How is simple energy savings calculated when upgrading lighting to LED?

• A. Energy saved = (old watts - new watts) × hours used per day × days.
• B. Energy saved = old watts × hours per day.
• C. Energy saved = new watts × days.
• D. Energy saved equals the difference in lumens.

Answer: (D)

When you upgrade to LED, the savings come from using less power, not just producing more light. Energy savings is calculated from how much power you save each hour the light is on, multiplied by how many hours and days it runs. In formula form: energy saved = (old watts − new watts) × hours used per day × days. This gives energy in watt-hours, which you can convert to kilowatt-hours by dividing by 1000.

Lumens measure brightness, not energy use, so the difference in lumens doesn’t determine energy savings. LEDs can produce more light per watt, so you can maintain or increase brightness while using far less power. Example: replacing a 60 W bulb with a 9 W LED and using it 4 hours per day for a year saves (60 − 9) × 4 × 365 = 74,460 Wh ≈ 74.5 kWh.