Determining kW Use
You can estimate the power use in kilowatts of an appliance from the nameplate information. If the nameplate indicates the appliance's rated wattage to be 1200 watts at 120 volts, then divide the wattage by 1000 to convert to the kilowatt power usage. If you've been curious what a 100 watt light bulb uses in kilowatts, divide 100 by 1000 and get 0.1 kilowatts.
The power used by an appliance can be measured by observing the meter-disk revolutions on the regular kilowatt-hour meter at the service entrance. This method has the advantage of accurately measuring the watts or watt-hours used by equipment under actual loading and service conditions. This method can also be used to measure the power use of the service, or for any desired number of combinations of loads operating at one time.
Every regular kilowatt-hour meter has a flat aluminum disk with a black mark along its edge. This disk turns when energy is being used by the service. Meters also have a meter constant called the K sub h which is shown on the meter nameplate. This K sub h constant indicates how many watt-hours are used for each revolution of the meter disk. A constant of "Kh = 7.2" means that for each revolution of the disk, 7.2 watt-hours of energy have been used. The actual number associated with the constant will vary with different meters. Common values are 3.6 and 7.2.
To determine how much power is used by counting meter-disk revolutions, proceed as follows:
1. Make sure the appliance to be checked is the only operating device on the service. This may require finding the circuit that supplies the appliance you wish to check in the breaker box and shutting off all the other breakers. This assures that the appliance will be the only device operating and using electricity.
2. Disconnect, or power off, any other equipment that is on the same circuit with the appliance to be checked.
3. With a watch, determine the time it takes for the meter-disk to make a certain number of complete revolutions. Choose any number of disk revolutions you desire. Depending on the appliance you're measuring, the number of revolutions will vary. If you are measuring a smaller, less power consuming device, the time to move the disk may be quite lengthy. There is also the issue of devices like refrigerators and air conditioner compressors that cycle on and off. Make sure you measure these appliances while they are on, not when they have cycled off.
4. To calculate the kilowatts of power used by the appliance, divide the number of disk revolutions by the time in seconds. Multiply this times 3.6 and multiply again times the K sub h factor from the meter. This is the amount of power actually used by the appliance in kilowatts.
As an example, the meter was timed with everything off in the house except the air conditioner. The meter disk made 10 complete revolutions in 30 seconds and the k sub h on the meter was 7.2. The power use, in kilowatts, of the air conditioner is found by dividing 10 revolutions by 30 seconds, multiplying this times 3.6 times 7.2 which equals 8.6 kilowatts. Remember, though, an air conditioner doesn't run all of the time - it cycles on and off.