When is it economical to turn off air conditioning if weather temporarily cools down?

Question

We live in Portland, OR and get hot spells every two weeks or so. Does anyone have a rule of thumb of when it is more economical to turn off the AC vs. just keep it on? It usually gets to mid 60's at night and highs of around 80. Hot spells get up to low 90's.

Answer 1

I would never run the A/C at night if it was in the 60s and only in the day if I was there in the house when it was in the 80s and 90s.If you have a programmable thermostat set it to turn on and hour or so before you come home.

It's a myth that leaving the AC on while you're away at work uses less energy than turning it on when you get home.  Here's why:

Heat goes to where it's not. That's why heat from outside goes into your cooler home. With the AC off, at some point your house will be so hot it can't absorb any more heat. When you come home and turn the AC on, the AC removes all that heat.

But if the AC is on when you're gone, then you've turned your house into a heat magnet. But keeping it artificially cool, there's no limit to the amount of heat it can absorb. It can always absorb more heat. And your AC has to remove that heat constantly. Your AC kicks in and removes some of that heat, then the house is cooler so it sucks in more heat from outside, so your AC kicks in again and removes that heat, and so on.

This means that throughout the day, your house has absorbed way more than one houseful of heat. And your AC had to remove it all. By contrast, with the AC off all day, then it has to remove just one houseful of heat when you come home and turn it on.

Let's say you leave the AC off, and your house absorbs 20k BTU's of heat and then stops, because that's all it can absorb.

Now let's say that you have the AC running instead. The house absorbs 5k BTU's of heat, so the AC kicks in and removes it. Then it absorbs another 5k BTU's, and your AC kicks in and removes that. Repeat that process several times during the day.

The actual numbers will vary, and I haven't tested this to see exactly how much the penalty for leaving the AC on during the day is, but there is zero question that running the AC all the time uses more energy than turning it on when you get home. This is not a gray area, it's simple physics, and no person with any knowledge of this subject disputes it. Running the AC when you're not home wastes energy, period.

Answer 2

Air conditioning is a lifesaver when the humidity and the temperature climb. But running an AC unit consumes considerable energy.

Once the Air conditioner has done its job and a room’s air environment is at a humane level, 72 to 78 degrees f and 35 to 60 percent humidity, consider switching off the AC and turning on a fan.

Answer 3

The less it runs, the less energy it uses.  Conservation 101.