"neither fans nor AC should be operating when you are not at home. Fans make us feel cooler by convecting hot air away from our bods. If our bods are not there, the fan's energy is wasted."

That is simply not true in many instances. A fan may be bringing in cooler air from another location such as a basement or cool evening air and needs to run during all the cool hours to work effectively.

In my case, I use a strategically place double window-mount fan that use 50-75 watts. If used well in an area that cools off below indoor temperature at night it can cool off 1000 sq ft on low power for most nights. We live in a dry area where the summer days are often 80-95 degrees and the nights are 60-75 degrees.

The key is to place it at the coolest and lowest end of the structure and open the highest windows at the opposite end. Keep windows near the fan closed. It will push cool air through the entire place. We can cool our house off by about 10 degrees F on most days.

Keep ALL windows completely closed when the temp outside is hotter than inside. The instant the outside temp drops below the inside temp, open the windows and turn the fan on and LEAVE it on until the temps reverse again.

So, 50-75 watts with a fan on for less than 12 hours a day vs. 1000 watts for AC and there ain't no contest. The fan is the clear winner.

Our electric bill right now for a 2 person 2,000 sq ft house with a highly computerized office that is used all day is about $30-40/month in the summer.

What about those of us that leave small, furry roommates at home during the day? My apartment is usually fine with fans and the windows open. However, on days when it gets above 80 and with high humidity here in DC, I tend to opt to shut the windows and put the air condidtioning on low. If I could teach the dog to get up and adjust the fans, this wouldn't be an issue . . .

OK OK maybe in a dry climate.  Have you been to NYC in the summer?  There's no cool dry night air to suck in.  Usually it feel like a sauna outside.  That's just my $.02.  

All of NYC - and that is all five boroughs, not just the second-to-least populated Manhattan (that's right - Manhattan is the second to least populated borough, most NYers live elsewhere, like about 6 out of 7 of them) is either on an island or an isthmus, so humidity is the real issue around here, all day, all night.  If you are not right down by the beach, and are therefore not the direct beneficiary of onshore/offshore breezes (which is the vast majority of residents) then the air is pretty still and heavy most of the summer.  Thunderstorms offer false hope, since they may drop the temp a few degrees, but leave even more steamy air - usually a worse trade-off.

Anyway, leaving your fan on all day is not going to effectively alleviate this problem.

Always amazed at how much mis- and dis-information Umbra can spout in one column with that pseudo-folksy style.

Quoting the DOE to get wattage of fans? And that extreme range...a 250w fan is like those giant 48" industrial air movers you see in factories. A table fan may be 50w.

Then the incorrect "no one seems to care about watts (on a/cs)" What? Every a/c ad. and certainly the a/c nameplate shows wattage used. Iae, the computation of watts=btu/eer is something even my 9 year could do, it's hardly "too many numbers."

Also, who uses 3 (250w!!) fans in a room to replace one a/c as noted in the comparison? The comparison of energy used should be 50-90w (fan) to 700-900w (a/c.)

Obviously, the a/c provides different comfort (not least the reduction of humidity which is in most cases more important than temp,) not mentioned by her.

Thanks for providing a perfect answer.

As for the person with dogs, you may not like this answer but they might be happiest with a little kiddie pool that they can plop in.

I suspect fan's would still win, but an AC probably does not need to operate constantly.  The peak wattage draw of an AC unit will not match the average draw for a 24 hour period.  A cheap device like a kill-a-watt could be used to measure the 24hr KW usage for a small window unit and compared to a fan operating for the same duration.

While it is nice to say we should use fans over ACs, sometimes that is just not realistic.  Come on people, no matter how green you are, I know you cannot not honestly tell me on a day when it is 90+ degrees and humid, you would just "chill" in your house with a fan and feet in a bucket of water.

Umbra faild to discuss programmable AC units.  On days mentioned above I need my AC especially after gardening outside and to relieve my allergies (or I could suffer with a constant headache from the humidity).  When the AC is on, I always have it programmed to control itself at the highest temperature I can comfortably stand.

For someone who mentioned fans to be used to bring cool air from a basement, I'm sorry to say but that is impossible.  Heat rises, and as my bedroom is on the second floor I can tell you there is no cool air moving up my stairs.

As far as the pets, if it is hot and you are uncomfortable, I'm sure they are since they are most likely covered in fur.  I too keep the air on for my pets on very hot days (and it is programmed).

Many people here have mentioned their pets, and this is where environmentalism goes politically correct, because the environmental response to questions about pets would be: "Don't have them."

It is astonishing that people ostensibly concerned about global warming, and who are willing to force others to change their lifestyles with regard to cars, housing, clothing, flying, and even limiting the number of children people can have feel free to own animals that have extensive carbon footprints due to their protein diets, energy used in heating and cooling, and the zillion-dollar pet products industry.

If environmentalists were honest, they'd tell you straight up that the great green future is one of no pet-ownership. But they won't admit that, since it would lose a significant fraction of their support from animal lovers who also love their pets.

It's true that people are devoted to their animals, once they have them- just as they are to their houses, cars and other chattels. And it is also true that people ought to consider, carefully, whether to  acquire them in the first place. Actually, KenGreen did us all a favor by pointing out the environmental  hypocrisy in keeping pets, in times of diminishing resources and keener competition for them. Pets will  once again (as in previous times) be kept exclusively by the wealthy oligarchs who run things: the bankers, the politicians, the megacorporations that own the media in this country, and who make athletes, rock and film stars dance to their tunes. Real animal lovers know that the wild ones are those who ought to be protected and loved, first.

