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How To Calculate Watts For Analyzing Energy Efficiency Projects


In Order To Learn How To Calculate Watts, You First Need To Understand "Watt's" a Watt? 

Yes, I know I spelled What wrong above.  I will likely get in trouble for the spelling from my wife, the teacher.  But it is for a good reason, energy efficiency.  First let's define a watt.....

Official Definition of a Watt

watt (wät)

The basic unit of electric, mechanical, or thermal power in the SI and MKS systems, equal to one joule per second or 10 ergs per second ( of a horsepower): for electric power it is equal to one volt-ampere.

OK, that's the official definiton, now let's focus on defining this in a way that makes sense to you.  The watt is the starting point for how you are charged for energy, so a watt is tied directly to your wallet.

Lowering Wattage is Key To Improve Energy Efficiency

Taking a few minutes to learn about how watts relate to energy efficiency is an important exercise for those who don't understand electricity.  Counting watts is the key to lowering the cost of energy in your home or place of business.

How to Save?  It's Simple Math.....Let's Take A Look At How To Calculate Watts...

Most people don't realize that in order to improve energy efficiency, you must how to calculate wattage reduction.  This is as simple as a mathematical subtraction problem.  

Determining the difference in wattage can have a significant impact.  Understanding this concept will help you make more informed buying decisions when purchasing products that use electricity in your home or business.

The 100 Watt Incandescent Bulb vs. The 23 Watt CFL Equivalent

For an example, we are going to compare two bulbs.  The images below show a 100 Watt standard incandecent bulb on the left and a 23 Watt compact fluorescent bulb on the right. 


Both of the bulbs above are suitable replacements and can be utilized for the same application.  While the old style 100 Watt bulb will likely be cheaper to purchase upfront, over time the compact fluorescent will save you money.  How much difference? 

Finding the Difference in Wattage

This is where the simple subtraction problem comes in.  Using the two bulbs above, we utilize the wattages to do a simple mathematical problem.  

Take the 100 Watt incandescent and Subtract the 23 Watt Compact Fluorescent and you find a difference of 77 Watts

(100 Watts - 23 Watts = 77 Watts Difference).  

With this information we can now determine how much each of the bulbs will cost us annually.   

Calculating Kwh

Your likely saying to yourself "but my energy company charges me per kilowatt hour (kwh)".  Now the math gets a little more complicated, but still manageable.  In order to determine the cost difference between the two, we utilize the wattages to determine the kwh.  We do this with the following equation:

Fixture Quantity x Fixture Wattage = Total Watts

Total Watts / 1000 = Kilowatts

Kilowatts x (Hours of Usage) = Kilowatt/Hr

Kilowatt/Hr x (Cost of Energy) = Annual Cost

It Costs How Much?

Let's assume you have the following:

10 Lights (Fixture Quantity)

100 Watt Bulbs (Fixture Wattage for Each) - or -

                                 23 Watt Bulbs (Fixture Wattage for Each)

2000 Hours (Hours of Usage)

$.10 Per Kilowatt/Hr (Average Energy Rate Charged)


How much does it cost annually for 10 - 100 Watt Bulbs?

10 (Fixtures) x 100 (Watts Each) = 1000 Watts

1,000 Watts / 1000 = 1 kw

1 kw x 2000 hrs per year = 2000 kwh (Kilowatt)

2000 kwh x .10 = $200 to operate 10 lamps with 100 Watt Bulbs


How much does it cost annually to operate 10 - 23 Watt Bulbs?

10(Fixtures) x 23 (Watts Each) = 230 Watts

230 Watts / 1,000 = .23 Kw (Kilowatt)

.23 Kw x 2000 hrs per year = 460 kwh (Kilowatt Hour)

460 kwh x .10 = $46 to operate 10 lamps with 23 Watt Bulbs


"Watt's" the difference? $154 Annual Savings

100 Watts - 23 Watts = 77 Watts


$200 - $ 46 = $154 Annual Savings

$154 / 10 Bulbs = $15.40 per bulb

Good Return On Investment?

You could spend up to $15.40 per bulb and get 100% payback in the first year. 

Many of the bulbs last up to 4 years.  If the bulbs pay for themselves in year 1, that gives you 3 more years of savings, or $462 of savings from years 2, 3 and 4. 

You invest $154 and in 4 years you get $462 in return.  That is a 300% return on your money. 

How much would you have in a bank account if you placed it in a regular savings account?

I can guarantee you it won't return 300%, maybe 3%. 

"Watt" are you going to buy next time? 

Want To Learn More?

Check out our FREE eBook that takes a much more indepth look at how energy efficiency makes sense for most business owners.  Just click on the book below....



Hi Jimmy, 
I stumbled onto your website while looking for answers to the real power consumption of CFL’s and LED bulbs. In reading this page everything is very straightforward except you’re taking the wattage of these bulbs at the value printed on them. I have not found these numbers to be correct when tested. I’ve tested several different bulbs, below are just a sample of a few. Calculation formula examples W = PF × V × A: 
Incandescent Bulb 
1x122.20.493 = 60.29 Watts 
CFL bulb 
This CFL is marked: 27 W 0.450 A 
Actual readings: 1x122.32x0.370= 45.26 Watts 
LED bulb 
ChiChinLighting® Dimmable E12 6w LED E12 Candelabra Base Candle Bulb Light Bulbs 60w 60 watt 6000k 
Actual reading: 1x121.69x0.200 = 24.34 Watts 
If there is something I am not understanding here, please enlighten me. 
Posted @ Tuesday, July 15, 2014 1:10 PM by Jack
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Posted @ Wednesday, August 26, 2015 5:28 PM by eruninstic1979
@ Jack... What you're measuring is the VA power (Apparent Power) NOT the True Power (Watts.) 
You will have to do the complex calculations to see the true power, this way you will see the correct results which will be much closer to what is written on the bulb. 
Ohms law does not quite work the same for AC Voltage, as it does not take into account the phase shift between the voltage and current in the AC waveform. This occurs due to the load you are powering, which is an inductive load (CFL) as opposed to a resistive load (filament bulb.) 
Hope this helps
Posted @ Wednesday, September 16, 2015 10:44 AM by John
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