For a given lighting level, metal halide lamps use about 75 - 80% less energy than conventional incandescent or tungsten halogen lamps.
Metal halide lamps also provide suitable colour for most merchandising applications.
For a given lighting level, high pressure sodium lamps use about 50% less energy than conventional mercury vapour lamps.
High pressure sodium lamps emit a golden yellow colour - are most suitable for exterior lighting and low-usage warehousing.
Installing metal halide lamps instead of incandescent lamps in a store reduces the total electrical consumption by about 30 - 40%.
To find the net effect on the total electricity bill, multiply the lighting part of the bill (40%) times the saving (80%) to find the net reduction (32%).
Similarly installing high pressure sodium lamps instead of mercury vapour lamps reduces a factory's total consumption by 5-12%.

To find the energy costs for lighting energy consumption in the example below, it is necessary to know:

• Electrical energy costs: 7p/kWh
• Lamps are burning 10 hours per day, 5 days per week, 52 weeks per year - 2600 hours per annum
• Lamps in the building total: 1000

Replacing 500W tungsten halogen lamps with 70W metal halide lamps would save £39,130 in energy costs per year, calculated as follows:
Tungsten halogen lamps of 500W rated at 4,800 lumens consume £54,600 of electrical energy each year.
Metal halide lamps of 70W (85W including ballast) rated at 5,200 lumens consume £15,470 of electrical energy each year.
Energy cost for tungsten halogen £54,600 Energy cost for metal halide - £15,470 Annual energy cost savings £39,130

The same quantity, electricity costs and burning hours are used in this second example. Substituting 400W high pressure sodium lamps and ballasts for 1000W mercury lamps saves £112,840 in energy costs for almost the same amount of light. Mercury lamps of 1000W (1050 circuit watts) emit 58,000 initial lumens and consume £191,100 of electrical energy each year. High pressure sodium lamps of 400W (430 circuit watts) emit 47,500 initial lumens and consume £78,260 of electrical energy each year.
Energy cost mercury £191,100
Energy cost HPS £78,260
Annual energy cost savings £112,840

When looking at the potential for saving energy costs it is necessary to check that the current lighting system is providing sufficient light for the visual tasks.
Operator efficiency should always take preference over lighting efficiency.
There are two approaches to saving energy with lighting:
1. One for one lamp substitution: Substitute lower wattage, higher efficiency lamps for present lamps - keep same luminaires - lowest initial cost.
2. Replace present lighting system luminaires and lamps with a more efficient lighting system. Higher capital cost but greater long term savings.