Lighting Systems
What this domain tests
Lighting questions test your command of efficacy, illuminance, power density, and controls. You should be able to compute savings from a retrofit without overstating them.
Core rules
- Efficacy = lumens per watt (lm/W); LED products commonly exceed 100 lm/W.
- Illuminance is measured in lux; 1 footcandle = 10.76 lux.
- Lighting power density (LPD) is W/ft² and is capped by energy code.
- Include lamp wattage, ballast/driver losses, and ballast factor in fixture input watts.
- L70 is the hours until output falls to 70%; it drives maintenance and replacement economics.
Worked example
A 200-fixture office has 4 × 32 W lamps plus 12 W ballast loss = 140 W per fixture, operating 3,000 h/yr. Existing energy = 200 × 140 × 3,000 ÷ 1,000 = 84,000 kWh. A 45 W LED retrofit uses 200 × 45 × 3,000 ÷ 1,000 = 27,000 kWh. Savings = 57,000 kWh; at $0.10/kWh that is $5,700/yr, plus reduced relamping labor.
Traps
Counting lamp watts only and ignoring ballast factor; assuming 8,760 operating hours; ignoring occupancy sensors and daylight dimming; and forgetting that lower LPD can still fail illuminance or glare criteria.