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What is a Kilowatt Hour?

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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. For example:
Hours used
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

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Electricity Monitor and Control System

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 was done in the table above.

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

Cost of electricity = (wattage * hours used) * price per KWh / 1000

For example, let's say you leave a 100-watt bulb running continuously (730 hours a month), and you're paying 15 cents ($0.15) per kWh. Your cost to run the bulb all month is:

100-watt x 730 hours * $0.15 per kWh  / 1000= $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.)

You can't always trust the wattage printed on the device, 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 listed wattage for a fridge.

watts vs. watt-hours

watt and watt-hour cannot be compared. Watts are a unit of power, which is to say the rate of energy being used every second. Watt hours are a unit of energy.

Watt hours are often used because it is a meaningful way of charging people for electricity. Electric bills are charged per kWh. With regard to generating electricity, 200Wh would be a total amount of energy generated, but 200W would be 200J every second - the total would have to be calculated by multiplying the 200W by the time in hours the generator is running for.

Many people 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 frequently asked, 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 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 that instant.

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 it by 1000 to get the kilowatt-hours, which is how the power suppliers charge the bill.  Example: 100-watt bulb x 2 hours / 1000 = 0.2 kWh.

How much does electricity cost?

The cost of electricity depends on where you live, how much you use, and possibly when you use it. The electric company measures how much electricity you use in kilowatt-hours, abbreviated kWh. Your bill might have multiple charges per kWh (different charges per-kWh) and you have to add them all up to get the total cost per kWh.

Electricity rates vary widely. The rates may vary even from the same provider. The only way to know what you're actually paying is to check your bill carefully.

The average cost of residential electricity was 12cents per kWh in the U.S. in April 2009, and ranged from 7cents per kWh in North Dakota to 26cents per kWh in Hawaii (from the DoE, which also has historical rates). But average rates are misleading, because most utility rates are tiered, meaning that excessive use is billed at a higher rate. This is important because your savings are also figured for the highest tier you're in. For example, let's say you pay 9cents per kWh for the first 500 kWh, and then 16cents per kWh for use above that. If you normally use 900 kWh a month, then every kWh you save reduces your bill by 16cents. Because savings happen at the highest-billed tier, those writing about saving electricity generally should not use the average rate, since the savings rate will usually be higher.

Electricity Usage Monitors

The basic figures contained within a monthly or quarterly electricity bill do not give you much information as to where your electricity is going - they just tell you how much you have used in total during that period and how much totally you need to pay. Therefore it is well worth considering purchasing an electricity usage monitor and using it to see exactly where all your hard-earned money is going.

One great way to find out how much electricity each of your household appliances and electronic devices uses is with a wireless electricity power/energy monitor, which shows you in real time exactly how much money your total home or office electricity usage is costing you. These monitors can help you reduce your electricity consumption by as much as 20% simply by showing you what you are using. Sailwider-SmartPower is a developer and manufacturer of electricity power monitor and controlling system.

uni-directional (1-way) electricity power energy monitors

Most electricity energy monitors in the market are uni-directional (1-way) only, that means you can only get energy consumption information from the monitor. The bi-directional (2-way) electricity power monitoring and control system from Sailwider-SmartPower makes the user not only able to monitor the electricity usage, but also can easily remote control the connected electrical appliances wirelessly, providing great convenience to electricity efficiency management.

bi-directional (2-way) electricity power energy monitoring and control system


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