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# How to Quickly Calculate the Annual Expense for Electricity to Operate Any Light Fixture Knowing how to quickly calculate the annual light fixture electricity cost will help you decide which light fixture to select. For instance, an LED light fixture may use 50 watts of energy, while an equivalent metal halide version may use 112 watts of energy. Even if the sticker price of an LED light fixture is 20% higher than its metal halide equivalent, the LED fixture may be a better long-term bargain. To intelligently choose between LED and metal halide fixtures, you need to know how to calculate the annual electricity cost of each.

## How Do I Find My Light Fixture Electricity Cost?

The operating cost of a traditional lamp includes electricity and the price of replacing the light every 10,000 to 15,000 hours of use, depending on the average life expectancy of the specific lamp. LED lamps, boards, and chips can operate for over 100,000 hours before needing to be replaced. Still, the biggest and most important cost of operating a light fixture is electricity. To find the cost of electricity required to operate a light fixture, you will need to first find the cost of your electricity per kilowatt hour (kWh).

## Find the Cost of Electricity per Kilowatt Hour (kWh) To find your cost of electricity per kWh, you will need a copy of your most recent electricity bill. With bill in hand, here is how to calculate the cost per kWh:

1. Find the cost of electricity from your monthly electricity bill. Use the total cost, as it allows you to include all of the charges that are being assessed. For example, the total cost of a National Grid bill in Massachusetts includes a customer charge, distribution fee, transition charge, energy-efficiency charge, renewable energy charge, a basic service charge, and a sales tax. Many utilities will also include a fee for electricity used during “peak demand.”
2. Find the total number of kilowatts used on your invoice.
3. Divide the total cost of electricity by the kilowatt hours.
4. You have now solved for the cost per kWh.

## Determine the Hours of Use for the Luminaire

Once you have the value for the cost per kWh, determine the hours per day the fixture will be used and then how many days per week it will be used. You will need this data to use the chart below.

## Calculating the Cost of Electricity to Operate a Luminaire 1. Scan down the left column and locate your cost per kWh.
2. Scan to the right to find the appropriate “Hours/Day” column.
3. Within that column, find the correct days of operation per week (5, 6, or 7). The cell that lines up with both the cost-per-kilowatt-hour row and the “Hours/Day” and “Days/Week” columns is your operating cost per watt per year.
4. Multiply the operating cost per watt, per year, times the total watts used by the light fixture.
5. You have calculated the light fixture electricity cost for that luminaire.

## An Example of Calculating the Annual Light Fixture Electricity Cost

We’ll now walk through an example. Let’s say you’re torn between two wall packs. Either way, both wall packs will be used seven nights a week for about eight hours each night, both emitting similar light, but with very different technologies—which will be the most electricity-efficient choice? Let’s find out.

• Find the cost of electricity from your monthly electricity bill. See the following sample electricity bill. Total cost (“AMOUNT DUE”) is \$1,342.28.
• Find the total number of kilowatt hours used on your invoice. On the invoice below, “Total Energy” is 5,850 kWh.
• Divide the total cost of electricity by the kilowatt hours. \$1,342.28 / 5,850 = .23
• You have now solved for the cost per kWh. Electricity at this location costs \$0.229, which we can round up to \$0.23/kWh.
• Scan the left column of the “Annual Cost Per Watt” chart to find your cost per kWh. \$.023 is the third-to-last row.
• Scan to the right to find the appropriate “Hours/Day” column. The wall pack will be used for eight hours a day. This puts us in one of the first three columns.
• Within that column, find the correct days of operation per week (5, 6, or 7). The cell that lines up with both the cost per kilowatt hour row and the “Hours/Day” and “Days/Week” columns is your operating cost per watt per year. The wall pack will be used seven days a week. Our operating cost is \$0.67 per watt, per year.
• Multiply the operating cost per watt per year times the total watts used by each light fixture. For a PSMH wall pack: \$0.67 x 150 = \$100.50. For an LED wall pack: \$0.67 x 21 = \$14.07
• You have calculated the cost of electricity per year to operate a luminaire. The LED wall pack will be \$86.43 less expensive per year than the MH wall pack. An LED wall pack is by far the more electricity-efficient choice. But how long will it take to pay off? Considering LEDs often have a rated life of 100,000 hours, they can be expected to last for an astonishing 34 years. That makes for almost \$3,000 saved over the life of the fixture—and that’s before the cost of replacement drivers, replacement ballasts, and replacement lamps for the PSMH fixture.

## Cost per Watt Calculator

Do you know how much you’re spending on energy? More importantly, how much money are you losing each year? By switching to more energy-efficient lighting like LED wall packs or LED bollards, you could help lower your light fixture electricity cost.

Unfortunately, 22% of America’s facilities have never undergone any type of lighting upgrade, or haven’t had any work done in over 10 years, according to a survey in the April 2013 issue of Today’s Facility Manager. That means 1.1 million facilities are wasting energy and money. By taking the above steps, you can begin to calculate how much you’re spending each year. For example, energy is priced at approximately \$0.15 per kilowatt hour in the Boston area. A walkway in a commercial area has 15 bollards equipped with 70-watt metal-halide lamps and ballasts. If the luminaires are on seven days a week for ten hours a day, it would cost \$0.55 for each watt and \$577.50 in total annual costs.

But what if they decide to switch to LED lighting? The same bollards are equipped with 15-watt LED modules and drivers, which emit about the same amount of light. If the luminaires are on seven days a week for ten hours a day, it would still cost \$0.55 for each watt but only amount to a total annual cost of \$123.75. For each year they delay switching to efficient lighting, they’re losing \$453.75—only on the bollards!