Cost per hour to run 16 SEER 2 ton a/c? (Same as space heater?!)
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  1. #1
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    Aug 2009
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    Baltimore, MD
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    Cost per hour to run 16 SEER 2 ton a/c? (Same as space heater?!)

    Hello. I would like help figuring the cost per hour to run my central A/C.

    I got a new 16 SEER, 2 ton Carrier air conditioning system installed last year. I am trying to figure out the cost per hour to run it. I need to know how much I should allow my blood pressure to go up when the thing runs for hours at a time during really hot days.

    I am coming up with 1500 watts per hour (costing 16.5 cents/hr at a utility charge of 11 cents/KwH). Interestingly, this is the same as a standard ceramic space heater set on continuous high. Am I correct?

    My calculation is as follows: 16 SEER means that 1 watt of electricity produces 16 BTUs of cooling. At this ratio, it will take 1500 watts to produce 24,000 BTUs (2 tons).

    I do understand that the SEER is theoretical based on optimal conditions and flawless installation and there might be a small variations due to nominal BTUs vs real BTUs.

    That said, is 1500 watts/hour the correct theoretical number? Even with the variations mentioned above, can I probably assume my real usage will be within 10% of this theoretical number?

    Thanks!

  2. #2
    Join Date
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    Look on the data plate of the condenser (outdoor unit) for RLA = Rated Load Amps.

    That is the highest current that the system will draw on the hottest day.

    By the way, you can get electricity slightly cheaper than BGE's rate at Washington Gas & Electric (www.WGES.com). BGE's summer rate will be 10.8 cents per kw-hr for generation. WGES is 10.2 cents. That's a whopping $6 savings per 1000 kw-hrs consumed, but better than nothing.

    Take care.

  3. #3
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    Quote Originally Posted by alkydrinker View Post
    My calculation is as follows: 16 SEER means that 1 watt of electricity produces 16 BTUs of cooling.
    That is the definition of Energy Efficiency Ratio (EER), not SEER.

    SEER is a weighted average of EER's at outdoor temps from 67F to 102F, in 5 degree increments. The lowest 2 temps (67F and 72F) account for almost half of the value of the SEER calc.

  4. #4
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    If I am not mistaken EER is the number you want to use in your calculation, not SEER. For example a 2-ton Trane XL16i heat pump is rated at 16.0 SEER, 11.9 EER. This model delivers somewhat less than 24,000 BTU/hr, and other brands are said to offer better EER in many cases for the same SEER.

    I took a different approach and looked up claimed wattage in Trane "Performance Data" for the above system. It is rated to draw 1900-2000 watts.

    Hope this helps -- Pstu

  5. #5
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    Here is Technical Operating Data for a 16 SEER / 13.5 EER Goodman single stage a/c at 400 CFM per ton, 75F Indoor Dry Bulb temp, 67F Indoor Wet Bulb temp:

    Ambient
    Temp--------KW's consumed---BTU's

    115F-------------1.90---------21,200
    105F-------------1.84---------22,900
    95F--------------1.76---------24,100
    85F--------------1.67---------24,700
    75F--------------1.56---------25,300
    65F--------------1.44---------25,900

    As I look over the data sheet for various IDB's and IWB's, the min KW is 1.36 and the max is 1.99.

    Take care.

  6. #6
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    Quote Originally Posted by gary_g View Post
    Here is Technical Operating Data for a 16 SEER / 13.5 EER Goodman single stage a/c at 400 CFM per ton, 75F Indoor Dry Bulb temp, 67F Indoor Wet Bulb temp:

    Ambient
    Temp--------KW's consumed---BTU's

    115F-------------1.90---------21,200
    105F-------------1.84---------22,900
    95F--------------1.76---------24,100
    85F--------------1.67---------24,700
    75F--------------1.56---------25,300
    65F--------------1.44---------25,900

    As I look over the data sheet for various IDB's and IWB's, the min KW is 1.36 and the max is 1.99.

    Take care.
    Thanks Gary, that helps alot...so even though my understanding of SEER is a little confused, I was in the ball park with 1.5kw/hr...though it looks like 1.8kw or so would be a closer to reality after factoring in some unknowns and inefficiencies. Comes out to about 20 cents an hour.

    If it ran 10hrs a day, that's $60/month in electricity....not too bad.

  7. #7
    Join Date
    Jan 2004
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    Use the SEER, not the EER.

    Your 2 ton is a 2 stage. So it will operate a lot differently then a single stage 16 SEER.

    If you have the Infinity control. Your A/C will run long times. But will be switching back and forth from first stage to second stage, and back to first stage.
    In first stage, it may only be using 1 KW, but be removing 19,000+ BTUs of heat(or roughly 80% of second stage capacity at the same outdoor temp.).
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