LED Lighting Savings Calculator

This calculator quantifies the saving from swapping old bulbs for LEDs. You enter the number of bulbs, the old and new wattages, daily usage and your tariff, and it returns the annual energy and money saved, the CO2e avoided, and how quickly the LED bulbs pay for themselves.

How it works

Energy use is wattage times hours, so the saving is the wattage difference run over a year, converted to kWh and then to money and carbon:

watts_saved = (old_W − led_W) × bulbs
kWh_saved   = watts_saved / 1000 × hours_per_day × 365
bill_saving = kWh_saved × tariff_per_kWh
co2_saved   = kWh_saved × grid_factor (≈0.21 kgCO2e/kWh)
payback_yrs = (led_cost × bulbs) / bill_saving

Because LEDs draw a fraction of the power of the bulbs they replace, the watts saved per bulb is large and the saving scales directly with how many hours the lights are on.

Worked example

Suppose you have ten 60 W incandescent bulbs in a living room and kitchen, each running about 4 hours a day. You replace them with 9 W LED equivalents at around £5 each, and your electricity tariff is £0.28/kWh.

watts_saved = (60 − 9) × 10 = 510 W
kWh_saved   = 510/1000 × 4 × 365 = 745 kWh/year
bill_saving = 745 × £0.28 = £208/year
payback     = (£5 × 10) / £208 = 0.24 years (about 3 months)

After 3 months the bulbs pay for themselves, then they deliver around £208 in savings every year for their remaining 15,000+ hour lifespan. That is the equivalent of many years of free electricity for those ten fixtures.

Prioritising which bulbs to replace first

Not all bulbs are equal targets. A few guidelines:

• Replace high-hours-per-day lights first. A kitchen downlight on 6 hours daily saves far more in year one than a bathroom bulb on 30 minutes.
• Target high-wattage bulbs. A 100 W incandescent replaced by a 12 W LED saves 88 W per hour — more than double the saving of replacing a 40 W bulb.
• Halogens are underrated targets. Kitchen ceiling halogens are often 35–50 W each, run all day, and are directly replaceable by GU10 LED spots at 5–7 W.

Matching brightness: lumens, not watts

When buying LEDs, match by lumens, not watts — a lower-watt LED can deliver the same light as a higher-watt incandescent. Rough equivalents:

Choosing by lumens ensures you keep the same light quality while cutting the wattage — and therefore maximises the saving this calculator measures.