Question About Air Conditioner Power Consumption

[aussiebebe]

Hi all,

Apologies if my non-scientific mind is annoying, but I wanted information about air con power consumption. Please post a link if this has been covered elsewhere. I've tried to find out by reading website like www.howstuffworks.com and the like but i wanted to understand more about how air conditioners work.

My bedroom is 24 square meters and I'm using an LG 12,000BTU air con unit. If I set the thermostat low to 20 degrees the compressor stays on, whereas if I set it higher, to say 27, the compressor turns off when the room is cool enough, and then flicks back on, off,on,off,on.etc.

Is there a difference in power consumption between these two scenarios?

If the compressor is always on does this use more power, all things being equal? Does it consume more power if the thermostat is set lower, or is it either on or off?

What's the most energy-efficient way to use an air-con?

[Crossy]

Your 12,000 BTU aircon will use about 1200 Watts of electricity when cooling (compressor running). That means about 1.2 units per hour. It will use very little (50 Watts or so) when the compressor is not running (just the fan in the indoor unit).

Obviously the less the compressor runs, the lower the overall power consumption, so with the thermostat set to 27C (compressor part time) it will use less juice than if it's set to 20C (compressor full time).

Most energy efficient mode? Off of course

Seriously, the warmer you can stand the room, the less power you'll use.

If it has a 'dry' or 'dehumidify' mode you could save on power using that and setting a higher temperature. The lowered humidity will make the room FEEL cooler

[trogers]

My bedroom is 24 square meters and I'm using an LG 12,000BTU air con unit. If I set the thermostat low to 20 degrees the compressor stays on, whereas if I set it higher, to say 27, the compressor turns off when the room is cool enough, and then flicks back on, off,on,off,on.etc.

First, 12,000 BTU is too small for your bedroom of floor size 24m2. What is the ceiling height of your room? I doubt the room can be cooled to 20C in the day, probably only in the dead of night (4-5am), when outside temp falls to 25C. Which direction does the window or balcony in the bedroom faces (north, south, east or west)?

Second, the on/off state of the compressor is for only non-inverter air-con. Inverter models have a function to reduce the speed of the compressor instead of switching it off.

Edited July 27, 2009 by trogers

[WeeGB]

My bedroom is 24 square meters and I'm using an LG 12,000BTU air con unit. If I set the thermostat low to 20 degrees the compressor stays on, whereas if I set it higher, to say 27, the compressor turns off when the room is cool enough, and then flicks back on, off,on,off,on.etc.

First, 12,000 BTU is too small for your bedroom of floor size 24m2. What is the ceiling height of your room? I doubt the room can be cooled to 20C in the day, probably only in the dead of night (4-5am), when outside temp falls to 25C. Which direction does the window or balcony in the bedroom faces (north, south, east or west)?

Second, the on/off state of the compressor is for only non-inverter air-con. Inverter models have a function to reduce the speed of the compressor instead of switching it off.

Hi, is there a formula for calculating area size for air conditioners? How many BTUs are necessary per sq metre, for example. We're looking into putting air conditioning in our house here (Phetchabon), but I don't know the first thing about it, and though I've tried to read up on it, frankly I'm baffled.

Thanks for your help.

[johnnyk]

I don't understand what m2 has to do with it.

If two rooms are 10X20 but one has an 8-foot ceiling and the other has a 12-foot ceiling isn't it obvious that one has a lot more air to push around?

The a/c isn[t cooling the floor!

Calculate the volume then it can be discussed sensibly.

Edited July 28, 2009 by johnnyk

[stgrhe]

My bedroom is 24 square meters and I'm using an LG 12,000BTU air con unit. If I set the thermostat low to 20 degrees the compressor stays on, whereas if I set it higher, to say 27, the compressor turns off when the room is cool enough, and then flicks back on, off,on,off,on.etc.

First, 12,000 BTU is too small for your bedroom of floor size 24m2. What is the ceiling height of your room? I doubt the room can be cooled to 20C in the day, probably only in the dead of night (4-5am), when outside temp falls to 25C. Which direction does the window or balcony in the bedroom faces (north, south, east or west)?

Second, the on/off state of the compressor is for only non-inverter air-con. Inverter models have a function to reduce the speed of the compressor instead of switching it off.

Just like to add that electrical motors use a lot of current at the starting phase, which makes on/off mode uneconomical, hence the reason for using inverters. However, inverter models are more expensive.

[stgrhe]

I don't understand what m2 has to do with it.

If two rooms are 10X20 but one has an 8-foot ceiling and the other has a 12-foot ceiling isn't it obvious that one has a lot more air to push around?

