Swapping a bulb for an LED is one of the few household changes where the arithmetic is genuinely simple and the payback is genuinely fast. The US Department of Energy states that residential LEDs, particularly ENERGY STAR rated products, use at least 75% less energy and last up to 25 times longer than incandescent lighting. That is a large enough gap that the interesting question is not whether to switch but which fittings to switch first, and what the change is actually worth in money.
This calculator from Arb Digital answers that in the terms a household bill uses: kilowatt-hours, currency and months. It takes the wattage you have, the wattage you would move to, how long the lights are on and what you pay per unit, then reports the annual saving, how quickly the purchase pays for itself and what the total comes to over the lamps' rated life.
What This LED Savings Calculator Does
It computes the running cost of your existing bulbs and of the LED replacements, subtracts one from the other, and expresses the gap annually and across the LED's rated hours. The payback figure divides the purchase cost by the annual saving, so it tells you how long the bulbs take to pay for themselves out of the electricity bill alone. The energy-cut percentage shows the reduction in power draw, which is the same whatever your tariff.
Boundaries with the closest tools already on the site. The electricity bill calculator totals a whole household's consumption across every appliance rather than isolating lighting. The appliance wattage calculator estimates the draw of a device you do not have a figure for. The room lighting lumens calculator answers how much light a room needs, which is the question you should settle before choosing a replacement wattage. This page is purely the before-and-after cost comparison for a lamp swap.
How to Use It
- Count the bulbs in one batch. Group fittings that run for similar hours; a kitchen and a rarely used loft light should be calculated separately.
- Enter the existing wattage. Read it off the bulb. If the bulb is unmarked, a standard pear-shaped household incandescent is usually 40, 60 or 100 watts.
- Enter the LED wattage you would buy. Choose it by lumens to keep the same brightness, then read the wattage from the same box.
- Put in your real electricity rate. Divide a recent bill's total by the kWh used to get the all-in figure, which is usually higher than the headline unit rate.
- Set the purchase price and rated life. Both are on the packaging, and both change the payback and lifetime figures substantially.
The Formula / How It's Calculated
Energy in kilowatt-hours is power in kilowatts multiplied by hours: kWh = (watts ÷ 1,000) × hours. Annual hours are the daily figure times 365. So the annual saving in energy is (old watts − new watts) ÷ 1,000 × hours per day × 365 × number of bulbs, and the annual saving in money is that figure multiplied by the rate per kWh. Payback is the total purchase cost divided by the annual money saving.
Worked example, matching the defaults. Twelve bulbs move from 60 W to 9 W, on five hours a day, at $0.17 per kWh, costing $3.50 each. The saving per bulb is 51 W, so 12 bulbs save 612 W, or 0.612 kW. Annual hours are 5 × 365 = 1,825. Energy saved is 0.612 × 1,825 = 1,116.9 kWh a year. At $0.17 that is $189.87 a year. The bulbs cost 12 × $3.50 = $42, so payback is $42 ÷ $189.87 = 0.221 years, about 2.7 months. At 1,825 hours a year the 25,000-hour rating runs 13.7 years, giving roughly $2,601 of gross saving and about $2,559 net of the purchase price. The energy cut is 51 ÷ 60 = 85%.
Match on Lumens, Not on Watts
The single most common error in a bulb swap is buying by wattage. Watts measure the electricity a lamp consumes; lumens measure the light it produces. The whole point of LEDs is that the relationship between the two changed, so a wattage that meant "bright" for an incandescent means nothing for an LED. As a rough guide, 800 lumens replaces a 60 W incandescent, 1,100 lumens replaces a 75 W, and 1,600 lumens replaces a 100 W.
Buying a 60 W-equivalent when the fitting had a 100 W bulb produces a room that feels dim, and the usual response is to conclude that LEDs are poor rather than that the replacement was undersized. Read the lumen figure on the front of the box, which is now the primary rating on packaging in most markets, and treat the "equivalent to X watts" line as marketing shorthand. The lumen to lux calculator converts total light output into illuminance on a surface, which is what matters for a work area.
Colour temperature is the second half of the decision and has no effect on cost. Around 2700 K reads as warm white and resembles an incandescent; 4000 K is neutral; 5000 K and above is stark and better suited to a garage or workshop. Mixing temperatures within one room is what makes a retrofit look wrong even when the brightness is correct.
Why Heat Is the Hidden Half of the Saving
An incandescent bulb releases the overwhelming majority of its energy as heat rather than light — this is precisely why it is inefficient. That heat does not vanish. In a cooled building it becomes a load the air conditioning has to remove, so every watt of lighting costs you slightly more than the watt itself. Replacing the lighting in a space that is actively cooled therefore saves twice: once on the lamp and again on the cooling.
