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Lighting Energy Savings Calculator

See how much energy, money and CO₂ you'd save each year by swapping old bulbs for LEDs, and how quickly the new bulbs pay for themselves.

Level: Grades 6–7

Printable worksheet

Lighting Energy Savings Calculator

Know the answer you need? Pick an input to work backward.

Inputs

The watts printed on the old bulb, for example 60 W for a standard incandescent.

The actual watts on the LED's package. A "60 W equivalent" LED usually uses 8–10 W.

Your price per kilowatt-hour, from a recent bill. Include delivery charges if they're billed per kWh.

Used to work out how long the new bulbs take to pay for themselves.

Default: US average from EPA eGRID2023 (767.209 lb/MWh). Regional and utility figures differ, so use your own if you know it.

Result

Money saved per year

$94.94

Swapping 10 bulbs from 60 W to 9 W saves about 558.5 kWh and $94.94 a year.

Energy saved per year
558.45 kWh
Yearly cost with the old bulbs
$111.69
Yearly cost with the new bulbs
$16.75
Saved per month (average)
$7.91
Cut in lighting energy
85%
CO₂ avoided per year
428.33 lb
CO₂ avoided per year (metric)
194.29 kg
Cost of the new bulbs
$30.00
Payback time
3.8 months

See the step-by-step solution ↓

Step-by-step solution

  1. Find how many hours a year the lights are on: hours per day × days per year

    3 h×365=1,095 h

  2. Find the old bulbs' yearly energy: bulbs × watts × hours, then ÷ 1,000 to turn watt-hours into the kilowatt-hours on your bill

    10×60 W×1,095 h1,000=657 kWh

  3. Find the new bulbs' yearly energy the same way

    10×9 W×1,095 h1,000=98.55 kWh

  4. Subtract to find the energy you save each year

    657−98.55=558.45 kWh

  5. Find the money saved each year: the energy saved × your electricity rate

    558.45 kWh×$0.17 /kWh=$94.94

  6. Find the payback time: what the new bulbs cost ÷ what they save each year, then × 12 to get months

    $30.00$94.94×12=3.7920083424 months

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

See how much you'd save each year by switching your old light bulbs to LEDs. A bulb's running cost depends on three things. They are how many watts it draws, how many hours it's on, and what you pay for each kilowatt-hour (kWh) of electricity. An LED that gives the same light as a 60 W incandescent usually draws 8 to 10 W. So a swap can save a lot, especially for lights that stay on for hours.

This calculator compares a set of old bulbs with their replacements. It shows the energy and money saved each year and what each setup costs to run. It also shows the cut in lighting energy and the carbon dioxide (CO₂) you avoid, using a grid rate you can change. And if you enter the bulb price, it tells you how many months the new bulbs take to pay for themselves.

Where it's used

Why this math matters outside the classroom:

  • Home budgets. A family swapping ten 60 W bulbs for 9 W LEDs used 3 hours a day saves about $95 a year at $0.17 per kWh.
  • Office retrofits. A facilities manager finds forty 32 W tubes replaced by 15 W LED tubes, on 10 hours a day, save 6.8 kWh a day.
  • School projects. Students measure classroom light use and convert watts and hours into kilowatt-hours and dollars.
  • Unit rates. Shoppers compare a $3 LED that saves $9.49 a year with a $5 one that saves the same to see which pays back first.
  • Carbon estimates. A sustainability club multiplies 558 kWh saved by 0.767 lb per kWh to report about 428 lb of CO₂ avoided.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.b. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Enter how many bulbs you're replacing and the wattage of the old and new bulbs.
  2. Enter how many hours a day they're on and how many days a year.
  3. Enter your electricity rate from a recent bill. You can also add the price of each new bulb.
  4. Read the yearly savings, the payback time and the CO₂ avoided. Change the CO₂ rate if you know your local grid's figure.