The a/c isn[t cooling the floor!

Calculate the volume then it can be discussed sensibly.

True, but one should also consider the location of the room, south, north etc, whether it is on an upper or lower floor, as well as the insulation factors of the windows/walls.

[aussiebebe]

Second, the on/off state of the compressor is for only non-inverter air-con. Inverter models have a function to reduce the speed of the compressor instead of switching it off.

Ahh! That answers my question - I didn't understand about inverters. Thanks for that. I understood that setting the thermostat lower would use more power, but I didn't understand how or whether the compressor would work differently depending on the desired temperature. I don't have any more questions but,

Calculate the volume then it can be discussed sensibly.

True, but one should also consider the location of the room, south, north etc, whether it is on an upper or lower floor, as well as the insulation factors of the windows/walls.

I do see the error of my ways. My room is on the second floor, it's 3.8m wide 6.4m long and 2.5m high which I think is a volume of 60m3. The window faces, I think, North and the windows are wooden framed. We have curtains and a suspended ceiling. My air con works fine, I just wanted to know about power usage. Thanks for the replies

[Crossy]

HomePro use a quick-and-dirty table to size your aircon, it makes a vast number of assumptions but here it is anyway

BTU - - - - - - - Sq Meters

9,000 - - - - - - 9-15

12,000 - - - - - 16-20

18,000 - - - - - 21-30

24,000 - - - - - 31-35

30,000 - - - - - 40-50

36,000 - - - - - 50-55

OR

Floor area (in square metres) X by 600 gives you the size of the aircon (in BTUs).

If you're going to want arctic conditions you'll need the next bigger unit than this table suggests.

Whatever you do, don't think 'bigger is better', a big aircon cycling slowly will not produce the drying effect that helps you feel cooler.

[trogers]

Actually, sizing air-con depends on 5 factors: volume of space to be cooled, heat transmission/insulation of perimeter elements, uncooled air temperature and desired temperature, humidity, and density of human occupants and their activity.

Cooling is not just bringing temperature down, but humidity as well. A conference room or gym fully occupied will require more cooling power, then a bedroom that sleep 3.

For your bedroom with standard ceiling height of 2.5m and windows facing north, you should be using 16,000BTU. Sizing it smaller means your compressor has less time to stop and rest, thus reducing its life span. If window faces west, I would use 20,000BTU for effective cooling during the afternoons.

Edited July 28, 2009 by trogers

[katabeachbum]

My bedroom is 24 square meters and I'm using an LG 12,000BTU air con unit. If I set the thermostat low to 20 degrees the compressor stays on, whereas if I set it higher, to say 27, the compressor turns off when the room is cool enough, and then flicks back on, off,on,off,on.etc.

First, 12,000 BTU is too small for your bedroom of floor size 24m2. What is the ceiling height of your room? I doubt the room can be cooled to 20C in the day, probably only in the dead of night (4-5am), when outside temp falls to 25C. Which direction does the window or balcony in the bedroom faces (north, south, east or west)?

Second, the on/off state of the compressor is for only non-inverter air-con. Inverter models have a function to reduce the speed of the compressor instead of switching it off.

12.000 BTU works fine up to 35 m2 if ceiling is less than 2,6 meters, ceiling insulated, double walls or insulated walls, hardwood floor, heatreflecting windows (glass or curtains), gaskets in doorframes. I have a Sharp doing this job in a masterbedroom.

[noudb]

Hi, is there a formula for calculating area size for air conditioners? How many BTUs are necessary per sq metre, for example. We're looking into putting air conditioning in our house here (Phetchabon), but I don't know the first thing about it, and though I've tried to read up on it, frankly I'm baffled.

-------------------

Yes there is, I've used it many times, it is a collection of formulas (originating from a engineering handbook) that calculate the heat-load on the room from the outside and inside based on some standard figures for insulation and transmission effects of materials.

In general for condominiums it comes down to the quick and dirty method mentioned above.

The most important factors are (in order of effect on heat load):

1. Size of windows in the room, (heat radiation coming in from sunshine and transmission at night)

2. Size of walls/ceilings that get sunshine from the outside (heat transmission coming in)

3. Required temperature difference inside vs outside

4. The use of the room, i.e. using at night only when there is no sunshine

5. equipment in the room, fridges, lights and number of people.

For example, a big bedroom 24sqm with one small window used at night at about 24 Degree C can easily be cooled with a 11-12 kBTU unit.

