What this calculator does
Swapping a 60 W incandescent for a 9 W LED cuts the electricity for that fitting by 85%. At five hours a day and 16 cents a kilowatt-hour, the old bulb costs $19.36 a year to run and replace and the LED costs $3.12, a saving of $16.24 a year from one bulb.
Replacement cost is the part worth including and it usually is not. An incandescent bulb lasts around 1,000 hours, so at five hours a day it needs replacing nearly twice a year. An LED rated at 15,000 hours lasts over eight years at the same usage. The bulbs are a small line next to the electricity, but they run the same way and the comparison is incomplete without them.
The formula
Annual hours are the daily hours multiplied by 365.25. Each bulb costs its wattage in kilowatts multiplied by those hours and by the electricity price, plus the price of a bulb multiplied by how many times it has to be replaced in that period. The saving is the difference, and the payback period is the LED purchase price divided by the monthly saving.
| Term | Meaning |
|---|---|
| Watt | Power drawn. An LED giving the same light as a 60 W incandescent typically draws 8 to 10 W. |
| kWh | A kilowatt-hour, the unit electricity is billed in: one kilowatt drawn for one hour. |
| Bulb life | Rated hours before failure, around 1,000 for incandescent and 15,000 to 25,000 for LED. |
| Payback period | How long the saving takes to cover the purchase price of the LED. |
The inputs explained
| Field | What to enter |
|---|---|
| Old bulb wattage (W) | The wattage of the bulb being replaced. A traditional incandescent is 40 to 100 W, a halogen somewhat less for the same light. |
| LED wattage (W) | The wattage of the LED. Take it from the packaging rather than from the incandescent-equivalent figure printed next to it. |
| Hours used per day | Hours the light is on per day. This drives the result more than anything else. |
| Electricity price ($/kWh) | Your electricity price per kilowatt-hour, from a recent bill rather than a national average. |
| Old bulb price ($) | What a replacement of the old bulb costs. |
| Old bulb lifetime (hours) | Rated life of the old bulb in hours. About 1,000 for a standard incandescent. |
| LED bulb price ($) | What the LED costs to buy. |
| LED lifetime (hours) | Rated life of the LED in hours, usually on the packaging. |
When to use it
Deciding which fittings to change first
The saving scales with hours used, so the answer is always the lights that are on longest. A hallway light on twelve hours a day pays back in weeks while a spare-room light may take years.
Working out whether to replace a working bulb
The question is whether the saving beats the cost of the LED, not whether the old bulb still works. The payback period answers it directly, and at heavy usage it is usually short enough that waiting costs more than switching.
Estimating a whole-house figure
Run the calculation for a representative bulb at each usage level and multiply by how many fittings are like it, rather than averaging everything into one figure that describes no actual light.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much do daily hours change the saving?
The same swap at a range of daily usage levels.
| Hours per day | Annual savings | Old bulb annual cost | Payback period for the LED |
|---|---|---|---|
| 1 h | $3.25 | $3.87 | 14.8 months |
| 3 h | $9.74 | $11.61 | 4.9 months |
| 5 h | $16.24 | $19.36 | 3.0 months |
| 8 h | $25.99 | $30.97 | 1.8 months |
| 12 h | $38.98 | $46.46 | 1.2 months |
Questions
Is it worth replacing a bulb that still works?
It depends almost entirely on how much the light is used. At five hours a day the payback here is three months, so yes comfortably. At one hour a day it is nearly fifteen months, which is still a saving but not an urgent one. Replacing the heavily used fittings first gets most of the benefit for a fraction of the effort.
Do LEDs really last as long as the packaging says?
Often not quite, and the reason is usually heat. The rated life assumes reasonable ventilation, and an LED in an enclosed or recessed fitting runs hotter and fails earlier, sometimes much earlier. The electronics in the base generally fail before the LED itself does. Lowering the rated life here is a fair way to allow for a difficult fitting.
What wattage LED replaces a 60 W bulb?
Around 8 to 10 W for a similar brightness, which is roughly 800 lumens. Compare lumens rather than watts, since wattage describes what a bulb consumes and not what it produces, and the efficiency varies between LED products.
Does the saving depend on where I live?
Yes, through the electricity price, which is the input with the largest effect after usage hours. The default of 16 cents per kWh is illustrative only. Take the figure from a recent bill, dividing the total by the kilowatt-hours used, since that captures supply charges and tariffs that a headline rate does not.
For what a single appliance costs to run over a year, see the electricity bill calculator. For comparing bulb brightness rather than consumption, see the lumens to watts calculator.