What is a Kilowatt?

What is a Kilowatt?

A Kilowatt is a unit of power.  It’s actually 1000 watts of power. Whenever you see the term “kilo” or “K” it is an abbreviation for 1000. A Kilogram is 1000 grams, a Kilowatt is 1000 watts and $1K is $1000.

So we know that a Kilowatt is 1000 watts of power but what’s power?

It may seem confusing since we use the word power in a lot of different situations. We might say something like “that truck has a 400 horse power engine”,  “I need more light for my desk so I need a more powerful light bulb” or “that weight lifter is pretty powerful since they lifted 300 pounds”.  Or in the words of captain Kirk, “Scotty we need more power!” to out run the Klingons, that is. He was referring to more power to operate the warp drives and/or the shields. That power was converted to motion or the ability to deflect photon torpedoes.

“Scotty – We need more power!”

Physics defines power as the rate at which energy is used, transferred or transformed.  The rate at which a light bulb, for example, transforms electricity into light and heat is measured in watts. But wait, we’re talking about rate here which involves time so shouldn’t there be a dimension of time in the watt? Well there is because a watt is a joule per second (1 watt = 1 joule/sec).  Okay this just keeps getting deeper. What is a joule? A joule is a unit of energy that is defined in Physics as the work or energy expended to apply a force of 1 Newton for 1 meter. But we’re talking about electricity so what does that have to do with applying force over a distance?

Well there happens to be a similar calculation for electricity where a joule equals the electrical energy required to pass an electric current of one ampere through a resistance of one ohm for one second. Or in other words, the work required to produce one watt (joule/sec) of power for one second.

The rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit time. We find the watts that a bulb uses printed on the top of the bulb.

Electric companies charge you for the amount of electrical power you use in a given amount of time which is referred to as energy. They use a term called A Kilowatt Hour or KWH for short. If you use a kilowatt of power for one hour you have consumed 1 KWH of electrical energy and they charge you some amount for each KWH used in a month. Where I live, the rate is $0.10/KWH.

Electric Meter Measuring Kilowatt Hours

So let’s assume that I have a 100 watt bulb in a light fixture on my front porch and I never turn it off. What does it cost to operate every month?

Phot Source: unplggd.com


100 watt / 1000 = 0.1 KW. So the bulb will use 1KWH of energy every 10 hours (1KWH/0.1KW = 10 hours).

30 days/month x 24 hours/day = 720 hours/month  divided by 10 hours/KWH = 72 KWH/month x $0.10/KWH = $7.20/month.

If I use a sensor that turns my light on when the sun goes down and turns it off when the sun comes up, I might only use the bulb for 12 hours saving me $3.60 every month.

If I replace my 100 watt incandescent bulb with a 25 watt Compact Fluorescent Light (CFL for short) and I still use my daylight sensor I reduce my power consumption by 75% so it now only costs me $0.90/month ($3.60 x .25) so the savings are dramatic.


Incandescent and Compact Flourescent (CFL) Light Bulbs

A 100 watt bulb left on all the time costs $7.20/month or $86.40 /year.

The 100 watt bulb with a daylight sensor costs $3.60/month of $43.20/year.

The CFL bulb at 25 watts for the same amount of light with the sensor in $0.90/month or $10.80/yr.

The savings are dramatic not to mention the amount of natural resources that are consumed.  It takes about 1 pound of coal to generate 1KWH of electricity (56% of all electricity is generated using coal). So how much coal do our different bulbs use in a year?


100 watt bulb on all the time uses  876 pounds of coal in a year.

100 watt bulb with day light sensor uses 438 pounds of coal in a year.

25 watt CFL with daylight sensor uses 110 pounds of coal in a year.


Sunflower’s existing coal-fired power plant at Holcomb, Kansas (Photo courtesy Ohio Citizen Action)

It’s  interesting to think about just how much coal is used to generate our electricity not to mention the amount of pollution created in the process of burning coal to produce electricity.

Piles of Coal

Might also be of interest: What is Energy?