You want to purchase a new luminaire. How much energy will it use and how much will the energy it uses cost? Buyer beware! The watts used have a basis in the lamp or lamps, but it is not based exclusively on the wattage of the lamp.

Why does it matter? Energy is expensive. You may think the luminaire you purchased for your business uses 400 watts but the real usage may be 458 watts. Based on electricity priced at $.15/kWh and a luminaire being used 12 hours a day 5 days a week, the difference 58 watts would cost and extra $27.26 per year. Most businesses use many fixtures so the unexpected cost could be significant.

What are lamp watts? Lamp watts are the watts that a lamp is rated for. For instance a 400-watt metal malide lamp is rated for 400 watts. A 1000 watt high pressure sodium lamp is rated for 1000 watts. One might expect that to be the end of the story, but it is not.

What are system watts? Most lamps, with the exception of basic incandescent lamp, which runs on line voltage, use a ballast. The ballast takes the voltage provided and changes it to the correct format to light the lamp and keep it lit. The ballast requires watts too. System watts are the total watts the lamp and the ballast use together. The system watts for a 400 watt metal halide is about 458 watts depending on the ballast.  When you purchase a new luminaire or compare one lighting technology to another, be sure to ask for the system watts. That way you know which system is actually more energy efficient and what the system will actually cost to operate.

Metal Halide (MH)
High Pressure Sodium (HPS)
 System Watts (approximate) 
100w MH 142 watts
175w MH 210 watts
250w MH 295 watts
400w MH 461 watts
1000w MH 1080 watts
250w HPS 295 watts
400w HPS 460 watts
750w HPS 840 watts
1000w HPS 1100 watts


To learn more about the difference between lamp watts and system watts, contact an Access Fixtures lighting specialist.