# Operating Cost of Amps and Watts?

acehApril 30, 2011

Is there much difference in operating cost between 8.8 Amps, (925 Watts) and 7.0 Amps, (740 Watts)?

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brickeyee

Watts (actually Watt-hours) are what you are billed for.

Commercial service may have power factor used in determining what a Watt-our is billed as).

So yes, it costs more to operate something that has a higher Watt load, but you still have to factor in time since Watt-hours are what is billed.

April 30, 2011 at 8:19PM
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aceh

I'm trying to compare the operating cost between 2 window air conditioners. I realize that the higher amps and watts will cost more but would it be a BIG difference in the operating cost?

April 30, 2011 at 8:36PM
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kudzu9

Assuming the units have the same operating efficiency, an air conditioner rated at 925 watts will use 25% more energy than one rated at 740 watts. So, your power bill for the more powerful appliance will be 25% higher (or a little more if the higher usage pushes you into a more expensive use category). If you are talking about new units, I believe they come with labels that lay out the operating cost, and you could make a direct comparison.

April 30, 2011 at 9:39PM
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yosemitebill

This is a rather simple calculation to perform but sometimes appears complicated if you are not familiar with the way electricity is billed.

925 watts used for 8 hours a day would be 7.4 kilo-watt- hours (kWh), which is what you are billed in. Using an average of 30 days a month, that would equal 222 kWh.

Here in California where I live, Tier 1 rate is 12.23 cents per kWh right now, this time of year. So, the first AC unit would cost about \$27.15 a month to operate. The other 740 watt unit would cost \$21.72 a month.

But then what is the BTU rating, what is the square footage, or more importantly, the cubic footage you wish to cool, how much will each unit have to actually run over an 8 hour period?

There is no simple answer to how much will it cost but with minimal usage the above numbers might give you some direction.

April 30, 2011 at 10:09PM
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aceh

Thanks alot yosemitebill. From your calculations, it doesn't seem to be a big difference.

FYI: The 740 watt unit is an 8,000 BTU air conditioner rated up to 350 sq ft and the 925 unit is a 10,000 BTU A/C rated up to 500 sq ft. I need to cool an area of 325 sq ft., however I live in New Orleans and the summers here get pretty HOT and HUMID and I was thinking that the 8,000 BTU unit would be cutting it pretty close to the maximum 350 sq ft (again I need to cool 325 sq ft). Both units are the same brand and they look identical except for the BTU's and other specs such as CFM, fan RPM etc.

April 30, 2011 at 11:02PM
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weedmeister

Will the AC be on the sunny side of the house or shade?
Do you have 8ft ceilings or more?
Do you have several windows in this room or just one?
Is this a top floor room with attic above or a downstairs room?

May 1, 2011 at 12:56AM
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aceh

weedmeister,

The AC side will have sun for approximately 5 hrs a day.
Ceilings are 8 1/2 feet.
4 windows (double pane insulated for 2 rooms separated by a 6 ft 6 inch wide arch).
Ground level floor with large walk around attic. Insulation between the attic floor and ceiling below. Electric attic fan which automatically turns on at a certain temperature.

May 1, 2011 at 4:49AM
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aceh

CORRECTION: I meant to say electric ROOF Fan (NOT attic fan) which turns on automatically at a set temperature.

May 1, 2011 at 5:03AM
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brickeyee

The larger capacity air conditioner is not going to run as long as the smaller unit.

If each unit has the same load and efficiency it will come out pretty even.

The on time of even a window unit depends on the load it is trying to cool.
Just be aware that an oversize unit will not run long enough to dehumidify, and you can end up cool and clammy.

May 1, 2011 at 8:47AM
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aceh

brickeyee,

Based on the information I gave it's best for me to stick with the 8,000 BTU unit? (I was thinking about going to the 10,000 BTU unit) New Orelans can get extremely HOT and HUMID during the summer months.

P.S. I was under the impression that a unit which has long run times during the HOT period was struggling to keep up. But from what I've been hearing this is a good thing?

May 1, 2011 at 12:30PM
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brickeyee

Without a better assessment of you cooling load there is no reliable way to determine what size unit is needed.

