Kill A Watt Calculator

This calculator is for the Kill A Watt electricity usage monitor. It’s an excellent tool to measure your electricity usage, but to get the data it gives you into dollar terms, you must perform your own calculations. This calculator will automatically do that for you and give you the device’s cost per hour, day, month, year and 50 years. To calculate a device’s energy use, please input the fields below. For a tutorial and definitions of the terms please read below the calculator.

Kill A Watt Calculator
Enter the kilowatt-hours (kWh) used:
Enter the time the meter has run: Hours: Minutes:
Enter the local kilowatt hour (kWh) rate: Look up your local rate
Enter the average hours per day the device runs:
The kWh this device uses per month is
The cost of this device per hour is
The cost of this device per day is
The cost of this device per month is
The cost of this device per year is
The cost of this device per 50 years is


Kill A Watt Calculator Tutorial

First, you will want to plug your Kill A Watt into an outlet and then plug a device into the Kill A Watt, such as this fan below:

Kill A Watt Calculator Example

You’re going to want to run the device for at least a few hours. The larger sample size you get in hours, the more accurate your data will be when you run the calculator. Under the screen, on the right side of the device, is the kWh/hour button which is purple and highlighted by the red arrow:

Kill A Watt Time

By pressing this button, you can toggle between the time and kWh the device has used. Above, the time is listed as 8 hours and 13 minutes. You would fill that information into the calculator above. Now toggle the button to display the kWh, such as below:

Kill A Watt Calculator kWh

Now input the kWh used into the calculator above. For your local kWh rate, you can look it up in your utility bill or find a general estimate by clicking the link next to the input box. This is the Energy Information Administration’s average price per state. For the average hours per day the device runs, you’ll need to give your best estimate to how often you use a device. For something like a refridgerator, this will be 24 hours. For something like a television, it might be just 3 or 4 hours. Click Calculate Device’s Cost, and voila!

You now know how much your device costs over five different time periods: per hour, per day, per month, per year, and per 50 years. As well, the calculator will tell you the monthly kWh that device uses. This is helpful when comparing the device to your monthly utility bill.

Kill A Watt Calculator Formula

The calculator first divides the kWh used by the time ran. This gives us the kWh per one hour of use. Then:

kWh per hour X local kWh rate X average hours per day ran X length of time = cost of device

Example: The fan above generated 0.22 kWh over 8 hours and 13 minutes. This is 0.026775 kWh per hour.

0.026775 kWh per hour X 0.13 cents per kWh in Santa Barbara X 12 hours run per day X  8766 hours (aka 365.25 days to account for leap years) = $22.28 to run the fan per year.

NOTE: A month is considered 30.4166667 days (this is 365/12). A year is considered 365.25 days to appropriately account for leap years.

If you like the calculator, please share! If you haven’t bought a device yet, get one on Amazon for cheap.


9 Responses to “Kill A Watt Calculator”
  1. Jaret says:

    This is a fantastic thing! Is there any possibility a guy could use the source code of this calculator or is it restricted? This is very useful and I would like to use the source to calculate power consumption over several devices which I use for my business. I would not share the source code unless explicitly given permission by the author. Is it possible?

  2. Richard says:

    The local rate page publishes 9.69 cents/kWh for my area. Is that entered into the calculator as .0969 or 9.69 in the ‘Enter the local kilowatt hour (kWh) rate’ field?

  3. bill says:

    Hmmm. I think your calculator is not functioning correctly.

    If I tell you how much a load has used over a time period then the amount of time it’s in use per day is not required. You already have the data. I only need to know the load time per day if I only provide the hourly consumption.

    • Bill Vosti Bill Vosti says:

      Hey Bill,

      Asking the time of use per day is to get a sense of how much the device would cost over time. Asking for the meter readouts of kWh used & time run help to normalize a kWh value for that device, which I then can multiply over time to see what it costs. Let me know if you have any more questions,


  4. Jess says:

    Just purchased a P4400 and want to make sure I have the calculations correct.

    I plug in a Delonghi 12000 BTU room air condiionar into the P4400.

    3.27 kWh
    5:05 hours

    >>> .644 kWh per hour

    Ulitity rate:$0.0835 per kWh

    .644 x $0.0835 = $0.053774/hr x 12 hours per day = $0.645 per day

    .$0.645 x 30 days = $19.35/month


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