In general I would say if the room is affected by direct sunshine and the AC is used in daytime, the quick and dirty method is quite okay to use,

If you really want the formulas PM me, it is quite some things to write up and people might be bored with it

[oldgit]

Actually, sizing air-con depends on 5 factors: volume of space to be cooled, heat transmission/insulation of perimeter elements, uncooled air temperature and desired temperature, humidity, and density of human occupants and their activity.

Cooling is not just bringing temperature down, but humidity as well. A conference room or gym fully occupied will require more cooling power, then a bedroom that sleep 3.

For your bedroom with standard ceiling height of 2.5m and windows facing north, you should be using 16,000BTU. Sizing it smaller means your compressor has less time to stop and rest, thus reducing its life span. If window faces west, I would use 20,000BTU for effective cooling during the afternoons.

I would add to that heat gains from lighting and machinery i.e. computers and televisions.

[quiksilva]

If I have a room which requires say, 36,000 BTU (by the home pro rough guide thingy) would I best off getting two smaller units or one large one?

[Crossy]

If I have a room which requires say, 36,000 BTU (by the home pro rough guide thingy) would I best off getting two smaller units or one large one?

Conventional wisdom suggests two smaller units

Better efficiency during the cooler periods (run only one).

You at least get some cooling if one unit fails.

Better distribution of cooled air around your room.

[lopburi3]

20c is a meat locker. If you need that a high BTU unit will be required. But 25c should be comfortable here for a bedroom. I recently changed from an 18,000 btu unit to 9,000 btu and 25sm rooms are fully cool 24 hours a day. The 18k unit is available but has not been needed since Panasonic 9k unit installed. And it really does not run more than the 18k unit did. If you have any insulation or shade the HomePro size chart is too big. It is much more confortable having dry air at 25 than wet air at 22.

[WeeGB]

Thanks to all for the replies.

Now all I have to do is remember where I put my tape measure.

Life doesn't get easier as one gets older does it.

[Naam]

I don't understand what m2 has to do with it.

If two rooms are 10X20 but one has an 8-foot ceiling and the other has a 12-foot ceiling isn't it obvious that one has a lot more air to push around? The a/c isn[t cooling the floor! Calculate the volume then it can be discussed sensibly.

wrong assumption! it depends at which height the unit is mounted. only ignorant installers will mount the unit in a room with 12' ceilings high up and "disturb" as well cool the warmer air in this area. if unit is mounted at an appropriate height (max 8' / 240 cm) than the volume has not too much impact on cooling demand.

[Naam]

If I have a room which requires say, 36,000 BTU (by the home pro rough guide thingy) would I best off getting two smaller units or one large one?

Conventional wisdom suggests two smaller units

Better efficiency during the cooler periods (run only one).

You at least get some cooling if one unit fails.

Better distribution of cooled air around your room.

Crossy said it all

[Naam]

Actually, sizing air-con depends on 5 factors: volume of space to be cooled, heat transmission/insulation of perimeter elements, uncooled air temperature and desired temperature, humidity, and density of human occupants and their activity.

correct

Cooling is not just bringing temperature down, but humidity as well. A conference room or gym fully occupied will require more cooling power, then a bedroom that sleep 3.

correct

For your bedroom with standard ceiling height of 2.5m and windows facing north, you should be using 16,000BTU. Sizing it smaller means your compressor has less time to stop and rest, thus reducing its life span. If window faces west, I would use 20,000BTU for effective cooling during the afternoons.

bullshit

[trogers]

I don't understand what m2 has to do with it.

If two rooms are 10X20 but one has an 8-foot ceiling and the other has a 12-foot ceiling isn't it obvious that one has a lot more air to push around? The a/c isn[t cooling the floor! Calculate the volume then it can be discussed sensibly.

wrong assumption! it depends at which height the unit is mounted. only ignorant installers will mount the unit in a room with 12' ceilings high up and "disturb" as well cool the warmer air in this area. if unit is mounted at an appropriate height (max 8' / 240 cm) than the volume has not too much impact on cooling demand.

Until someone coughs!

Seems somebody is suggesting an efficient air-con design that gives cooler toes and hot heads...as the room cools down...

Edited July 29, 2009 by trogers

[trogers]

If I have a room which requires say, 36,000 BTU (by the home pro rough guide thingy) would I best off getting two smaller units or one large one?

Conventional wisdom suggests two smaller units

Better efficiency during the cooler periods (run only one).

You at least get some cooling if one unit fails.

Better distribution of cooled air around your room.

Crossy said it all

I do not see how a long rectangular room can achieve better cool air distribution with only one air-con unit running at one half of the room during cooler weather. Best way to cool a large room is to use one air-con unit with ducting, rather than a wall type.