HVAC systems have design loads based on he long term average of the local weather.

When the weather hits the design limit the equipment will be on nearly 100% of the time.

If the weather is outside the design load it cannot keep up.

If the weather is inside the design load the equipment cycles on and off.

Only larger commercial systems are designed to dehumidify AND cool, and have sensors for both.
They are complicated and expensive, and often have throttling systems that allow the system to more closely match the loads.
Residential systems are normally 'bang-bang' systems.
The equipment is off or on.
They have no ability to throttle the system capacity.

Residential AC users only temperature as the control parameter.
The system runs at full capacity when it is on, and when the temperature falls below the set-point the system is turned off.
For heating it is often pretty good, for cooling it can create problems.
A larger than required AC system (or one operating inside the design limit but not at it) will come on for a short time, drop the temperature, and then switch off.
Humidity is only removed when the system is running as a by product of cooling the air.
The more oversize the system is to the cooling load, the worse the performance.

There are 'split systems' (outside condenser and inside evaporator) that even have two compressors in them.
The smaller compressor is used when the load is not as large.
The smaller compressor allows the system to run longer resulting in better humidity control.
When it can no longer keep up, the larger compressor is used.
Some units are coming out with variable speed built into a single compressor to allow its capacity to be varied.
Window units are not this sophisticated.

You need to try and size the unit more closely to the loads, and then you have to put up with some cold and clammy when the weather is not as bad, or not maintaining the set point at the hottest times.

Since window units are relatively cheap, getting more than a per-square-foot load calculation is about all you may find.

May 1, 2011 at 1:23PM
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weedmeister

With 4 windows and a hot attic above, I would go with the larger unit.

The longer run times are ok, if not preferable, if the temperature in the room is staying constant or slowly dropping.

May 1, 2011 at 4:43PM
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aceh

In testing the unit today, I set the temperature on the unit to 72 degrees. Outside temp is 88-90 degrees, inside room temp for the desired cooling area (325 sq ft) was 82 degrees. Unit ran for 2 hours and 56 minutes before shutting off and reaching the set temperature of 72 degrees. How does this seem?

I'm concerned when the summer really gets started here in New Orleans when temperatures easily reach 98 degrees and higher with unbeleviable sticky, HIGH humidity the unit just won't be able to handle that.

May 1, 2011 at 5:51PM
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Billl

If you were going to install a whole house system, you would need to have a pro in to do the calculations. They would factor in everything from square footage, to outside temps, to insulation levels etc and size a system appropriately. Really, the level of insulation is critical since that determines how quickly the heat gets into your room and how hard the unit will have to work to get that heat back out.

For a window unit, you should probably just get the one that is 1 size up from the minimum recommendation. There will be a small increase in cost of the system is oversized and cycles on and off too frequently. However, that is better than being undersized and not being able to maintain the temp you are after in your climate.

May 2, 2011 at 8:57AM
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aceh

I returned the 8,000 BTU unit and I will get a 10,000 BTU unit which is rated to cool up to 500 sq ft. I need to cool 325 sq ft plus I'm taking into consideration the New Orleans hot, extremely humid summers.

I really appreciate all the input on this subject. Thanks again.

May 2, 2011 at 2:10PM
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aceh

Now that I will get the 10,000 BTU unit I'm curious about something. The specs says the unit uses 925 watts, does this mean as long as it's running regardless as to whether it's on Cool, with fan speed HI ,LO or Auto it still uses 925 watts?

May 5, 2011 at 12:33AM
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brickeyee

"The specs says the unit uses 925 watts, does this mean as long as it's running regardless as to whether it's on Cool, with fan speed HI ,LO or Auto it still uses 925 watts?"

The maximum power is normally specified.

That should be fan on high, compressor running.

For most variable speed motors of the type used the fan uses less power on lower speeds.

Compressors in window units are normally single speed devices, but as the compressor cycles on and off the power draw varies.

May 5, 2011 at 9:05AM
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aceh

Thanks brickeyee.

May 5, 2011 at 12:51PM
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