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Program your thermostat for ComEd hourly electricity savings

Significant savings is available by programming your thermostat to schedule your air conditioning.

The largest fixed cost in your ComEd hourly electricity bill is usually the “Capacity Charge.” The Capacity Charge is a fixed monthly cost based on your electricity usage during the ten hours in the previous summer where the system-wide and ComEd-area load was highest.

By pre-cooling your house by a few degrees you can avoid running your air conditioner during these peak hours, which will reduce your fixed Capacity Charge every month next year. Additional savings comes from avoiding higher rates during the summer that typically occur between noon and 6 pm.

For more information see Introduction to ComEd hourly electricity pricing. Continue reading to learn how much you can save and how to program your thermostat.

How much can I save?

A typical energy efficient “3 ton” air conditioner cooling a moderately sized house will use about 2.5 kW while it’s running. The worst case would be for it to be running for all five peak hours.

By programming your thermostat you can ensure that it won’t run and save $171 per year on your Capacity Charge using the 3 ton example. This savings is based on the Capacity Charge cost/kW that is $5.69001 effective June 2010.

Option 1: A thermostat program that maximizes savings…

The maximum savings can be achieved by pre-cooling your house between midnight and 6 am. Rates are typically lowest during this period so the greater heat gain because your house is cooler is more than offset by low electricity rates.

Our house temperature rises about one degree per hour on the hottest summer days. Although it gets warm in the afternoon on a hot day, this program is a good match for our energy cost savings goals vs. our tolerance for heat. Other houses may be less energy efficient or have different sun exposures that may significantly affect how quickly your house heats up without air conditioning. Or you may have a different WAF level than I do.

Following is a starting point for such a program:

  • 12:30 am – 72 degrees (pre-cool at low rates)
  • 9:00 am – 75 degrees (maintain cool temps before afternoon heat)
  • 1:00 pm – 90 degrees (ensure air conditioner won’t run)
  • 9:30 pm – 76 degrees (some cooling for sleep comfort)

Some thermostats will start cooling in advance to reach the programmed temperature around the time scheduled. In this case the program should be adjusted to take this into account. For example, if the thermostat will start cooling 1.5 hours early the 12:30 am and 9:30 pm times should be moved back to 2:00 am and 11:00 pm. The 9:00 am and 1:00 pm settings can be left as is because the early cooling only affects changes to lower setpoints.

Option 2: A thermostat program that optimizes comfort…

Following is a suggested program that will reduce temperature variation but still avoid large Capacity Charges:

  • 12:30 am – 75 degrees
  • 2:00 pm – 90 degrees
  • 4:00 pm – 77 degrees
  • 8:00 pm – 76 degrees

Historically the peak hours that the Capacity Charge is based on usually occur between 2 pm and 4 pm. To be safest it’s best to avoid running your air conditioner between 1 pm and 5 pm. To do this you would change the 2:00 pm and 4:00 pm times to 1:00 pm and 5:00 pm.

If your thermostat has a feature that starts cooling in advance of the scheduled time be sure to adjust the program as noted in Option 1. You want to ensure the air conditioner doesn’t run between 2 pm and 4 pm.

Option 3: A thermostat program that balances savings and comfort…

Following is a suggested program that could be used as a starting point between the savings and comfort programs. This program avoids high Capacity Charges and also reduces air conditioner run time during high cost periods.

  • 12:30 am – 75 degrees
  • 1:00 pm – 90 degrees
  • 6:00 pm – 78 degrees
  • 9:00 pm – 76 degrees

What else can I do?

An additional safeguard is to sign up for the “Load Guard” option on ComEd’s hourly electricity web site This option automatically switches your air conditioner off for a two-hour period of time any time electricity rates are high.

Note that for most air conditioners Load Guard only turns off the outside condenser unit leaving your indoor fan running. Your indoor fan can use significant electricity and also generates heat so the primary “line of defense” should be programming your thermostat to avoid the typical peak periods.

The Load Guard option requires that you also sign up for the Central Air Conditioning Cycling program, which provides an additional savings of $20 or $40 per year. If you program your thermostat as recommended above it is likely your air conditioner will already be off if AC Cycling is triggered and you will end up saving the $40 without any additional impact on your comfort.


  1. Rex says:

    Excellent Information and article – Glad you mentioned the “Load Guard” because that is a likely and probable candidate for the 5CP’s that we are measured against for the “Capacity Obligation” calculation.

    Capacity Charge – The most painful charge and subsidized by RRTP customers to the benefit of BES customers. Sounds like a great promotion to get customers to come over to RRTP or BES-H (sarcastic of course)!

  2. Kristy says:

    I am the wife, and I have a high WAF. What I have is a low HAF. I just changed our settings – we’ll see how long it takes for him to notice. :)

  3. Scott says:

    FYI I updated the recommended Option 1 9:00 am setting from 74 to 75 degrees and the Option 3 12:30am setting from 74 to 75 degrees.

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