[trogers]

Actually, sizing air-con depends on 5 factors: volume of space to be cooled, heat transmission/insulation of perimeter elements, uncooled air temperature and desired temperature, humidity, and density of human occupants and their activity.

correct

Cooling is not just bringing temperature down, but humidity as well. A conference room or gym fully occupied will require more cooling power, then a bedroom that sleep 3.

correct

For your bedroom with standard ceiling height of 2.5m and windows facing north, you should be using 16,000BTU. Sizing it smaller means your compressor has less time to stop and rest, thus reducing its life span. If window faces west, I would use 20,000BTU for effective cooling during the afternoons.

bullshit

If that is bullshit, I imagine you are cooling your 36m2 room with a 1 horsepower window unit.

[Crossy]

If I have a room which requires say, 36,000 BTU (by the home pro rough guide thingy) would I best off getting two smaller units or one large one?

Conventional wisdom suggests two smaller units

Better efficiency during the cooler periods (run only one).

You at least get some cooling if one unit fails.

Better distribution of cooled air around your room.

Crossy said it all

I do not see how a long rectangular room can achieve better cool air distribution with only one air-con unit running at one half of the room during cooler weather. Best way to cool a large room is to use one air-con unit with ducting, rather than a wall type.

Nobody said it did, you don't necessarily get all the benefits all the time

That said, yes a ducted system would be better for air distribution. But these are rare in domestic situations in Thailand, take space for the ducts and in order to be most efficient in cooler weather would need to be inverter units (costly).

And of course, if it fails you get no cooling

[trogers]

Nobody said it did, you don't necessarily get all the benefits all the time

That said, yes a ducted system would be better for air distribution. But these are rare in domestic situations in Thailand, take space for the ducts and in order to be most efficient in cooler weather would need to be inverter units (costly).

And of course, if it fails you get no cooling

Non-inverter units can also be used. We have been running round ducts to linear diffusers in the 20cm space above false ceilings of 60m2 lobbies of low-rise condo buildings, using two 30,000BTU units.

[Crossy]

Non-inverter units can also be used. We have been running round ducts to linear diffusers in the 20cm space above false ceilings of 60m2 lobbies of low-rise condo buildings, using two 30,000BTU units.

Not exactly a domestic situation (the diffusers look so much more professional in a commercial area), but out of interest, how does the cost of such an installation (say 36,000 BTU) with the diffusers, ducting etc. compare with having two 18,000 BTU regular split units?

[trogers]

Non-inverter units can also be used. We have been running round ducts to linear diffusers in the 20cm space above false ceilings of 60m2 lobbies of low-rise condo buildings, using two 30,000BTU units.

Not exactly a domestic situation (the diffusers look so much more professional in a commercial area), but out of interest, how does the cost of such an installation (say 36,000 BTU) with the diffusers, ducting etc. compare with having two 18,000 BTU regular split units?

About 25-30% more, depending on shape of room. But ducting gives more efficient air distribution than a ceiling type, and require less space above false ceiling.

[trogers]

Your 12,000 BTU aircon will use about 1200 Watts of electricity when cooling (compressor running). That means about 1.2 units per hour. It will use very little (50 Watts or so) when the compressor is not running (just the fan in the indoor unit).

Obviously the less the compressor runs, the lower the overall power consumption, so with the thermostat set to 27C (compressor part time) it will use less juice than if it's set to 20C (compressor full time).

Most energy efficient mode? Off of course

Seriously, the warmer you can stand the room, the less power you'll use.

If it has a 'dry' or 'dehumidify' mode you could save on power using that and setting a higher temperature. The lowered humidity will make the room FEEL cooler

The usual advice to save power consumption, ventilate the room with natural air for 15-20 mins to remove any heat and humidity buildup before turning on the air-con. This is especially so when turning on the air-con in the early evenings.

[quiksilva]

I take Troger's point on effciency air distribution, as you might imagine I see this quite a bit in the office market, but fitting a system like that into my house is not really practical, and I'm not convinced that it would offer enough value for money so as to be worthwhile.

Yes I asked regarding my house, so it looks like I will be getting a second unit for my living room. I have been running a Daikin inverter unit for the past few years and whilst its been tolerable most of year, it never gets truly cool, and can get too warm during the day especially in the warmer months.

I will probably get another similar sized inverter unit, which might be a little more than I need, but rationalise this as thinking that the load on both units would be much lower.

[trogers]

I installed two 18,000BTU inverter units in the 4.5m x 13m living and dining area of one of my condo. Ceiling height is 3m with two ceiling fans. Both need to be turned on to cool the entire space, or just one if only spot cooling is required, say, during dining or watching TV.