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How to Calculate Total Fixture Wattage for Fluorescent Lighting


Step 1 - Determine Your Type of Fluorescent Fixture


In order to calculate your total fixture wattage, you need to know what the wattage is of the existing system you have in place.  There are numerous variables, but typically we see T-12 fluorescent lighting in most places.  Even if you are aware of that you use T-12's, knowing what type of T-12 fixture is very important.  Here are the 3 that we run into most

Step 2 - What Size are the Fluorescent Fixtures?

There are primarily 3 sizes of fluorescent fixtures; They are:

  • 2' x 2' - Typically a square type of fixture.  Found most often in office situations. This style fixture has a U-shape bulb.
  • 2' x 4' - Rectangular fixtures that are typically used as drop in's in office situations.  They are sometimes used in manufacturing environments as direct lighting over a particular area.These fixtures have 4' bulbs and most have 3-4 bulbs per fixture.
  • 8' - Long and narrow is the best description of these types of fixtures.  Typically seen in 2 - 8' bulb configurations.
(Quick Tip: If you are unsure of what type of fixture you have, T-12, T-8 or T-5, take a look in your storage room, look at the bulb or box, or ask your maintenance staff what type of bulbs they are using)

Step 3 - Identify the Quantity of Each Fluorescent Fixture

Once you have identified the type of fixture you have in your facility(you may have many types) total up the number of each type in each area.  If you have a lot of fixtures to count, get some help. You can easily get lost in your count walking around looking up in the ceiling (oh ya, don't trip on anything either!)

Now that you have your fixture count, you are ready to calculate your total wattage. 

Step 4 - Identify the Wattage for Each Type of Fluorescent Fixture

There are a couple of different ways to figure out the wattage of the fixtures in your facility.  One is complicated and the other is not so complicated. 


If you really want to be exact and have lots of time on your hands, then you could theoretically go to each and every fixture to find out what type of ballast you have.  This would be more exact as then you could determine your ballast factor, type of ballast and other relevant information. 

This way would require a tremendous amount of time and effort which isn't necessary since companies like General Electric and most utilities adopt averages for calculations. 

Not So Complicated:

You don't have to be exact when determining your fixture wattage.  You should be able to determine from a distance what the type of fixture is and then determine the wattage. From there you can use averages like the ones that we use when doing an energy efficiency assessment. 

Here are the fixture wattages that we typically use:

  • 89 Watts - 2'x2' -U-shape Lamp

  • 140 Watts - 2'x4'(3 Lamp)

  • 178 Watts - 2'x4'(4 Lamp)

  • 158 Watts - 8'(2 Lamp) 

  • 178 Watts - 8'(4 - 4' Lamps)

By using the averages, we are able to get a good comparison against newer technologies with lower fixture wattages.

Step 5 - Multiply Your Quantity of Fixtures by the Wattage of Your Existing Fixture.

Let's say you have 100 fixtures and they are 8' - 2 Bulb T-12's:

100 fixtures x 158 Watts = 15,800 Total System Watts (Assuming this is the only type of fixture you have...)

Want one more complicated?

Okay, how about you have an office with (20) 2' x 4' 4 bulb T-12's, a work area with (78) 8' - 2 Bulb T-12's:

20 fixtures x 178 Watts =  3,560 Total System Watts

78 fixtures x 158 Watts =  12,234  Total System Watts

3,560 Watts  + 12,234 Watts =  15,884 Total System Watts

Step 6- Forget the Calculations and Call Hovey Electric.....

If your still confused about all of this, here are some resources for you to consider:

See my article "Watts" a Watt? If you want to see how to take this calculation further and calculate your energy savings from a new system upgrade...or...


If you would like to learn more about the benefits of energy efficiency for your business, just download our ebook, by clicking on the book below.  It's FREE and available right away, get your copy.... 



Related Articles:

How To Choose The Right Type Of Energy Efficient Fluorescent Ballasts

Where Is The Best Place To Use T5 Lighting Technology?

