Energy Saving - Calculations
What the heck is a kilowatt hour?
Before we see how much electricity costs, we have to understand
how it's measured. When you buy gas they charge you by the gallon.
When you buy electricity they charge you by the kilowatt-hour (kWh).
When you use 1000 watts for 1 hour, that's a kilowatt-hour.
The wattage will be printed on the device or its label. To get
kilowatt-hours, take the wattage and divide by 1000 to turn it into
kilowatts, and then multiply by the number of hours you're using the
item. That's exactly what I did in the table above. If you'd rather
not do the math then my handy calculator will do all the work for
you. That same page also has a list of the wattage for most
If your device lists amps instead of watts, then just multiply
the amps times the voltage to get the watts. For example:
2.5 amps x 220 volts = 550 watts
(your country probably uses 120 volts instead of 220.)
You can't always trust the wattage on the label, because many
devices don't use the full listed wattage all the time. For example,
the compressor in a refrigerator doesn't run constantly, only
sometimes, so you can't go by the label for a fridge. My calculator
takes this into account by listing the average wattage for fridges.
The most accurate way to find the average wattage of a device is to
measure it with a watt-hour meter.
Watts vs. watt-hours
Many of my readers get confused about the difference between
watts and watt-hours. Here's the difference:
* Watts is the rate of use at this instant.
* Watt-hours is the total energy used over time.
Here's a question I frequently get, which makes no sense:
"You say that some device uses 100 watts. What period of time is
It's not for any period of time, because watts is a rate at that
instant. One might as well ask:
"The speedometer in my car says I'm going 35 miles an hour. What
period of time is that for?"
It's not for any period of time. You're going 35 miles an hour at
The difference is:
* We use watts to see how hungry a device is for power. (e.g.,
100-watt bulb is twice as hungry as a 50-watt bulb.)
* We use watt-hours to see how much electricity we actually used
over a period of time.
So, just multiply the watts times the hours used to get the
watt-hours. (Then divide by 1000 to get the kilowatt-hours, which is
how your utility charges you.) Example: 100-watt bulb x 2 hours ÷
1000 = 0.2 kWh.
Formula : Watt/1000 * cost/kWH =
cost per hour
Note: atifkamal.com provides information
here for illustration only, without warranty either expressed or
implied. This includes, but is not limited to, the implied
warranties of merchantability or fitness for a particular purpose.