Central AC vs. Window AC Electricity Cost

[FirstNissan]

I just moved to an apartment complex, its the first time that I have had central air, prior to this its always been window AC units.

My question: I have a dog. He needs the AC on during the summer months, 9 hours during the day. Running my window AC back in NJ cost me an extra $10 per month for the daytime use. About how much can I expect to pay using central AC? Is it going to be cost-effective to get a window AC and turn off the central air during the day for the dog?

Thanks!

 

[440Magnum]

If it was properly installed and maintained, the central unit will be significantly more efficient than a window unit.

Window units have the advantage that you can shut the doors to every other room and just cool the one room you need. If you can do that, then it might be worth having a window unit. But window units are noisy- I've got one in our upstairs room to help the central unit out in the heat of summer, and although its one of the best and quietest on the market (Friedrich) its still pretty loud.

 

[SLCraig]

Something I'm interested in too. My condo has central air and I've only used it maybe 5-6 days this year but I am worried what it will cost me in say July/August vs maybe getting a window unit and only cooling my bedroom for comfy sleeps.

 

[azjake]

If you're away at work you can dial up the central A/C a few degrees to save money without causing harm to your dog. Do some online research or talk to a vet to see what would be an acceptable temp.

 

[rshaw125]

What medical condition does your dog have?

My AC thermostat automatically turns the temp to 80 while I'm at work. It turns it down to 75 30 minutes before I get home.
My Beagle does not know the difference.

 

[CarbonSteel]

Originally Posted By: 440Magnum
If it was properly installed and maintained, the central unit will be significantly more efficient than a window unit.

Window units have the advantage that you can shut the doors to every other room and just cool the one room you need. If you can do that, then it might be worth having a window unit. But window units are noisy- I've got one in our upstairs room to help the central unit out in the heat of summer, and although its one of the best and quietest on the market (Friedrich) its still pretty loud.

+1 **Most** central units are more efficient, but as 440 alluded to a window unit allows you to only cool the space that you need or want to versus the entire home--thus the run time for the compressor will be low for a single room (which is what uses the majority of the electricity). Many of the new units (both central and window) are much more efficient than those from the past so it could depend on what you have...

 

[Virtuoso]

Central A/C, while more efficient as a whole, also cools a much larger area; has bigger compressors and fans in general.

I did notice in our apartments that a single window unit added about $20/month to our electric bill. Liberal usage.

We have a 3 year old, 14 SEER single-stage central A/C unit that adds somewhere between $40-100 to our monthly electric bill; much of that depends on ambient temperatures and who's home.

This year may not be as expensive since we had our windows replaced mid-summer last year.

YMMY

 

[daves87rs]

About the same here...

 

[JHZR2]

That's an expensive dog if you're cooling an entire uninhabited home for it.

It's all about cooling load and heating load.

Take the current and voltage rating for the whole central system (outdoor part plus indoor air handler), and the current rating for the window unit.

Current * voltage = power (watts).

Assume consistent heat load so each will work proportionately to cool it.

Say your window unit is 600W and the central is 4000W, thats your ratio.

Assume each actually runs 5 hours per day.

0.6kW* 5 hr = 3kwh= $0.45/day
4kW*5 hr = 20kwh = $3/day

It's a rough estimate for you, just get the real numbers and plug and chug.

 

[rjundi]

I would give it a month or two and see what electric bill entails.

When you leave the apartment just leave temp on something like 75-78F which will not bother most dogs even health effected and minimize energy usage.

 

[rjundi]

I would give it a month or two and see what electric bill entails.

When you leave the apartment just leave temp on something like 75-78F which will not bother most dogs even health effected and minimize energy usage.

 

[rjundi]

Originally Posted By: JHZR2
That's an expensive dog if you're cooling an entire uninhabited home for it.



Some folks pay lots more in vet bills/meds than cooling bills for pets. Just happens unfortunately.

 

[Schmoe]

Get a programmable thermostat. Program it not to drop below 79 or so. Once the house is at that temperature, shouldn't run no where near as much. You'll spend more money trying to "catch up" by cranking the central HVAC system on in the evenings as opposed to leaving a window unit running all the time. Plus, you got to think, the dog will probably be sleeping most of the time and won't get his core temperature up very much so you really don't need to keep the room in the mid 70's.