T8 vs. T5: How To Determine Which Light Is The Right Light

Energy Efficient Fluorescent Lighting Options


Dear sir, 
I want your help. 
Our office room size will be 12' X 12' X 10' height. 
We want use 4 X 18W Florescent lamp. 
How many lights required for this office we will calculate. 
Thanks and regards, 
Posted @ Monday, August 13, 2012 1:54 AM by binoj
Dear sir,  
I want your help.  
Our Dining Hall size will be 13.5m X 17.75 X 5 height.  
We want use 4 X 18W Florescent lamp.  
How many lights required for this office we will calculate.  
Thanks and regards,  
Posted @ Wednesday, September 12, 2012 3:53 AM by Marwa
Our school has fluorescent lighting that uses 48-4 foot tubes per classroom. How much does it cost for 1 hour of electricity usage? When is it advantageous to turn the light switch off for short periods of time when leaving classroom or when to just leave them on?
Posted @ Tuesday, November 13, 2012 1:29 PM by joyce ohlau
If anyone has PG&E, like me, you can log into their website and view your daily usage. Very informative stuff. 
I've noted certain things I've done at certain times and gone back to view the history. Pretty neat to see how much your A/C and other appliances use. 
Other electric companies may have the same type of usage reports. You just have to log into their website and check.
Posted @ Thursday, December 20, 2012 2:51 PM by Tom
does it matter when calculating if fixtures are 277 volts
Posted @ Sunday, January 27, 2013 9:48 AM by dave
I have a small project to work on 
I need to calculate how many light fixture I do need for this size hallway(apartment building) 
It is possible to email me the result and the formula for the calculation 
I may have 2 more project that I'm working on  
Thank you so much 
Thank you
Posted @ Wednesday, January 30, 2013 9:48 AM by Paolo
I have a small project to work on  
I need to calculate how many light fixture I do need for this size hallway(apartment building)  
It is possible to email me the result and the formula for the calculation  
Thank you so much  
Thank you
Posted @ Saturday, February 02, 2013 3:58 PM by Zia Raman
<a> Why dont you just get an LED retrofit, its way better energy savings and they are 10yr warranty... never have maintenance cost.. WAYYY better.
Posted @ Thursday, February 21, 2013 7:12 PM by Just go LED
what is the starting power consumption of fluorescent tube light ( T-12 & T-8 ) ?( Starting current or voltage )
Posted @ Friday, August 16, 2013 7:10 AM by ravinder
I am thinking of converting 33 older magnetic T12 8' 2 lamp fixtures over to Electronic ballasted 8' T8's. Can you help caculate the energy savings to see if its worth while doing?
Posted @ Wednesday, November 06, 2013 12:35 PM by Steve
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Posted @ Thursday, January 30, 2014 6:16 AM by Decorative Lighting
Our church recently purchased an old Wal-Mart building and we would like to replace the strip lighting with 4 bulb recessed lights The room is 64x64x13 feet. How many fixtures do we need. Thank you
Posted @ Saturday, February 15, 2014 2:40 PM by John Mitchell
how to select watt vs feet vs dia of florescent lamp
Posted @ Tuesday, March 18, 2014 6:17 AM by p.priyadharshini
I have converted a garden garage into an Botanical Art Studio. I have windows on the East facing side. The room is 11ft by 15 ft and 7ft in height. I am looking to used Graphica Pro Triphoshor tubes as they will give me daylight effect, but I don't know how many or wattage required. Any advice would be appreciated. Thanks
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Posted @ Saturday, October 18, 2014 2:15 AM by kmart coupons
I have 4 foot led 18w bulb what is current rating sir 
Posted @ Wednesday, December 17, 2014 7:13 PM by rakesh arun
why is suitable an electronics ballast
Posted @ Thursday, December 18, 2014 8:45 AM by Mahavir Singh
Our shop building is about 100' × 150' we are assessing our hibay liting costs if we upgrade our lites. We currently have 20 400w lites and 118 dual tube 8' flour. Fixtures These are mounted at a hieght of 22'. I am thinking of retrofiting the t12 fixtures to t8 any comments or advice would be appreciated.Thank you
Posted @ Sunday, January 04, 2015 11:18 AM by Steve Manning
what are two voltages commonly used for fluorescent light fixtures?
Posted @ Thursday, March 05, 2015 1:41 PM by Lucy
I have a garage with 304 (40W), 48 inch tubes. They are on 24 hrs per day. What is daily cost in northeast 
Florida? Thanks
Posted @ Thursday, March 26, 2015 8:48 AM by Richard
Usually our workshop produce grille light 4X18W T8 type is 595X595X80mm and 603X603X80mm,these actual size are available.
Posted @ Sunday, April 19, 2015 9:21 PM by Miller
To Richard, Thursday, March 26, 2015: I would hope that with LEDs now widely available for almost everyone at much lower pricing, that you are calculating your "tube" solution to REPLACE them. You would probably save 50-80% on your electric bill by using LEDs. Good luck.
Posted @ Wednesday, April 22, 2015 3:45 PM by Ken
im looking for some 2x2 fluorescent fixtures with f40t12cw u tubes I been using the fixtures I have for about 20 years and I like them can you help me thank you. LARRY
Posted @ Tuesday, May 12, 2015 12:25 PM by larry evans
Want your help how many watts the day light fluorescent 4' lamp willtake if I switch on for the whole day long
Posted @ Sunday, May 24, 2015 9:23 PM by khin Swe Thet
I have been told that if we remove two bulbs from a 25 watt T-8 fluorescent fixture with four bulbs, we will not save much or any electricity because the ballast will still be doing the work for four bulbs. Would be there be any savings by removing two bulbs, and if so, approximately how much?
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