heat about and draining your wallet.
Getting good enough insulation is not trivial. But the difference it can make can be dramatic. Many cheap modern homes are little short of dreadful. Especially those built in mild climates where builders can get away with it.
Also the capacity of the air conditioner itself. If it’s barely adequate for the heat load of the room, then it may not be able to pull the temperature down in a reasonable amount of time. Doubly so if the room isn’t well insulated, and it gets a blast of sun in the evening. My apartment is a 100 year old uninsulated brick building, which faces west, so if I don’t have my a/c units turn on by 3:00 in the afternoon then they won’t be able to handle the evening load.
Also, the OP stated that they have a portable AC. Those are the worst for efficiency because they blow conditioned air out through the condenser, thus sucking hot air into the room through cracks, windows, doors, etc. Technology Connections has a video about those too.
It depends upon the amount on insulation and the method of cooling.
If the room only gets to 80 F during the day, I agree with the person who mentioned turning the window unit on an hour before you go to bed.
In Arkansas, where it is 95F and our insulation is suspect, if we can’t get the temperatire down to the 70s during the day, we can’t get the room comfortable for several hours.
There may occasionally be special circumstances that change the answer, but in general, @Telemark got it in one: You want your climate control keeping rooms comfortable only while you’re in the room, and to run as little as possible in any given room while you’re not in it.
Not sure, I’d agree with that. Down here in the south, we have mold problems if the AC (which does humidity control as well as temperature control) is turned off for long durations.
Well, OK, let me rephrase that: Run climate control in any given room only while there’s someone or something in that room that would benefit from the climate control. That encompasses not letting pipes freeze in the North, and keeping mildew down in the South, and keeping pets comfortable, and so on.