How much would a costantly-on AC cost?

September 5, 2011 10:46 AM

Can you calculate the electricity usage of one AC in one room and the percentage it would be in terms of the electricity bill?

My two roommates and I get along superbly well, and we are in a 3 bedroom apartment.

We have an AC in the living room/dining room/kitchen (which is kind of a big, open, square space), and roommate A has an AC in her room. Roommate B and I don't have AC, but we use fans.

When roommate A installed the AC, she had agreed that she will contribute more to the electricity bill, since her room will be using more electricity than ours. But roommate B and I soon find that she leaves her AC on 72 pretty much all the time, even when she is out of town for 3 or 4 days. She told us specifically to not turn her AC off. She sometimes leaves the door open hoping that the cool air will get to us.

The living room AC is on a lot these days, too, but at 78 degrees, and it is off when we all leave the house. The AC is 8000 BTU and we have about 300 square foot of space.

I would like to know if there's any way I can calculate the electricity usage of her AC? I know the square foot, the exact model of the AC. She keeps it on 24-7, at energy saving mode, at an average of 72 degrees.

I want to get a sense of how we should divi up the electricity bill-- I don't want to charge her by the volt, but if her AC is costing as much as the rest of the house, then obviously contributing a few buck more would not do.

posted by atetrachordofthree to home & garden (14 answers total)

You can get a Kill-A-Watt device (this is just the first Amazon hit, not necessarily an endorsement of the exact product), and it will give you a reading in kilowatt-hours, which you can compare to the usage on your monthly electricity bill for pretty much an exact answer.
posted by brainmouse at 10:49 AM on September 5, 2011 [3 favorites]

There's no practical way to calculate this, because there are too many variables in play. It would depend on the insulation in the walls, the weather, etc. Far easier to measure it with a device like the Kill-A-Watt mentioned above.
posted by jon1270 at 10:52 AM on September 5, 2011

Two things:

To avoid a lot of calculation, you could compare your current electric bill to your past ones, then split the increase with her (which would be generously in her favor, as your A/C isn't used as much as hers, but it would be fast, easy, and hard-to-deny).

Also, if you don't want to spend money on a Kill-A-Watt type device, you could use this formula to calculate roughly what the unit uses.
posted by sarling at 11:32 AM on September 5, 2011

That calculator will give an upper bound, really— you don't know how many watts the AC is actually drawing (its plate tells you how many watts it can draw, maximum).
posted by hattifattener at 11:35 AM on September 5, 2011

P.S. Energy Saver Mode doesn't conserve energy the entire time the unit is running - it just shuts off the compressor when the room is cool enough. Even then, the unit is still consuming power (albeit less) because the fan is still running. And if the room/house doesn't cool down enough, the compressor never shuts off - effectively rendering Energy Saver mode useless. The size of the house, insulation in the walls, etc. don't really factor into it - if the compressor is running, it's running. And if it's running, it's costing you money.

A combination of calculation and comparing old and new electric bills should put you in the ballpark, at least.
posted by sarling at 11:38 AM on September 5, 2011

Thanks guys. We don't have a past bill though--- we moved into this new house together and a few days later she was getting the AC in her room.

If it helps, the AC in the living room and in her room are identical. Also, her room is about 76 square foot, compared to 300 in the living room. She keeps it at 72 degrees, and the front room at 78.

Would we be able to calculate the difference these two AC's would cost based on that info, all other factors are presumably the same or very similar?
posted by atetrachordofthree at 11:53 AM on September 5, 2011

If you have your electricity bills from the winter, or can see a usage history online at your electric company's website, you can compare the kilowatt-hours used before and after AC and come up with a good estimate of how much they add to the bill. I think I'd offer to split the increase 60-40 with her.

Barring that, given the information from this blog post, I would ballpark an 8,000 BTU AC, running 24 hours a day at 72° at 10 kWh/day or 300 kWh per month. (He ran a 6,000 BTU model 14 hours a day at 72°, so 4.46/14=x/24 = 7.65, 7.65/6000=x/8000 = 10.2)
posted by ob1quixote at 11:57 AM on September 5, 2011

Without a Kill-a-Watt the best you can do is ballpark it. Start with some assumptions. This whole calculation is multiplicative, so if my assumptions are low or high just multiply the answer by the same factor to correct (though, if you don't know all of these, keep in mind that a low guess one place may offset a high guess somewhere else).

1500W AC unit (when compressor on)
Compressor running 50% of the time
On 24/7
Residential power costs 12 cents/kW-hour

So that's 1500W * 0.5 * 24h/day = 18 kW-hour per day
18 kW-hour * 30 days/month = 540 kW-hour per month
540 * $0.12 = about $65/month in electricity

Ways this is probably wrong: Need correct electricity cost in your area; need correct compressor wattage; need correct duty cycle for compressor (if it's not terribly hot outside, or the room is well-insulated, or the room is small, the factor of 0.5 above could easily by 0.25 or 0.1, with end costs of $32 and $13 respectively). If her door is closed most of the time I'd guess that her compressor runs about 1/4 of the time yours does.

When the compressor isn't running (but the fan might be) the AC unit probably consumes less power than a desk fan, and I wouldn't worry about it. You really only care about compressor time for this kind of estimate. Even with a Kill-a-Watt there are so many factors that change day-to-day performance that you'd need to use the Kill-a-Watt all month to know the real answer (that is, you can't extrapolate from a single day or even a single week). The temperatures, etc. they are set to are meaningless because you're never going to solve this by working forwards from the laws of thermodynamics.
posted by range at 11:59 AM on September 5, 2011

so, if I am reading this post and other information right, the size of the room only effect the electricity usage in the sense that the compressor does not have to be on as long before the room reaches the temperature, is that correct?
posted by atetrachordofthree at 12:04 PM on September 5, 2011

Yes! And if it's an overly hot room, it could be the size of a broom closet and STILL use more electricity than the communal living space if the compressor stays on.
posted by sarling at 12:06 PM on September 5, 2011

Awesome! and the point of the temperature is also relative in the sense that I can use temperature to help me ballpark how long the compressor would need to stay on? (i.e. if it's 80 degrees outside, then to maintain a room at 72 = more compressor time, because the room's natural tendency is to slowly climb back up to 80, so the compressor will be on longer than if we want to maintain the room at 80? )

Okay, now I understand why there are so many variables!
posted by atetrachordofthree at 12:12 PM on September 5, 2011

Regarding range's calculation above, I looked at several 8,000 BTU AC units, and they all seem to be between 700-850 watts, with many being 740 watts. I would be surprised if the units in your apartment are 1,500 watts. Could you look for a plate or sticker on the unit and see how many watts it is rated?
posted by ob1quixote at 12:22 PM on September 5, 2011

atetrachordofthree: "Okay, now I understand why there are so many variables!"

Speaking of which, Consumer Reports has an extremely detailed worksheet to calculate the cooling load for a given room and the associated cost.
posted by ob1quixote at 12:38 PM on September 5, 2011

In some locations, you can actually check a Kill-a-Watt device out of the local library! It's worth looking into, as it will give you far better info the speculative calculation.
posted by itstheclamsname at 2:40 PM on September 5, 2011 [1 favorite]