What the inputs mean

Old bulb wattage (each)
The watts printed on the old bulb, for example 60 W for a standard incandescent.
New bulb wattage (each)
Use the actual wattage printed on the LED package, not the 'equivalent' wattage. A '60 W equivalent' LED usually draws 8 to 10 W.
Electricity rate
To get your real rate, divide your bill's energy charges by the kWh you used. You enter it yourself because prices vary by utility, plan and season.
Price of each new bulb (optional)
Used to work out how long the new bulbs take to pay for themselves.
Grid CO₂ per kWh
The default, 0.767 lb per kWh, is the US national average from the EPA's eGRID2023 data. Grids that burn more coal give off more per kWh. Grids with more hydro, nuclear, wind or solar give off less.

Formula and method

hours per year = hours per day × days per year
kWh per year = bulbs × watts × hours per year ÷ 1,000
kWh saved = old kWh − new kWh
money saved = kWh saved × rate
payback (months) = bulbs × bulb price ÷ money saved per year × 12
CO₂ avoided = kWh saved × grid emission rate

Watts times hours gives watt-hours. Divide by 1,000 and you get kilowatt-hours, the unit on your electricity bill. Take the difference between the old and new setups and multiply by your rate. That's your yearly saving.

Payback compares what the new bulbs cost up front with what they save. It ignores the money you'd have spent replacing old bulbs that burn out sooner. So the real payback is usually a little quicker.

Worked example

You replace ten 60 W bulbs with 9 W LEDs that cost $3 each. They're on 3 hours a day, every day, and electricity costs $0.17 per kWh.

  1. Hours per year: 3 × 365 = 1,095 h.
  2. Old bulbs: 10 × 60 × 1,095 ÷ 1,000 = 657 kWh; new bulbs: 10 × 9 × 1,095 ÷ 1,000 = 98.55 kWh.
  3. Energy saved: 657 − 98.55 = 558.45 kWh, worth 558.45 × $0.17 = $94.94.
  4. Payback: $30 ÷ $94.94 × 12 = 3.8 months.
  5. CO₂ avoided at 0.767 lb/kWh: 558.45 × 0.767 = 428.3 lb (about 194.3 kg).

Result: The swap saves about $94.94 and 558 kWh a year and cuts lighting energy by 85%. It pays for itself in under four months.

Understanding the result

Lights that are on for many hours a day save the most. A bulb used ten minutes a day may take years to pay back. One that's on all evening pays back in months.

The CO₂ figure is an estimate based on averages. What a particular saving really avoids depends on which power plants cut back when demand drops. That varies by region and time of day.

Assumptions and limitations

  • Bulbs draw their rated wattage whenever they're on, and the hours of use don't change after the swap.
  • The electricity rate stays the same for the year.
  • All bulbs in the comparison are the same type and are used for the same hours.
  • It doesn't include the cost of replacing old bulbs, which LEDs avoid because they last far longer.
  • It ignores the small amount of heat incandescent bulbs give off, which slightly changes heating and cooling bills.
  • You enter the rates and emission factors. The defaults are examples, not a quote for your home.

Frequently asked questions

How much does one 60 W bulb cost to run?

On for 3 hours a day, it uses 60 × 1,095 ÷ 1,000 = 65.7 kWh a year. That's about $11.17 at $0.17 per kWh. A 9 W LED doing the same job uses 9.855 kWh, about $1.68.

What wattage LED replaces a 60 W bulb?

Look for an LED giving about 800 lumens, the light output of a typical 60 W incandescent. Lumens measure brightness. Such bulbs usually draw 8 to 10 W. Compare lumens, not watts, when choosing brightness.

Where does the CO₂ figure come from?

The default is the US average rate of 767.209 lb of CO₂ per MWh (megawatt-hour, or 1,000 kWh) from the EPA's eGRID2023 data. It covers the total output of US power plants. That's 0.767 lb per kWh. Your utility's or region's figure may be higher or lower.

Is it worth replacing a bulb that still works?

For lights used for hours each day, the energy saving usually pays for the new bulb within a year. For lights you rarely use, it can take much longer. So many people replace those only when they burn out.

Sources

How this calculator is checked

Method: Energy = power × time (kWh = W × h ÷ 1,000); cost = energy × rate; default emission factor from US EPA eGRID2023 national average CO₂ output rate (767.209 lb/MWh). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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