LED vs Incandescent Savings Calculator

How it works

A bulb's electricity use is its watts times the hours it is on. An LED gives the same light (lumens) as an incandescent for a fraction of the watts, so replacing like-for-like brightness cuts the energy by the ratio of the two wattages. The calculator multiplies each bulb's watts by the number of bulbs and by hours per day × 365, divides by 1,000 for kWh, and prices the kWh at your rate.

Bulbs also wear out. Over a year, a bulb on for H hours uses up H ÷ rated life of its life, so the yearly replacement cost is the bulb price times that fraction. Incandescent bulbs rated around 1,000 hours are replaced far more often than LEDs rated 15,000 hours or more.

The simple payback is the extra purchase cost of the LEDs divided by the yearly electricity savings; if no electricity cost is saved (the LEDs draw as much as the old bulbs, the lights are never on, or the rate is zero) there is no payback; the avoided replacements are shown separately in the total savings and shorten the real payback further. If you do not know the LED's wattage, choose Lumens ÷ LED efficacy: the LED watts are the lumens you need divided by the LED's lumens per watt.

Formula

LED W (lumens mode)  = lumens ÷ LED efficacy (lm/W)
hours per year       = hours per day × 365
kWh per year         = W × bulbs × hours per year ÷ 1,000
kWh saved            = (W_current − W_LED) × bulbs × hours per year ÷ 1,000
electricity savings  = kWh saved × rate ÷ 100
replacements per year = bulbs × price × hours per year ÷ rated life
total savings        = electricity savings + current replacements − LED replacements
payback (years)      = (LED price − current price) × bulbs ÷ electricity savings

Example

Ten 60 W incandescent bulbs on 3 hours a day (1,095 hours a year) use 657 kWh a year; ten 9 W LEDs use 98.6 kWh. The 558.5 kWh saved is worth $100.52 a year at 18 ¢ per kWh ($118.26 against $17.74).

At $1 each and 1,000 hours, the incandescents cost $10.95 a year in replacements; the $2.50 LEDs rated 15,000 hours cost $1.83. Total savings are $109.65 a year, and the $15 extra for the LEDs is repaid by the electricity savings in 0.15 years, under two months.

Assumptions and limitations

• The LED is assumed to give the same light as the bulb it replaces; compare lumens on the packages. The efficacy and rated-life defaults are typical values, not the figures for a particular product.
• Rated life is a laboratory median; individual bulbs fail earlier or later, and frequent switching, heat and enclosed fixtures shorten it. Replacement cost is spread evenly over the years rather than paid when a bulb fails.
• Simple payback ignores interest, price changes and the value of avoided replacements, and uses the extra purchase cost of the LEDs only. A negative or zero price difference means the LEDs cost no more, shown as a payback of 0.
• The rate is a single flat price per kWh; the 18 ¢ default is illustrative. A year is 365 days of the same daily use.
• Heat given off by incandescent bulbs slightly reduces heating and increases cooling loads; that interaction is not modelled.

Frequently asked questions