Wikipedia:Reference desk/Archives/Mathematics/2012 July 7

= July 7 =

Air Conditioner running cost.
I have a room that's 934sq ft (feel free to round up) that I want to cool with a window mounted a/c. If I run it for 8hours a day, and about 30 days a month for let's say 4 months. (June, July, August, September) From some guides I found on the internet, it takes about 18000BTU's to cool it, but I was thinking maybe 2 10000's.  Now I'm not sure what it means, but let's say an average SEER rating, 12. My cost per kiloWatthour is about $.06, or 6 cents. Excluding the up front cost of the unit, how much per summer, or hour, or month, will this 1 18000BTU, or these 2 10000BTU cost?. I've tried to calculate this, but it's worse than I can manage. Feel free to ask for any extra information, I just want a rough estimate, aimed high. Also, my windows are 4ft wide, what would I do to fill the space? Bluefist  talk 23:36, 7 July 2012 (UTC)


 * It would depend a lot on how long you run it, how hot it is outside, and how low you set it. I think your .06 is off as well because they usually double that by the end of the bill. You could take your total bill and divide by your kw used and get a true cost. You could calculate the usage in hours at full power and estimate from there. Look at the sticker for 'wattage'. 1000 watts for each hour running = 1kiloWatthour = $.06 and then use your actual cost which is probably $.12 - $.15.--Canoe1967 (talk) 23:53, 7 July 2012 (UTC)


 * Well I guessed the cost, I can't find the electric bill. I don't have an ac, so I don't know the actual kWh.  From about 95 to 80F, I live in West Michigan, so I don't need it for a long time, but it's always hotter inside than out, even with fans. Like I said, I just want a rough high estimate.   Bluefist  talk  00:33, 8 July 2012 (UTC)

I think your maximum wattage is 1500-1800w on a normal 120 volt breaker and plug there. Most AC units would use that at max output. So it would cost 1.5 to 1.8 x .06 to .15 per hour to run at full bore. This would make it $.09 to $.27 per hour. Those are the extremes for a normal circuit with the above figures. Larger units would need a different plug and circuit with more power. Does this help at all?--Canoe1967 (talk) 01:04, 8 July 2012 (UTC)
 * To fill the space in the window you can just cut a thin piece of plywood to fit and then wrap it in foam or staple foam to it. Cut five when you have the saw out. One will be too big, one will be too small, one will be a spare, and the other two you can sell to the stranger at the gate.--Canoe1967 (talk) 01:18, 8 July 2012 (UTC)


 * Yes both of those answers helped. Do you think that 6 fans running at about 72 watts (120V*.6A) almost all day would equal near that amount?  Because that's what we're doing right now, and I think that A/C is cheaper, or at LEAST cooler, because on 95F weather, fans do nothing.  Bluefist  talk  01:27, 8 July 2012 (UTC)


 * I might have just figured this out, by my math it's $46.66 a month to run 6 fans 24 hours a day, give or take ~$5 for turning some off or maybe it uses more than it says. Bluefist  talk  01:47, 8 July 2012 (UTC)


 * On filling the gaps: I use Styrofoam alone, which works fine (much better than the plastic baffles they provide), and is an excellent insulator, as long as you tape up all the way around. However, if your windows are accessible (ground floor, near fire escape, etc.), this does pose a security issue, since the window lock no longer offers any protection with an A/C unit installed, and Styrofoam is easily knocked out by a robber.  StuRat (talk) 01:51, 8 July 2012 (UTC)


 * On the use of fans: During dry, cool nights, blow hot air out half the windows (upstairs is best, since heat rises) and cool air in the other half.  Keep them on 'til the sun rises, to get is as cool inside as possible.  During the day, aim them directly at people, and close all the windows.  Doing both of these should help, but, when the outside temps are too high or humidity is too high, fan's aren't enough. StuRat (talk) 01:51, 8 July 2012 (UTC)


 * Thanks for the help, but I don't have an upstairs, or basement. That's basically what we do already, but it got up to 98 this week (thanks heat wave), and usually hotter than outside.  Fans just can't cut it, especially for 3 cats, which I feel so bad for.  Bluefist  talk  02:18, 8 July 2012 (UTC)


 * In that case, point all the fans in on the cool side (East or North, in the evening) and out on the hot side, and leave all doors open. Counter-intuitively, don't open any other windows, and close the windows containing fans as much as possible, to prevent local circulation (cool air blowing in and right back out again). StuRat (talk) 03:34, 8 July 2012 (UTC)


 * They should like my cat does.--Canoe1967 (talk) 02:24, 8 July 2012 (UTC)


 * That's not a real cat, is it ? The nose looks fake. StuRat (talk) 03:29, 8 July 2012 (UTC)
 * Do you mean real as in alive or does dead and stuffed count? I may never tell. Actually that may be vaseline to prevent hairballs that makes her nose so shiny.--Canoe1967 (talk) 19:46, 8 July 2012 (UTC)
 * They all hate the fridge for some reason, only one of them can tolerate being near it even, and that's because we give him ham from it sometimes. Bluefist  talk  02:32, 8 July 2012 (UTC)


 * 0.6 x 72 x 24/7 would cost more than AC. 432 watts only but running far longer. Fans just move air they don't cool it. Most AC will shut down a lot of the time if you don't try to turn your place into a fridge. 74-78F is a comfy high setting that most would be ok with. You may want to look into an Evaporative cooler they may cost a little more up front, but use far less power than AC. They just run a small fan and not a huge AC compressor. You could probably google and make a homemade one. I haven't read the article yet, but ones I know of cost 300-600 and are on wheels so you can move them from room to room. They do need water added more often in heat though.--Canoe1967 (talk) 01:53, 8 July 2012 (UTC)