I've got a pet.  But then I live in AK where we've only had one day over 70 F so far this year.  I don't have any children, by choice.  A single child would use far more energy than my dog does.  And I only weigh 110 lbs so I myself use far less energy than the average overweight to obese 50 y.o. American woman.  I think I can get over any guilt I might otherwise feel about my dog.

As for bringing cooler air up from the basement: I used to do this all the time when I lived in un-airconditioned older houses in eastern MA.  Most basements in such houses have at least one window that opens, or a walkout door.  If you open that window or door in the evening, open the door to the basement, and put a strong window fan in an upstairs window, directing the air OUT of the room (i.e. on the exhaust setting), you will feel cool air flooding your house from the ground floor up.  Even in dwellings without basements a window fan set in the upper part of a double hung window can help bring cool evening air in while exhausting hot interior air.  While cold air is denser than warm and will not rise without help, a fan can easily overcome this density differential.

No, this won't work in dense urban areas where nighttime temps never drop below 80.

Just don't do what a former roommate of mine insisted on doing: opening her bedroom windows wide all day to the hot outside air while leaving a powerful room fan on while she was out at work.  No surprise that her room was at least 10 degrees hotter than the rest of the house by 5 PM . . .

All other thing being equal, a large, high ceilinged room, or a shady porch, will feel cooler than a small low room at the same temperature because our bodies actually absorb long wave radiation coming off solid objects like walls and ceilings.  It's time we started designing buildings with some thought about climate and comfort.  Time to bring back the screened porch for summer sleeping and after dinner living!

I have to agree with KenGreen that having pets is fundamentally wasteful...but I have three cats and a dog, all of which were street animals.  In every neighborhood I've lived in, I've volunteered trapping and neutering stray animals.  I have to tell you though, that it's actually wrong that pets need a "protein" diet.  It's only American pets that need a protein diet, just like American humans.  Dogs and cats from the rest of the world will eat leftover beans and rice, lentils, rats, mice, cockroaches, and lots of other stuff.  Dogs really don't need to be eating meat, unless it's scraps being thrown away.  

When it comes to cooling a house/apartment for the sake of the animals, don't bother.  Unless it gets incredibly hot in there, you're just being wasteful.  I'd say that as long as you leave water out for them every day, they'll be fine.  I mean...it's not like they're running around and working hard when you're not home...they're just sitting in a window or sleeping.  If it gets to 90 degrees, they may pant a bit but they'll be fine.

I have to and disagree with some of the pet comments above.  While yes, many pets will be fine with just a bowl of water, it isn't always as simple as that.  There are factors that have to be taken into consideration: age of the pet (older pets cannot tolerate heat as well), the temperature outside and inside, and the humidity.  On of the two cats I have (a rescue) does not tolerate fluctuations in temperature well.  I find that when the temp changes significantly, including with the seasons, she becomes sick.  Of course on a day where the temperature is 78 and the humidity is low, no the AC does not need to be on for pets.  On a day that it is 90 and the humidity is high, it would be cruel to just assume a pet is comfortable.  Remember, pets are "pets."  Many pets adapt to living indoors.  Even my rescue cat, who was a stray for a couple years, has now adapted to living indoors and I don't think she would tolerate being outside as much as she had in the past.  

But as some have stated, pets take up energy.  Is it a burden you are willing to deal with?  If no, then don't have a pet.

If you gave up the apartment and moved to the suburbs you could hang out in your backyard, or sprawl in a hammock.

I think all domesticated animals should get spayed even the ones that walk on two feet.  Then they should go to the store and buy themselves an air conditioner.

Maybe owning pets isn't the greenest choice in the world, but what is?  I say if environmentalists were REALLY honest, they'd tell you straight up that the great green future is one of no humans. But they won't admit that, since it would lose a pretty much all of their support from, well, everyone who isn't suicidal.

All environmentalists make some concessions, (just being alive is a big one).  Some of us own pets because they give us companionship and fill emotional gaps in our lives.  Others turn on the AC in the summer rather than use fans to be comfortable.  The point is there are no perfect environmentalists, we all have non-green things we do for our health or sanity, and no activity is completely exempt from some sort of environmental impact.  Everyone posting or reading these comments is using computer that is consuming electricity.  We could all save power by shutting our computers off right now and never turning them on again, but for one reason or another we've decided that the benefits to ourselves of using a computer outweighs the environmental costs.  Pet owners are no different, and shouldn't be made to feel guilty about the life enrichment they get from their animals.  Just as we focus on lessening the impact of our computers and AC units rather than getting rid of them, we can do the same for our pets by feeding them a healthy but more vegetarian diet, getting them spayed and neutered, avoiding needless pet product consumerism, etc.

Companion animals consume more in the US than most humans in the natural, undeveloped world. If you have a cat and drive a Prius, you have just negated the benefit of your Prius versus an SUV as far as the planet is concerned. Aside form the immorality of enslaving an animal for human pleasure, pets are just bad for Earth. Cats kill native species and spread disease. There is no such thing as a green pet master.