How To Calculate Watts For Metal Halide (HID) Lighting

Posted by Jimmy Hovey on Thu, Dec 22, 2011 @ 10:12 AM

Why Do I Need To Know The Wattage Of My Metal Halide Fixtures?

The only way to see if an energy efficient update to your facility makes sense is to calculate the cost of what you already have.  Your electric bill won't tell you this because the bill is a aggregate view of all of the electricity consumed from your facility, not just the lighting.

By calculating the cost of your existing fixtures, you are able to make educated decisions regarding upgrades to energy efficient lighting products. 

Here are the 5 steps involved with calculating the total wattage of HID lighting fixtures because this is what we see most when visiting customers.  Here are the steps you need to follow:

Step 1 - Determine Your Type of Fixture

This article assumes that you are aware of your metal halide fixtures.  But if not, here is what you need to be looking for:

The Bulb:

Metal Halide Bulb

250 Watt Metal Halide Low-Bay

400 Watt Low Bay Metal Halide

400 Watt Metal Halide High-Bay

High Bay 400 Watt Metal Halide


Step 2 - Determine the Size Of Your Metal Halides

What Is A Size?

Size, when talking about Metal Halide, the size actually refers to the wattage of the fixture.  So you likely have 175 Watt MH, 250 Watt MH, 400 Watt MH or 1000 Watt MH.  While the size indicates the wattage, it is not the actual wattage of the fixture ( I will explain in Step 4...)

Where To Find Your Size

Supply Room

Metal Halide come in many shapes and sizes.  One of the easiest ways to find out the size of your fixtures is to look on the supply room shelf.  You likely have a replacement bulb in there somewhere under a layer of dust. 

Maintenance Staff

If you have a maintenance person, they will be able to tell you what size of Metal Halide you have. Metal Halide are typically very high maintenance and if light quality is important to what you do, you have likely order either bulb or ballast replacements in the past year. 

Get A Ladder!

Climb on up there and get the number, that is if your comfortable on the ladder and you are not too high up.  Be careful, make sure someone spots your ladder and be sure to let them cool down.....these babies heat up to over 900 you won't want to be using the fixture as a balance point!

Hire A Professional Electrical Contractor

If you want to do your own calculations, but want to be exact, bring in your Electrical Contractor.  They work with these types of fixtures all the time.  An hour or two walking around your facility will give you a pretty good idea what you have. 

(TIP: Depending on the contractor, they may not be too excited about energy efficient products.  Contractors are typically a little slow to change, so if you get this sense, just let them know you want help with determining what you have.  Learn more about How Contractors Are Afraid Of Energy Efficient Products in a post I did earlier this week. If your in Michigan, Tennesse or Kentucky, we can help you out!)

Step 3 - Need Your Fixture Quantity

If You're Just Seeing If It Is Worth Pursuing, Just Get it Close

If you are just doing rough numbers for now, you can get a rough estimate simply by counting your fixtures.  If you already have a close number, then just use that at this stage.

If You're Ready To Submit Paperwork For Rebates, You Need To Be Exact

If you are getting ready to submit paperwork for a utility rebate, it is in your best interest to get an exact count. Most programs pay you for each fixture or total kilowatt reduction.  To get the most rebate amount, get some help counting. 

We have had 3 different people count in some facilities and each of us come up with a different number.  You may have to count a couple of times. 

The point here is you need a quantity that is close to work with to do your final calculation.

Step 4 - Determine Actual Wattage

Just because your Metal Halide is 175 Watt or 400 Watt, that doesn't mean that is the actual wattage of the fixture.  I guess it is easier to say 400 Watt rather than 458 Watt.  Here are the ACTUAL wattages of the most common types of Metal Halide that we see.

  • 175 Watt Metal Halide  Fixture = 205 Actual Watts
  • 250 Watt Metal Halide Fixture =290 Actual Watts
  • 400 Watt Metal Halide Fixture = 458 Actual Watts
  • 1000 Watt Metal Halide Fixture =1,081 Actual Watts
When doing your energy efficiency calculations, you will want to use the actual wattage to get an accurate picture of what you can save potentially on your energy bill. 

Step 5 - Multiply Quantity x Actual Wattage

Let's say you have (100) existing fixtures and you have determined they are 1000 Watt Metal Halide:

100 fixtures x 1,081 Watts = 108,100 Total System Watts (Assuming this is the only type of fixture you have...)

Step 6 - Compare Existing Total Watts With Proposed Total Watts

If you are considering an energy efficient upgrade, you should see a major difference in wattage consumption.  In this case, one alternative technology we would look at is inductive fluorescent.  We could easily replace the 1000 watt Metal Halides with 250 Watt inductive fluorescents.  Here is how we would compare:

100 (New Fixtures) x 265 Watts = 26,500 Total System Watts. 

Step 7 - Compare Old With New

Old Total System Watts - 108,100

New Total System Watts - 26,500

Wattage Reduction New System = 81,600 Watts

In this case, you would be reducing your total system wattage by over 80% which will translate into a significant amount of savings over time.  This may seem unbelievable to you but it's true and we wrote an article about the project titled:Energy Efficient Lighting for Dealerships and Large Parking Lots

Want To Learn More About Energy Efficiency

1. Read Some Related Blog Posts

We write extensively about different energy efficient technologies and how to calculate your energy efficient savings.  Here are a few suggested articles:

Learn How Watts Can Improve Energy Efficiency; "Watt's" a Watt?

Metal Halide:The "Shocking Truth" In The Life Of A Metal Halide Bulb

Types Of HID Lights And Replacements For |Energy Efficient Lighting| 

2. Or...Download Our Free E-book: Business Owners Guide To Energy Efficiency

The concept of energy efficiency is often mis-understood as a gimmick or scam.  We have created an e-book to help dispel that type of rhetoric.  The purpose of the book is to show you how you can see significant savings with new, updated energy efficient technologies. 

Download a copy to learn about the benefits of energy efficient technologies for your business. 

Topics: how to calculate watts, energy efficient lighting