Calculate how much switching to LED lighting saves per year in dollars and kWh, plus bulb replacement savings, payback period, and CO2 avoided.
LED Lighting Savings Formula
kWh = N * (Wo - Wl) / 1000 * h * d * 52.18
S = kWh * R
P = (N * C) / S * 12
- kWh = electricity saved per year (kilowatt-hours)
- N = number of bulbs replaced
- Wo = wattage of each current bulb (W)
- Wl = wattage of each LED replacement (W)
- h = hours the lights are on per day
- d = days per week the lights are used (52.18 is the average number of weeks in a year)
- S = money saved per year; R = your electricity rate per kWh
- P = payback period in months; C = price of each LED bulb
The savings mode of the calculator applies the first two formulas directly: it multiplies the wattage difference by your usage hours and electricity rate to find the yearly cost reduction. If you open the advanced options, three refinements are added. Entering bulb prices and lifetimes adds replacement savings, which spread the cost of buying bulbs evenly across their rated life, so a 1,000 hour incandescent that burns out several times a year is charged against your old setup. Entering an LED price also produces the payback period P, the number of months before the upfront cost of the LED bulbs is recovered. Turning on the cooling bonus adds the electricity your air conditioner no longer spends removing bulb waste heat, calculated as kWh * (cooling months / 12) / 3, which assumes an air conditioner with a coefficient of performance of 3. The brightness match mode works in the other direction: it converts your current bulb wattage to lumens using the typical efficacy of its technology, then divides by LED efficacy (about 90 lumens per watt) to tell you which LED wattage gives the same light.
Savings by Bulb Swap and the Air-Conditioning Bonus
The table below shows the yearly savings from replacing one bulb, assuming 3 hours of use per day and an electricity rate of $0.17 per kWh. Both bulbs in each row produce roughly the same brightness.
| Swap (same brightness) | Brightness | kWh saved per year | Saved per year |
|---|---|---|---|
| 100 W incandescent to 15 W LED | ~1,600 lm | 93.1 | $15.83 |
| 75 W incandescent to 11 W LED | ~1,100 lm | 70.1 | $11.92 |
| 60 W incandescent to 9 W LED | ~800 lm | 55.9 | $9.50 |
| 43 W halogen to 9 W LED | ~800 lm | 37.3 | $6.33 |
| 14 W CFL to 9 W LED | ~800 lm | 5.5 | $0.93 |
Almost every watt an old bulb uses beyond its LED replacement ends up as heat in the room. When those lights sit in air-conditioned space, your cooling system spends additional electricity pumping that heat back out, so the true savings are larger than the direct energy math shows. The table below gives the extra savings to add on top of the direct figure, based on how many months per year you run air conditioning and a typical AC coefficient of performance of 3.
| Months of AC cooling per year | Extra savings on top of direct savings |
|---|---|
| 3 months | about 8% |
| 4 months | about 11% |
| 6 months | about 17% |
| 9 months | about 25% |
| 12 months (year-round AC) | about 33% |
If you heat with electric resistance heating in winter, bulb waste heat was offsetting a little of your heating bill, which trims the net benefit slightly during the heating season. With gas heating or mild winters the effect is small enough to ignore.
Example Problems
Example 1: You replace ten 60 W incandescent bulbs with 9 W LEDs. The lights run 4 hours per day, 7 days per week, and you pay $0.17 per kWh. Each LED costs $3.00. First find the yearly hours: 4 * 7 * 52.18 = 1,461 hours. The electricity saved is kWh = 10 * (60 – 9) / 1000 * 1,461 = 745.1 kWh. The money saved is S = 745.1 * 0.17 = $126.67 per year. The payback period is P = (10 * 3.00) / 126.67 * 12 = 2.8 months, so the bulbs pay for themselves before the first season ends.
Example 2: You want the LED equivalent of a 75 W incandescent bulb. Incandescent bulbs produce about 15 lumens per watt, so the bulb puts out 75 * 15 = 1,125 lumens. At a typical LED efficacy of 90 lumens per watt, the matching LED is 1,125 / 90 = 12.5 W. In a store, look for a bulb labeled as a 75 W equivalent, rated near 1,100 lumens, which usually draws 10 to 13 W.
LED Lighting Savings FAQ
How much does replacing one bulb with an LED actually save? A single 60 W incandescent swapped for a 9 W LED saves about 55.9 kWh per year at 3 hours of daily use, which is $9.50 per year at $0.17 per kWh. Double the daily hours and you double the savings. On top of that, one LED outlasts roughly 15 incandescent bulbs, so you avoid a steady stream of replacement purchases.
Do LED bulbs really lower air conditioning costs too? Yes. An incandescent bulb converts roughly 90% of its power into heat, and in cooled space that heat becomes work for your air conditioner. Because a typical AC moves about 3 units of heat per unit of electricity, every 3 kWh of bulb waste heat you eliminate saves about 1 kWh of cooling electricity. That is why the calculator lets you add a cooling bonus based on how many months per year you run AC.
Why does the calculator match bulbs by lumens instead of watts? Watts measure electricity use, not light output, and each technology produces a different amount of light per watt: incandescents manage about 15 lumens per watt while LEDs deliver 80 to 110. If you match watts you will buy an LED that is far too bright. Match the lumen rating instead, or use the brightness match mode to convert your current wattage into the correct LED size.
