Light Bulb Savings Calculator Formula
Understand the math behind the light bulb savings calculator. Each variable explained with a worked example.
Formulas Used
Annual Savings
annual_savings = (annual_kwh_old - annual_kwh_new) * electricity_rateOld Bulbs Annual Cost
old_annual_cost = annual_kwh_old * electricity_rateLED Annual Cost
new_annual_cost = annual_kwh_new * electricity_rateVariables
| Variable | Description | Default |
|---|---|---|
old_watts | Old Bulb Wattage(W) | 60 |
new_watts | LED Wattage(W) | 9 |
hours_per_day | Hours Used per Day(hours) | 5 |
electricity_rate | Electricity Rate(USD/kWh) | 0.13 |
num_bulbs | Number of Bulbs | 10 |
annual_kwh_old | Derived value= old_watts * hours_per_day * 365 / 1000 * num_bulbs | calculated |
annual_kwh_new | Derived value= new_watts * hours_per_day * 365 / 1000 * num_bulbs | calculated |
How It Works
LED vs Incandescent Energy Savings
LED bulbs use 75-85% less energy than incandescent bulbs while producing the same light output.
Formula
Annual kWh = Wattage x Hours/Day x 365 / 1000 x Number of Bulbs
Annual Savings = (Old kWh - New kWh) x Electricity Rate
A 9W LED replaces a 60W incandescent with equivalent brightness (800 lumens). Over 10 bulbs used 5 hours daily, the savings add up quickly.
Worked Example
Replacing 10 incandescent 60W bulbs with 9W LEDs, used 5 hours/day at $0.13/kWh.
- 01Old annual kWh = 60 x 5 x 365 / 1000 x 10 = 1,095 kWh
- 02New annual kWh = 9 x 5 x 365 / 1000 x 10 = 164.25 kWh
- 03Annual savings = (1,095 - 164.25) x $0.13 = $121.10
Frequently Asked Questions
How long do LED bulbs last?
Quality LEDs last 15,000-25,000 hours (15-25 years at 3 hrs/day). Incandescent bulbs last only 1,000-2,000 hours. This further increases LED savings.
Are LEDs really as bright?
Yes. Compare lumens, not watts. A 9W LED produces about 800 lumens, equivalent to a 60W incandescent. LED color temperature (warm vs cool) is a separate choice.
What about CFL bulbs?
CFLs use about 75% less energy than incandescent (similar to LEDs) but contain mercury, have shorter lifespans, and take time to reach full brightness. LEDs are now the clear best choice.
Ready to run the numbers?
Open Light Bulb Savings Calculator