What is a kilowatt hour

What is the difference between Watts and Watt-hours?

watt-hours

Before we see how much electricity costs, we have to understand how it’s measured.  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.  For example:

Device
Wattage
Hours used
kWh
medium window-unit AC
1000 watts
one hour
1 kWh
large window-unit AC
1500 watts
one hour
1.5 kWh
small window-unit AC
500 watts
one hour
0.5 kWh
42″ ceiling fan on low speed
24 watts
ten hours
0.24 kWh
light bulb
100 watts
730 hours
(i.e., all month)
73 kWh
CFL light bulb
25 watts
730 hours
18 kWh

 

To get kilowatt-hours, take the wattage of the device, multiply by the number of hours you use it, and divide by 1000.  (Dividing by 1000 changes it from watt-hours to kilowatt-hours.)  That’s exactly what I did in the table above.

Here’s the formula to figure the cost of running a device:

wattage   x   hours used  ÷  1000  x  price per kWh  =   cost of electricity

For example, let’s say you leave a 100-watt bulb running continuously (730 hours a month), and you’re paying 15¢/kWh.  Your cost to run the bulb all month is 100 x 730 ÷ 1000 x 15¢ = $10.95.

If your device doesn’t list wattage, but it does list amps, then just multiply the amps times the voltage to get the watts. For example:

2.5 amps   x   120 volts   =   300 watts
(If you’re outside North America, your country probably uses 220 to 240 volts instead of 120.)

Exercise #1.  Go get your electricity bill and see how many kilowatt hours you used last month.

Exercise #2.  Assume that the lights in your kitchen and living room together use 400 watts.  How much does it cost if the lights are on 24 hours a day, for a whole month?  How much per year? Assume 15¢/kWh.

Answer

• 400 watts x 24 hours/day x 30.5 days/month = 292,800 Total Watt-hours

• 292,800 Wh / 1000 Wh = 293 kwh

• 293 kWh x 15¢/kWh = $44/mo.; $528/yr.

Exercise #3.  Assume your window AC uses 1440 watts.  How much does it cost to run it continuously for a month?  How much per year?  Assume 15¢/kWh.

Answer

• 1440 watts x 24 hours/day x 30.5 days/monh = 1,054,080 Total Watt-hours

• 1,054,080 Wh / 1000 Wh = 1,054 kWh

• 1,054 kWh x 15¢/kWh = $158/mo.; $1897/yr.

Watts vs. watt-hours

What is  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 that for?”

It’s not for any period of time, because watts is a rate at that instant. One might as well ask:

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 used over a period of time.  That’s what we’re paying for.

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.

Here is a guide to energy used by appliance

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