How to Calculate Wattage Use in a Minute

Electric power is expensive. In addition, electrical power often comes from the burning of fossil fuels. Fossil fuels are of limited supply and their combustion is detrimental to the environment. So there are plenty of reasons for wanting to keep electric power usage under control. The first step in getting anything under control is to better understand it. Understanding when you use electricity, and how much wattage you use, is the essential first step.

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Instructions

- 1
  Record the kilowatt-hour reading on your electric meter. For a digital meter, record the number that appears when the "kWh" symbol is lit. For an analog meter, read the dials. Be careful, because the dials move in opposite directions -- each one alternating between clockwise and counterclockwise. For example, you could have an initial reading of 17,721 kWh.
- 2
  Record the date and time at which you made a kWh reading. For the example, say you made the reading at 9 a.m. on August 3.
- 3
  
  Energy usage typically is very stable and relatively low during nighttime hours.
  
  Allow some time to pass. The time interval can be anything from one minute to 30 days; however, it's good to select an interval that is on the same order as the changes in electrical usage in your house. For example, your overnight electrical usage is probably relatively constant, but the usage when no one is home at noon will probably be much lower than when everyone comes home in the afternoon.
- 4
  Repeat Step 1 and record the date and time. Say you return on August 4 at 4:30 p.m. and you find a reading of 17,738 kWh.
- 5
  Subtract the reading from Step 1 from the reading in Step 3 and subtract the time in Step 3 from the time in Step 2. Express the time in minutes; that is an hour and a half would be 90 minutes.
  
  For the example, the difference in the kWh readings is 17,738 - 17,721 = 17 kWh.
  
  The time difference is 1 day, 7 hours, and 30 minutes. One day is 1,440 minutes and 7 hours is 420 minutes so the total is 1440 + 420 + 30 = 1,890 minutes.
- 6
  Divide the kWh difference by the time difference in minutes. That is the average amount of electrical energy you used during that time. If the time was an entire month, this will provide an accurate measurement of your average power usage, but will provide no insight into how your usage varies. On the other hand, if you selected a time interval of 10 minutes, your reading will be less accurate and will be more trouble to make.
  
  For the example, 17 kWh / 1,890 minutes = .009 kWh / minute; that's 9 watt-hours per minute.
  
  If you use .009 kWh in one minute, it means on average you turn on equipment that uses .009 kWh * 60 minutes/hour = .540 kW, or 540 watts.
- 7
  Repeat the entire procedure for several different time intervals. For example, you might want one interval to be the two hours between when you awaken and when everyone leaves for the day; the next interval could end when the kids come home from school, the next right before bed and then when you awaken.

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