What it shows
Three bulbs giving the same brightness draw very different power: about 60 watts for a filament bulb, 14 for a CFL and 9 for an LED. Using the lumens and watts printed on the packs and the price per unit from a bill, a ten-year cost comparison shows the LED is cheapest by far despite its higher price.
The science
Lumens measure the light a bulb gives out and watts measure the power it draws, so lumens per watt is its efficiency. A filament bulb turns most of its power into heat, a CFL excites a gas that makes a coating glow, and an LED turns current almost directly into light. Running cost is watts times hours times the rate per unit; lifetime cost adds replacements, since filament bulbs last about 1,000 hours, CFLs about 8,000 and LEDs 15,000 or more.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Packaging or label photos of a filament bulb, a CFL and an LED of similar lumens
- Shop prices for each
- The per-unit rate from an electricity bill
- A calculator or spreadsheet
- Graph paper
Safety
For this project: Reading labels only; do not open or test the bulbs. A CFL contains a small amount of mercury and must not be broken.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Pick three bulbs of about 800 lumens and note their watts, price and rated life from the packs.
Work out lumens per watt for each.
Assume four hours of use a day and calculate yearly units for each bulb.
Multiply by the per-unit rate for the yearly running cost.
Divide 14,600 hours (ten years at four hours a day) by each bulb's rated life to find how many bulbs are needed.
Add the purchase cost of all the bulbs to ten years of running cost.
Draw a line graph of cumulative cost against years for the three bulbs.
Mark the point where the LED line drops below the filament line, the payback time.
What you should see
A graph where the filament bulb costs over Rs 5,000 across ten years, the CFL about a quarter of that, and the LED the least, with the LED paying back its price within a few months.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does lumens per watt tell you?
How efficiently a bulb turns electricity into light.
2Why does a filament bulb get hot?
Most of its power becomes heat, not light.
3Why should a CFL not be thrown in ordinary waste?
It contains a small amount of mercury.
4What is payback time?
The time for the saving in bills to equal the extra purchase price.
Take it further
Count the bulbs in your school and estimate the yearly saving from switching all of them to LEDs.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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