Annual kWh saved = (old watts − LED watts) × bulbs × hours/day × days/year ÷ 1,000.
How the comparison works
The calculator compares the electrical power drawn by the existing bulbs with the power drawn by the replacement LEDs for the same lighting duration. For example, replacing eight 60-watt bulbs with eight 9-watt bulbs reduces the connected lighting load by 408 watts while all eight lights are on. The annual saving then depends on how long those lights are used.
What to include in the upfront cost
Enter the total purchase cost for all new bulbs. The simple payback result divides that upfront cost by the estimated annual energy-cost saving. It does not account for bulb replacement intervals, differences in light output, dimmers, electricity price changes, installation work, or discounting over time. If the replacement LEDs do not provide adequate brightness or fit the fixture, the comparison is not meaningful even if the wattage is lower.
Compare like with like
Check lumens for brightness, color temperature for appearance, and dimmer compatibility where relevant. Wattage measures power use, not brightness. Choose a replacement that provides the light you need and is rated for the fixture and location.
Interpreting a negative or small saving
If the new bulb wattage is equal to or higher than the old bulb wattage, the tool will not report an energy payback. That does not automatically make the bulb a poor choice; long life, light quality, or fixture requirements may still matter. Treat the result as a comparison of energy costs rather than a complete purchase recommendation.
Related tools and guides
For the basic relationship between watts and kWh, read Watts vs. kilowatt-hours. You can also use the Home Energy Planner to combine several devices in one estimate.