 * Fans can lower the temperature of inside air, if you use them to blow cooler outside air in (mainly at night). Also, this strategy only requires use of all the fans at night, which is maybe 1/3 of the day, in summer.  And evaporative coolers are mainly useful in dryer climates than Michigan. StuRat (talk) 01:58, 8 July 2012 (UTC)


 * I do agree about timing and windows on fans. Take advantage of cool night air, and then trap it inside. I just read the evaporative coolers article briefly. The one I saw working effectively was in Salmon Arm. I don't know what the humidity is there, but summers can be very hot. It didn't need a window mount either. Was a fan that blew air through really expensive mystery material that looked like thin straw. The straw just did the Capillary action thing from a tank that was filled below. 2-3 tanks per day when really hot, I think.--Canoe1967 (talk) 02:12, 8 July 2012 (UTC)


 * Those will also raise the humidity inside, which is only desirable if you live in an extremely dry climate, like a desert. A good way to benefit from evaporative cooling without the added humidity inside is to hose down the outside of the house during the hottest part of the day, or, better yet, aim a sprinkler at it. StuRat (talk) 03:26, 8 July 2012 (UTC)


 * Now some suggestions just for the cats:


 * 1) If they are long-haired, get them a short haircut. Makes a huge difference.


 * 2) Leave them cool water, maybe with some ice in it to keep it cool.


 * 3) Have a metal plate for them to sleep on, which will conduct heat away from them. They also sell those for cooling laptops.  (A ceramic tub or sink also works, so you might find cats sleeping there.) StuRat (talk) 03:39, 8 July 2012 (UTC)


 * One other suggestion, block windows facing the Sun with Styrofoam (white is best). This limits solar heating. StuRat (talk) 03:47, 8 July 2012 (UTC)


 * Also, you can use a hybrid of the various systems, as I do:


 * A) Fans at night are enough to cool it off all day, when not too hot.


 * B) When a bit hotter, also use fans blowing on yourself (or the cats), to supplement this night cooling.


 * C) When still hotter, or too humid to use fans at night, then I resort to window A/C units. StuRat (talk) 03:57, 8 July 2012 (UTC)


 * My in-laws live near Grosse Pointe, and it costs them about $40/mo to run two A/C units for a similar floor area. I don't know anything about the BTUs or efficiency. I live in Holland, and close enough to Lake Michigan that our air is about 5-10 degrees colder most days, and it costs about $30 to run our central air for about 1300sqft in this weather. We keep the house at 72 and actually crank it down to 68 or so at night - my wife is pregnant and gets uncomfortably hot at night otherwise. 209.131.76.183 (talk) 16:10, 12 July 2012 (UTC)


 * On a multi-story house, sleeping on a lower floor can keep you cooler in summer, since heat rises. If you only have a single floor, then having a fan blow in the window in the bedroom (with the door open and another window fan blowing out elsewhere) should keep you cool, when it's cool and dry outside.  If not, close the windows, aim a fan directly at your wife with minimal covers, and, if still necessary, turn on the A/C, but hopefully not set as low. StuRat (talk) 17:14, 12 July 2012 (UTC)


 * For long term thermal control, planting trees near the house can make a huge difference, in that they block light in summer, and, at least in the case of evergreens, keep heat from radiating away in winter. StuRat (talk) 17:17, 12 July 2012 (UTC)


 * Mention of the basement being cooler reminds me of another method. If you have a forced air furnace you can run just the fan to bring cool air up from the basement. Some even adjust the intake from upstairs return vents and force it to draw more cool air from the basement. Some wall controls have a setting for fan only and most furnaces have them inside. Many are just a white cylindrical button that you pull or push. I think most furnace fan motors are only 1/4 to 1/2 horsepower. 1 hp = approx 746 watts. That is shaft hp so you need to divide by the effiency to get the input wattage or use volts x amps. 1/2hp = approx 375 watts x .85 eff = .44Kw per hour.--Canoe1967 (talk) 17:17, 14 July 2012 (UTC)


 * You said to divide by efficiency, but that's a multiplication sign in your math. StuRat (talk) 05:59, 15 July 2012 (UTC)


 * That must be the way I was taught math. Efficiency is an inverse coefficient so when you multiply by it you actually have to divide.--Canoe1967 (talk) 07:50, 15 July 2012 (UTC)


 * Weirdness. I'd either write 375/0.85 = 441 or 441×0.85 = 375.  If you try your method on a calculator, it's not going to work. StuRat (talk) 08:10, 15 July 2012 (UTC)


 * I did not read the full discussion. But I remember my neighbour has a very modern Heat pump installed in his house. It warms the house in winter and cools it in summer. And he said it uses almost no energy. I know there are also some available which use no energy at all, with either a heat storage tank burried underground, or a downhole heat exchanger, but I guess they are more expensive because of the drilling needed. So I think should be possible to find some very efficient units which should lower the usage costs massively. --helohe (talk)  22:38, 15 July 2012 (UTC)


 * Heat pumps have their place, but they aren't the magic solution they appear to be:


 * 1) While they might not use any energy to produce heat or "coolth", but just move it from outside to inside, it still takes plenty of energy to pump that heat around.


 * 2) They tend to only work in a certain range of temperatures, meaning you may need a regular A/C and heater, too, which adds to the cost, complexity, etc.


 * 3) As you said, the installation costs are extreme. So much that the total operating cost over the life of the unit may well be more than A/C. StuRat (talk) 23:39, 15 July 2012 (UTC)