The three numbers, in the order they matter
A lighting swap has exactly three cost components and they are usually ranked backwards in people's heads. Largest is the electricity, which is watts saved multiplied by burn hours multiplied by the rate. Second is replacement bulbs over the period, which is where incandescent quietly loses badly because it dies every year or so. Smallest, almost always, is the purchase price of the LED, which is the only one visible on the shelf.
Ten fixtures going from 60 W to 9 W at four hours a day saves 744 kWh a year. At 16 cents that is about $119 annually, against roughly $30 of bulbs. The purchase price is paid back in about three months, and over ten years the difference between the two routes runs to four figures once replacement incandescents are counted. The shelf comparison of $2 against $6 is the least informative view of the decision available.
Why "60 watt equivalent" is not a wattage
Bulb packaging leads with an equivalence claim because that is the unit people still think in, but the equivalence is about light output, not power. What matters for light is lumens; what matters for the bill is watts. A traditional 60 W incandescent produces roughly 800 lumens. An LED producing the same 800 lumens draws somewhere around 8 to 10 W depending on its efficacy, and that actual figure is printed on the box in smaller type, usually inside the Lighting Facts panel.
| Light output | Incandescent | Halogen | CFL | LED |
|---|---|---|---|---|
| about 450 lm | 40 W | about 29 W | about 10 W | about 5 W |
| about 800 lm | 60 W | about 43 W | about 14 W | about 9 W |
| about 1,100 lm | 75 W | about 53 W | about 19 W | about 12 W |
| about 1,600 lm | 100 W | about 72 W | about 23 W | about 15 W |
Those LED figures are typical rather than fixed — efficacy varies meaningfully between products and improves over time, so read the box you are holding. The pattern in the table is what matters: halogen saves perhaps a quarter over incandescent, CFL saves about three quarters, and LED saves a little more than CFL while starting instantly, dimming properly and containing no mercury. The large step in this table is from incandescent to anything, not from CFL to LED.
Burn hours are per fixture, and they are wildly uneven
Averaging across a house gives a wrong answer in both directions. A kitchen or living room fixture might run five or six hours a day. A porch light on dusk-to-dawn runs ten or twelve. A bedroom lamp runs one. A basement stair light runs ten minutes. The savings scale directly with hours, so the fixtures worth converting first are not the ones with the highest wattage, they are the ones with the highest wattage multiplied by hours.
This is also what decides where a swap is not worth doing. A 60 W closet bulb switched on for two minutes a day burns about 12 hours a year and costs roughly 12 cents to run. No bulb price repays that. Convert the lights that are on, leave the ones that are not until they fail, and the arithmetic sorts the order for you.
What rated life means and what shortens it
An LED rated at 25,000 hours is not promised to work for 25,000 hours. The figure comes from a standardised projection — commonly the point at which output has dropped to seventy percent of initial, extrapolated from a shorter test — and it describes a population, not your bulb. Some fail early. The failure is also usually not the diode: it is the small switch-mode driver in the base, and heat is what kills it.
That gives a practical rule. LEDs live longest in open fixtures with air movement and shortest in sealed enclosures, recessed cans buried in insulation, and enclosed outdoor fittings that sit in the sun. Bulbs rated for enclosed fixtures exist and are worth the premium in those locations. If a particular fixture in your house has always eaten bulbs, it will eat LEDs too, and the lifetime column of this calculation should be adjusted downward for it rather than trusted.
A payback figure is a projection, not a result
Everything on this page after the annual energy saving depends on assumptions about the future: that your rate holds, that your usage holds, that the bulbs last. Rates move, sometimes sharply and sometimes by season. Usage changes when the household does. The honest way to read a payback period of three months is that the swap is so far ahead that no plausible movement in the inputs changes the conclusion — and the honest way to read a payback of eight years is that it might never arrive, because something else will happen first. Where the number lands near the boundary, run it again at a rate a few cents higher and a few cents lower and see whether the decision actually changes.
Questions people ask
How much does an LED bulb save per year compared with incandescent?
For a single 60 W incandescent replaced by a 9 W LED running four hours a day, about 74 kWh a year, which is roughly $12 at 16 cents per kWh. Ten such fixtures is about $119. The saving scales exactly with three things — the wattage difference, the hours the light is on, and your rate — so a porch light on twelve hours a night saves three times as much as the same bulb in a bedroom. Multiply the wattage difference by the annual burn hours, divide by 1,000, and multiply by your rate. That is the whole calculation.
How long does it take for LED bulbs to pay for themselves?
For a normally used fixture the answer is usually months rather than years, but it depends entirely on burn hours and it is a projection rather than a guarantee. A $3 LED replacing a 60 W incandescent at four hours a day saves about $12 a year, so it repays in roughly three months. The same bulb in a fixture used fifteen minutes a day saves about 75 cents a year and takes four years. Where the payback comes out long, the case for swapping rests on not having to change bulbs rather than on the electricity, and there is nothing wrong with leaving a rarely used fixture alone until the bulb fails.
Is it worth replacing CFL bulbs with LED?
Much less compelling than replacing incandescent, and often not worth doing while the CFL still works. A 14 W CFL against a 9 W LED at the same brightness saves 5 W, which is about $2.90 a year at four hours a day and 16 cents per kWh — a two-year payback at typical bulb prices before you count that you are throwing away a working bulb. The non-financial reasons are stronger than the financial ones: LEDs come to full brightness instantly, dim properly, tolerate being switched on and off frequently, work in cold, and contain no mercury. Replacing CFLs as they fail is the sensible middle course.
Do LED bulbs really last 25,000 hours?
The number is a projection about a population under test conditions, not a durability promise about your bulb, and real installations routinely fall short of it. Rated life is typically defined as the point at which light output has fallen to a stated fraction of initial — often seventy percent — extrapolated from a shorter measurement, and it assumes the ambient conditions of the test. Heat is what actually determines the outcome, and the driver electronics in the base fail long before the diodes do. An LED in an open fixture in a cool room may well go a decade. The same bulb in a sealed outdoor fitting or a recessed can packed in insulation can fail in two or three years, which is why enclosed-rated bulbs exist and why the lifetime figures in this calculator are editable.