The mirror argument is sometimes raised in cold climates: if the bulb heats the room, is the heat wasted? In practice, no sensible heating system is a light bulb. Electric resistance heating from a bulb is at best as efficient as a dedicated electric heater and far worse than a heat pump, and it is delivered at ceiling level where it is least useful. The seasonal offset is real but small, and it never reverses the saving. This calculator deliberately ignores both effects and reports only the direct lighting energy, which is the conservative figure.
Where the Payback Argument Actually Breaks Down
Payback is only impressive when the light is on. A bulb used ten minutes a day has almost no running cost to save, so the LED never repays its purchase price through electricity — it repays it through not needing replacement. That is a lifetime argument, not an energy argument, and it is much weaker for a lamp that is rarely switched on and therefore rarely fails.
Set the hours to 0.25 in this tool and watch the payback stretch to decades. The practical conclusion is to sequence a retrofit by hours of use rather than by room: outdoor and security lighting, kitchens, hallways and any fitting left on during the working day repay in weeks, while a wardrobe light can wait until the existing bulb dies. ENERGY STAR's guidance on certified light fixtures and downlights covers the same logic for fixed fittings, where the lamp and housing are replaced together.
The other genuine caveat is dimmers. Older mains dimmers were designed for a resistive incandescent load and can flicker, buzz or fail to dim smoothly with an LED drawing a fraction of the power. Either buy lamps explicitly rated for the dimmer you have or replace the dimmer with a trailing-edge type. Nothing about that changes the energy maths, but it is the most common reason a technically correct retrofit gets reversed. If you are budgeting a larger project, the payback period calculator handles staged spending across several years.
Reading the Rated Life Figure Honestly
A 25,000-hour rating is not a guarantee that the lamp goes dark at 25,001 hours. LED lifetime is normally quoted as the point at which output has fallen to seventy per cent of the original, because LEDs dim gradually rather than failing outright. In a room where all the fittings were replaced at once, this fade is nearly invisible; noticing it usually requires putting a new lamp next to an old one.
Early failure, when it happens, is almost always the driver electronics rather than the diodes, and heat is the usual cause. An LED lamp sealed inside an unventilated enclosed fitting runs far hotter than the same lamp in an open shade and will not reach its rating. If a fitting is fully enclosed, buy a lamp marked as suitable for enclosed fixtures. For converting between the energy units on a specification sheet, the energy converter handles kWh, joules and BTU, and the recessed lighting calculator covers spacing and count for downlights.
Arb Digital's free tools library covers the energy, cost and payback maths behind household and business projects, and our team is happy to talk through anything the tools cannot answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Replacing by wattage instead of lumens — a wattage rating tells you consumption, not brightness, and undersizing is the usual result.
- Using the headline unit rate — divide a real bill by the kWh it covers so standing and delivery charges per unit are included.
- Averaging hours across very different fittings — calculate high-use and low-use lights separately or the payback figure means nothing.
- Ignoring enclosed fittings and old dimmers — both shorten LED life or cause flicker, and neither shows up in the energy calculation.
- Assuming the saving continues forever — the lifetime figure runs only for the rated hours, which at heavy use may be five years rather than twenty.
Related Free Tools From Arb Digital
Use the electricity bill calculator for whole-home consumption, the appliance wattage calculator when a device has no published rating, the room lighting lumens calculator to size the light a room needs before you buy, the lumen to lux calculator for illuminance on a work surface, and the energy converter for switching between kWh, joules and BTU. Everything else is in the free online tools hub.
Frequently Asked Questions
It depends entirely on hours of use and your tariff. A 60 W bulb replaced by a 9 W LED, running five hours a day at $0.17 per kWh, saves about 93 kWh and roughly $15.80 a year. Twelve of them save around $190.
Divide the purchase cost by the annual electricity saving. Twelve bulbs at $3.50 each saving $189.87 a year pay back in about 2.7 months. A rarely used fitting can take years, which is why high-use lights should be replaced first.
Look for about 800 lumens, which is typically an 8 to 10 W LED. Match on the lumen figure rather than the wattage, because efficiency varies between products and the wattage alone tells you nothing about brightness.
The rating usually refers to the point at which light output has dropped to seventy per cent of the original rather than to total failure. Heat shortens it, so a lamp in a sealed enclosed fitting will not reach the rated figure unless it is specifically designed for one.
The saving is much smaller than replacing an incandescent, because a CFL already uses a fraction of the power. Enter your actual CFL wattage in the existing-bulb field and the calculator will show the real gap, which is often only a few watts per lamp.
Most likely the dimmer was designed for an incandescent load and does not work well with the low current an LED draws. Use lamps explicitly rated as dimmable with your dimmer type, or fit a trailing-edge dimmer intended for LED loads.
Only marginally. Heat from a ceiling lamp is delivered where it is least useful and is no more efficient than any electric resistance heater. This calculator ignores the effect in both directions and reports lighting energy only.
This calculator gives an estimate based on the figures you enter. Actual savings depend on your tariff, usage patterns and product performance, and it is not financial advice.
