Light Bulb Savings Calculator

What does this calculator do?

The Light Bulb Savings Calculator compares an old bulb (usually an incandescent or halogen) against a replacement — typically an LED — on the things that actually matter when deciding whether to switch: energy use, running cost, replacement cost, payback period, and CO2 avoided. Enter both bulbs' wattage, price, and rated lifespan, plus how you use them, and it works out whether the upgrade pays for itself and how fast.

How the numbers are worked out:

1. Annual energy use: kWh/year = watts × bulb count × hours/day × 365 ÷ 1000, for each bulb.
2. Annual energy cost: kWh/year × your electricity rate.
3. Replacement cost over the analysis period: (hours/day × 365 × years ÷ lifespan hours) × bulb count × bulb price — a fractional bulb count, since you rarely replace bulbs on a clean yearly schedule.
4. Total cost over the period is energy cost plus replacement cost; the year-by-year table spreads that evenly across each year.
5. Payback period is the extra upfront cost of the new bulbs ((new price − old price) × count, floored at $0) divided by the monthly energy savings — replacement cost is ongoing, not part of the initial outlay, so it isn't part of the payback math.
6. CO2 avoided: kWh saved per year × your grid's CO2-per-kWh factor (0.92 lbs/kWh is a common US grid average).

Worked example: swapping a 60W incandescent for an 8W LED, run 5 hours a day at $0.13/kWh, saves about 94.9 kWh and $12.34 a year — even though the LED costs more upfront ($3.50 vs $1.00), it pays that difference back in about 2.4 months from energy savings alone, and the incandescent needs ~18 replacements over 10 years against less than 1 for the LED.

How to use it

1. Enter the wattage for the old and new bulb — check the packaging or the base of the bulb.
2. Enter each bulb's price and rated lifespan in hours (also on the packaging, or the manufacturer's spec sheet).
3. Enter your hours used per day and the number of bulbs you're replacing (a whole fixture, a room, a house).
4. Enter your electricity rate from your utility bill, and how many years you want to project savings over.
5. Optionally adjust the CO2 factor if you know your local grid's carbon intensity — otherwise the US-average default is a reasonable estimate.

The summary table and year-by-year breakdown update as you type. Copy or download either table as CSV.

Why does the LED cost more upfront but still pay back so fast?

LEDs draw far less power for the same light output, so the electricity savings usually dwarf the price difference within months — even though the bulb itself costs 2–5x more than an incandescent. The payback period only counts that price difference against monthly energy savings; it ignores replacement cost, which is already baked into the separate total-savings figures.

Why are "bulbs needed" fractional?

The number of replacements over the analysis period is total operating hours ÷ rated lifespan, which is rarely a whole number — a bulb doesn't stop working exactly on your calculator's schedule. Treat it as an average replacement rate (e.g. 18.3 bulbs over 10 years means roughly 18-19 replacements), not a literal purchase count.

How accurate is this?

Real-world bulb lifespan varies with switching frequency, dimming, heat, and manufacturing quality — rated lifespan is a lab average, not a guarantee. Electricity rates often include tiered pricing, taxes, and fixed fees this tool doesn't model. Treat the output as a solid planning estimate, not an exact bill forecast.

Privacy

This calculator runs entirely in your browser. Your wattage, prices, lifespans, and electricity rate are never sent to a server, logged, or stored — the math happens locally on your device.