 

[PandaBear]

Depends on how big is your apartment and where you put your dog, and whether you have to buy and pay someone to install your window AC, and how long you plan to stay in that apartment. It may or may not be worth it.

A window AC is not as efficient as central AC, but like others said you are using it in a smaller area, so it may save you money even at lower efficiency. If you are just renting for a few years it may not worth paying for a window AC that cost $100 and then paying to install it for another couple hundred dollars.

 

[bigmike]

I have a dog at home, but I set my A/C to 85 degrees when I'm not there. Rarely does the house go over 82 (good insulation, on a slab, etc add to this), so it seems moot, but I set it anyway just in case. I know several people who leave their dog outside with a dog house, appropriate food, water, and they survive. The good ones add fans and shade. I have a soft spot for my pets, so I keep mine inside.

I don't see why you can't get a programmable thermostat. That could save you some money but keep your dog and house cool enough.

 

[FirstNissan]

Thanks for all the input!
My dog is my 4 legged furry kid, and he prefers the cold. We have always kept the apartment at 75 degrees even when we are out. 80 would probably work, but I prefer to keep him on the cooler side, paranoia maybe?

JHZR2- Thanks for those figures. Im going to look at the wattage of the AC compressor for my central unit, and then do the same math you did. If the figures are that drastic, im going to look into getting a small window unit for the living area.

 

[ddrumman2004]

Originally Posted By: rshaw125
What medical condition does your dog have?

My AC thermostat automatically turns the temp to 80 while I'm at work. It turns it down to 75 30 minutes before I get home.
My Beagle does not know the difference.


Doesn't know the difference? Have you asked him or her?

Sorry......had to ask!

 

[440Magnum]

Originally Posted By: JHZR2


Take the current and voltage rating for the whole central system (outdoor part plus indoor air handler), and the current rating for the window unit.

Current * voltage = power (watts).

Assume consistent heat load so each will work proportionately to cool it.

Say your window unit is 600W and the central is 4000W, thats your ratio.

Assume each actually runs 5 hours per day.

0.6kW* 5 hr = 3kwh= $0.45/day
4kW*5 hr = 20kwh = $3/day

It's a rough estimate for you, just get the real numbers and plug and chug.



The flaw in that assumption is the "assume each runs 5 hours per day." You could have a *tiny* window unit that draws very few watts, but also has so little cooling capacity that it runs 18 hours per day and STILL can't keep the temp below 80, whereas the central system could keep everything at 78F and run less than 4 hours total per day. Its comparing apples to orangutans.

 

[JHZR2]

That's why the assumption is that heat load is consistent and that the AC is right-sized to the space.

So that the cooling performance is equivalent. Then the ratio really amounts to the load for cooling one space versus the whole house.

Sure there can be a ton of variables, but central isnt just automatically wonderful because it is more efficient or cools easier. It is an energy draw aspect too. The central will draw more, and will cool more spaces. Do we need it? If the heat load is roughly consistent in the one conditioned space where the window unit sits versus the whole house (which the central has to roughly average over, depending upon placement and number of thermostats), then the performance will be similar, and the over.

IMO the most efficient approach is a ductless split system with 26 SEER units that can be throttled down substantially. Not sensible for an apartment since it is a permanent install.

 

[440Magnum]

Originally Posted By: JHZR2


IMO the most efficient approach is a ductless split system with 26 SEER units that can be throttled down substantially. Not sensible for an apartment since it is a permanent install.


Agreed, ductless split AC is wonderful from several aspects. The inverter-drive units are incredibly efficient, since a single condensing unit varies its capacity as needed to run anywhere from 1 to 4 individual evaporating units. Since you can have multiple indoor units for each outdoor unit, its feasible to have individual temperature control in every room and shut off rooms that aren't in use. They are QUIET.

But they have two big drawbacks. First, if you're in an area where natural gas is available as a heating source, you have to have a separate gas furnace. DS systems can run as heat pumps, but NO heat pump is as cheap as a natural gas furnace. Second, they're still novel enough (in north America, they've got a longer history elsewhere) that they tend toward stupid expensive.