A room air conditioner cools the whole room. This is extremely expensive. Why?
First, the extreme heat. The cooling power needed increases with the difference between the wall temperature and the air temperature. Look at the table below (best viewed on a tablet or laptop).
|Location||Wall Temp (°C)||Comfort Temp (°C)||Average
|Insulation||Watts for 1°C cooling||Steady State Cooling Power|
|South & East USA||25 – 30||24||3.5||High||150||0.5 kW|
|South East Asia||26 – 31||24||4.5||Medium||250||1.1 kW|
|Karachi||26 – 32||25||4.5||None||450||2 kW|
|Multan, Lahore||40 – 42||28||13||None||500||6.5 kW|
|Sukkur||41 – 43||28.5||13.5||None||500||6.8 kW|
Wall temperature – this corresponds to the typical outside air temperature on a warm evening when you would want air-conditioning. The walls heat up during the day and cool off slowly at night. You can check this for yourself. Place the palm of your hand on the wall at around 8 PM in the evening. If the wall feels warm or hot to the touch, then the temperature is over 36°C.
Comfort temperature – the temperature at which most people feel comfortable in a given climate. It varies with the season. People adapt to higher temperatures unless they live in air-conditioned environments all the time.
Building design – few if any Pakistan buildings have insulation, and many buildings have only a single brick or concrete wall which lets heat in almost as well as an open doorway.
The table shows that the night time steady state cooling power needed for a bedroom in Lahore is about 6500 Watts compared with 1125 Watts in Singapore, and 525 Watts in southern USA.
But, there is yet another factor which is easily overlooked: thermal mass of building materials.
Concrete and bricks used for Pakistan buildings store much more heat compared with insulated plasterboard or timber used in the USA. A room air conditioner has to cool the walls as well as the people. It takes 5 to 10 times more energy to cool concrete and brickwork than insulated plasterboard.
Cooling a plasterboard wall by about 4°C in the USA takes only 30 minutes. In Lahore, it takes more than 12 hours for a 2 ton air conditioner to cool brick and concrete walls by the 13°C needed there. That means that a bedroom air conditioner in Pakistan is running at full power all night long in Lahore and for 5 or 6 hours in Karachi.
A highly efficient air conditioner needs about 4 kWh electricity (4 units) to keep a typical bedroom cool in the USA, costing 40 cents, or USD 15 per month. The same air conditioner in Lahore will require 22 kWh and nearly all that energy is used just to cool walls down during the night and the cooling has to start all over again the next night. The cost will be about Rs.26000, USD 250 per month because a generator will be needed to provide power during load shedding. This includes electricity, fuel, repairs, maintenance and the eventual cost of replacing the generator every few years.
Your electricity costs in a coastal situation such as in Clifton and other areas of Karachi close to the sea can be significantly less than elsewhere. However the huge amount of salt dust carried by the onshore breezes accelerates corrosion of the outside unit of a split system air conditioner. You will have to replace the air conditioner more often.
Of course, you can reduce the running cost if you make some sacrifices.
By doing without a generator, and putting up with an hour or so of heat when load shedding comes two or three times during the night, you can reduce your monthly bill to about Rs.6000.
But why not reduce your monthly bill for air conditioning to just Rs.1500 by using Close Comfort and get luxurious and continuous sleep through load shedding? You will sleep better and never have to worry about the bill again.
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Inverter Air Conditioners – do you save electricity?
How do they work?
An old-style on-off air conditioner switches off the compressor once it senses that the room temperature is cool enough. Once the room temperature rises, the compressor comes on again for a few minutes.
An inverter air conditioner can vary the cooling power of the compressor which runs continuously. When it starts, the air conditioner runs at full power until the room has cooled down to the desired temperature. Then, in a well-insulated room, the steady state power consumption drops to as low as 400 Watts, sometimes less. When running at low – medium power, the air conditioner can be much more efficient than an old-style on-off air conditioner. That’s why inverter air conditioners can provide large energy savings. They are designed to run under these conditions most of the time.
A reality check – cooling the walls
Unfortunately, this is rarely possible in Pakistan because of the thermal mass of the walls explained above.
Take a look at the table below to calculate the total night time energy needed for air conditioning.
|Location||Wall Temp (°C)||Comfort Temp (°C)||Ave. Diff.||Wall Material||Hours to cool walls||AC Power Rating||kWh to cool walls||Steady state kWh||Total Energy|
|South & East USA||25 – 30||24||3.5||Plaster bd. rock wool||0.5||1200||0.6||3.4||4 kWh|
|South East Asia||26 – 31||24||4.5||Insulated bricks||1||1200||1.2||3.6||5 kWh|
|Karachi||26 – 32||25||4.5||Bricks, concrete||6||1800||10.8||2.1||13 kWh|
|Multan, Lahore||40 – 42||28||13||Bricks, concrete||>12||2400||21.6||0||22 kWh|
|Sukkur||41 – 43||28.5||13.5||Bricks, concrete||>12||2400||21.6||0||22 kWh|
Since the air conditioner has to spend most if not all night running at full power, an inverter cannot provide energy savings. In fact, most inverter air conditioners are less efficient than an old-style on-off air conditioner running at full power, so you may well use more electricity with an inverter air conditioner in Pakistan.
It is nothing to do with the design of the air conditioner. Building design and Pakistan’s climate determine the running cost.
There’s nothing much you can do about that, except invest in Close Comfort and reduce your monthly electric bill for night time air conditioning to about Rs.1500.
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