How to Calculate Total Fixture Wattage for Fluorescent Lighting

Posted by Jimmy Hovey on Tue, Mar 06, 2012 @ 08:03 AM

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

Topics: t5 retrofit, t5 vs. t8, Energy savings, calculate energy savings, energy efficiency, t8 vs. t5, calculating wattage, t8 fixtures, wattage